Add model mutex, make var_pool const by default

63 files changed, 851 insertions, 595 deletions

diff --git a/build2/algorithm b/build2/algorithm
index ad52fe1..552ea7f 100644
--- a/build2/algorithm
+++ b/build2/algorithm
@@ -72,7 +72,7 @@ namespace build2
   // indicate this.
   //
   void
-  match (action, target&);
+  match (slock&, action, target&);
 
   // Note that calling this function only makes sense if the target itself
   // doesn't have its own dependents.
@@ -84,7 +84,7 @@ namespace build2
   // detection. Note that this function does not touch the dependents count.
   //
   void
-  match_only (action, target&);
+  match_only (slock&, action, target&);
 
   // Match a "delegate rule" from withing another rules' apply() function
   // skipping recursive matches (thus the third argument). Return recipe and
@@ -93,20 +93,20 @@ namespace build2
   // execute_delegate().
   //
   pair<recipe, action>
-  match_delegate (action, target&, const rule&);
+  match_delegate (slock&, action, target&, const rule&);
 
   // The standard prerequisite search and match implementations. They call
   // search_and_match_*() versions below passing non-empty directory for
   // the clean operation.
   //
   void
-  search_and_match_prerequisites (action, target&);
+  search_and_match_prerequisites (slock&, action, target&);
 
   // If we are cleaning, this function doesn't go into group members,
   // as an optimization (the group should clean everything up).
   //
   void
-  search_and_match_prerequisite_members (action, target&);
+  search_and_match_prerequisite_members (slock&, action, target&);
 
   // The actual prerequisite search and match implementations. They call
   // search() and then match() for each prerequisite in a loop. If this
@@ -117,10 +117,11 @@ namespace build2
   // match) prerequisites that are not in the same or its subdirectory.
   //
   void
-  search_and_match_prerequisites (action, target&, const dir_path&);
+  search_and_match_prerequisites (slock&, action, target&, const dir_path&);
 
   void
-  search_and_match_prerequisite_members (action, target&, const dir_path&);
+  search_and_match_prerequisite_members (
+    slock&, action, target&, const dir_path&);
 
   // Unless already available, match, and, if necessary, execute the group
   // in order to obtain its members list. Note that even after that the
@@ -128,7 +129,7 @@ namespace build2
   // fallback rule matched).
   //
   group_view
-  resolve_group_members (action, target&);
+  resolve_group_members (slock&, action, target&);
 
   // Inject dependency on the target's directory fsdir{}, unless it is in the
   // src tree or is outside of any project (say, for example, an installation
@@ -138,7 +139,7 @@ namespace build2
   // rule's apply() function.
   //
   fsdir*
-  inject_fsdir (action, target&, bool parent = true);
+  inject_fsdir (slock&, action, target&, bool parent = true);
 
   // Execute the action on target, assuming a rule has been matched
   // and the recipe for this action has been set. This is the default
diff --git a/build2/algorithm.cxx b/build2/algorithm.cxx
index f3bb8a0..6c812f3 100644
--- a/build2/algorithm.cxx
+++ b/build2/algorithm.cxx
@@ -53,7 +53,7 @@ namespace build2
   }
 
   pair<const rule*, match_result>
-  match_impl (action a, target& t, bool apply, const rule* skip)
+  match_impl (slock& ml, action a, target& t, bool apply, const rule* skip)
   {
     pair<const rule*, match_result> r (nullptr, false);
 
@@ -153,7 +153,7 @@ namespace build2
                            << diag_do (ra, t);
                     }));
 
-                if (!(m = ru.match (ra, t, hint)))
+                if (!(m = ru.match (ml, ra, t, hint)))
                   continue;
 
                 if (!m.recipe_action.valid ())
@@ -180,7 +180,7 @@ namespace build2
                              << diag_do (ra, t);
                       }));
 
-                  if (!ru1.match (ra, t, hint))
+                  if (!ru1.match (ml, ra, t, hint))
                     continue;
                 }
 
@@ -212,7 +212,7 @@ namespace build2
                   // @@ We could also allow the rule to change the recipe
                   // action in apply(). Could be useful with delegates.
                   //
-                  t.recipe (ra, ru.apply (ra, t));
+                  t.recipe (ra, ru.apply (ml, ra, t));
                 }
                 else
                 {
@@ -240,7 +240,7 @@ namespace build2
   }
 
   group_view
-  resolve_group_members_impl (action a, target& g)
+  resolve_group_members_impl (slock& ml, action a, target& g)
   {
     group_view r;
 
@@ -249,7 +249,7 @@ namespace build2
     //
     if (!g.recipe (a))
     {
-      auto rp (match_impl (a, g, false));
+      auto rp (match_impl (ml, a, g, false));
 
       r = g.group_members (a);
       if (r.members != nullptr)
@@ -259,7 +259,8 @@ namespace build2
       // phase.
       //
       const match_result& mr (rp.second);
-      g.recipe (mr.recipe_action, rp.first->apply (mr.recipe_action, g));
+      g.recipe (mr.recipe_action,
+                rp.first->apply (ml, mr.recipe_action, g));
     }
 
     // Note that we use execute_direct() rather than execute() here to
@@ -275,7 +276,7 @@ namespace build2
   }
 
   void
-  search_and_match_prerequisites (action a, target& t, scope* s)
+  search_and_match_prerequisites (slock& ml, action a, target& t, scope* s)
   {
     for (prerequisite& p: group_prerequisites (t))
     {
@@ -283,29 +284,30 @@ namespace build2
 
       if (s == nullptr || pt.in (*s))
       {
-        match (a, pt);
+        match (ml, a, pt);
         t.prerequisite_targets.push_back (&pt);
       }
     }
   }
 
   void
-  search_and_match_prerequisite_members (action a, target& t, scope* s)
+  search_and_match_prerequisite_members (
+    slock& ml, action a, target& t, scope* s)
   {
-    for (prerequisite_member p: group_prerequisite_members (a, t))
+    for (prerequisite_member p: group_prerequisite_members (ml, a, t))
     {
       target& pt (p.search ());
 
       if (s == nullptr || pt.in (*s))
       {
-        match (a, pt);
+        match (ml, a, pt);
         t.prerequisite_targets.push_back (&pt);
       }
     }
   }
 
   fsdir*
-  inject_fsdir (action a, target& t, bool parent)
+  inject_fsdir (slock& ml, action a, target& t, bool parent)
   {
     tracer trace ("inject_fsdir");
 
@@ -334,7 +336,7 @@ namespace build2
     // Target is in the out tree, so out directory is empty.
     //
     fsdir* r (&search<fsdir> (d, dir_path (), string (), nullopt, nullptr));
-    match (a, *r);
+    match (ml, a, *r);
     t.prerequisite_targets.emplace_back (r);
     return r;
   }
diff --git a/build2/algorithm.ixx b/build2/algorithm.ixx
index 90a0789..7da189d 100644
--- a/build2/algorithm.ixx
+++ b/build2/algorithm.ixx
@@ -51,14 +51,16 @@ namespace build2
   }
 
   pair<const rule*, match_result>
-  match_impl (action, target&, bool apply, const rule* skip = nullptr);
+  match_impl (slock&, action, target&, bool apply, const rule* skip = nullptr);
 
   inline void
-  match (action a, target& t)
+  match (slock& ml, action a, target& t)
   {
     if (!t.recipe (a))
-      match_impl (a, t, true);
+      match_impl (ml, a, t, true);
 
+    //@@ MT
+    //
     t.dependents++;
     dependency_count++;
 
@@ -70,74 +72,79 @@ namespace build2
   {
     // text << "U " << t << ": " << t.dependents << " " << dependency_count;
 
+    //@@ MT
+    //
     assert (t.dependents != 0 && dependency_count != 0);
     t.dependents--;
     dependency_count--;
   }
 
   inline void
-  match_only (action a, target& t)
+  match_only (slock& ml, action a, target& t)
   {
     if (!t.recipe (a))
-      match_impl (a, t, false);
+      match_impl (ml, a, t, false);
   }
 
   inline pair<recipe, action>
-  match_delegate (action a, target& t, const rule& r)
+  match_delegate (slock& ml, action a, target& t, const rule& r)
   {
-    auto rp (match_impl (a, t, false, &r));
+    auto rp (match_impl (ml, a, t, false, &r));
     const match_result& mr (rp.second);
-    return make_pair (rp.first->apply (mr.recipe_action, t), mr.recipe_action);
+    return make_pair (rp.first->apply (ml, mr.recipe_action, t),
+                      mr.recipe_action);
   }
 
   group_view
-  resolve_group_members_impl (action, target&);
+  resolve_group_members_impl (slock& ml, action, target&);
 
   inline group_view
-  resolve_group_members (action a, target& g)
+  resolve_group_members (slock& ml, action a, target& g)
   {
     group_view r (g.group_members (a));
-    return r.members != nullptr ? r : resolve_group_members_impl (a, g);
+    return r.members != nullptr ? r : resolve_group_members_impl (ml, a, g);
   }
 
   void
-  search_and_match_prerequisites (action, target&, scope*);
+  search_and_match_prerequisites (slock&, action, target&, scope*);
 
   void
-  search_and_match_prerequisite_members (action, target&, scope*);
+  search_and_match_prerequisite_members (slock&, action, target&, scope*);
 
   inline void
-  search_and_match_prerequisites (action a, target& t)
+  search_and_match_prerequisites (slock& ml, action a, target& t)
   {
     search_and_match_prerequisites (
+      ml,
       a,
       t,
       (a.operation () != clean_id ? nullptr : &t.root_scope ()));
   }
 
   inline void
-  search_and_match_prerequisite_members (action a, target& t)
+  search_and_match_prerequisite_members (slock& ml, action a, target& t)
   {
     if (a.operation () != clean_id)
-      search_and_match_prerequisite_members (a, t, nullptr);
+      search_and_match_prerequisite_members (ml, a, t, nullptr);
     else
       // Note that here we don't iterate over members even for see-
       // through groups since the group target should clean eveything
       // up. A bit of an optimization.
       //
-      search_and_match_prerequisites (a, t, &t.root_scope ());
+      search_and_match_prerequisites (ml, a, t, &t.root_scope ());
   }
 
   inline void
-  search_and_match_prerequisites (action a, target& t, scope& s)
+  search_and_match_prerequisites (slock& ml, action a, target& t, scope& s)
   {
-    search_and_match_prerequisites (a, t, &s);
+    search_and_match_prerequisites (ml, a, t, &s);
   }
 
   inline void
-  search_and_match_prerequisite_members (action a, target& t, scope& s)
+  search_and_match_prerequisite_members (
+    slock& ml, action a, target& t, scope& s)
   {
-    search_and_match_prerequisite_members (a, t, &s);
+    search_and_match_prerequisite_members (ml, a, t, &s);
   }
 
   target_state
diff --git a/build2/b.cxx b/build2/b.cxx
index b06459b..54defb3 100644
--- a/build2/b.cxx
+++ b/build2/b.cxx
@@ -96,6 +96,8 @@ main (int argc, char* argv[])
            << system_error (errno, system_category ()); // Sanitize.
 #endif
 
+    ulock ml (model);
+
     // Parse the command line. We want to be able to specify options, vars,
     // and buildspecs in any order (it is really handy to just add -v at the
     // end of the command line).
@@ -362,7 +364,7 @@ main (int argc, char* argv[])
       //
       if (dirty)
       {
-        var_ovs = reset (cmd_vars);
+        var_ovs = reset (ml, cmd_vars);
         dirty = false;
       }
 
@@ -572,7 +574,7 @@ main (int argc, char* argv[])
 
             // See if the bootstrap process set/changed src_root.
             //
-            value& v (rs.assign ("src_root"));
+            value& v (rs.assign (var_src_root));
 
             if (v)
             {
@@ -650,7 +652,7 @@ main (int argc, char* argv[])
           // Note that the subprojects variable has already been processed
           // and converted to a map by the bootstrap_src() call above.
           //
-          if (auto l = rs.vars["subprojects"])
+          if (auto l = rs.vars[var_subprojects])
           {
             for (const auto& p: cast<subprojects> (l))
             {
@@ -874,7 +876,7 @@ main (int argc, char* argv[])
             trace << "  out_root:  " << out_root;
             trace << "  src_root:  " << src_root;
 
-            if (auto l = rs.vars["amalgamation"])
+            if (auto l = rs.vars[var_amalgamation])
               trace << "  amalgamat: " << cast<dir_path> (l);
           }
 
@@ -973,7 +975,7 @@ main (int argc, char* argv[])
 
           // Load the buildfile.
           //
-          mif->load (ts.buildfile, rs, ts.out_base, ts.src_base, l);
+          mif->load (ml, rs, ts.buildfile, ts.out_base, ts.src_base, l);
 
           // Next search and match the targets. We don't want to start
           // building before we know how to for all the targets in this
@@ -1007,7 +1009,11 @@ main (int argc, char* argv[])
                           ? out_src (d, rs)
                           : dir_path ());
 
-            mif->search (rs, target_key {ti, &d, &out, &tn.value, e}, l, tgs);
+            mif->search (ml,
+                         rs,
+                         target_key {ti, &d, &out, &tn.value, e},
+                         l,
+                         tgs);
           }
         } // target
 
@@ -1026,8 +1032,8 @@ main (int argc, char* argv[])
 
           action a (mid, pre_oid, oid);
 
-          mif->match (a, tgs);
-          mif->execute (a, tgs, true); // Run quiet.
+          mif->match (ml, a, tgs);
+          mif->execute (ml, a, tgs, true); // Run quiet.
 
           if (mif->operation_post != nullptr)
             mif->operation_post (pre_oid);
@@ -1041,8 +1047,8 @@ main (int argc, char* argv[])
 
         action a (mid, oid, 0);
 
-        if (mif->match != nullptr)   mif->match (a, tgs);
-        if (mif->execute != nullptr) mif->execute (a, tgs, verb == 0);
+        if (mif->match != nullptr)   mif->match (ml, a, tgs);
+        if (mif->execute != nullptr) mif->execute (ml, a, tgs, verb == 0);
 
         if (post_oid != 0)
         {
@@ -1057,8 +1063,8 @@ main (int argc, char* argv[])
 
           action a (mid, post_oid, oid);
 
-          mif->match (a, tgs);
-          mif->execute (a, tgs, true); // Run quiet.
+          mif->match (ml, a, tgs);
+          mif->execute (ml, a, tgs, true); // Run quiet.
 
           if (mif->operation_post != nullptr)
             mif->operation_post (post_oid);
diff --git a/build2/bin/init.cxx b/build2/bin/init.cxx
index 913c887..6bd2636 100644
--- a/build2/bin/init.cxx
+++ b/build2/bin/init.cxx
@@ -49,7 +49,7 @@ namespace build2
       //
       if (first)
       {
-        auto& v (var_pool);
+        auto& v (var_pool.rw (r));
 
         // Note: some overridable, some not.
         //
@@ -325,7 +325,7 @@ namespace build2
       // Load bin.config.
       //
       if (!cast_false<bool> (b["bin.config.loaded"]))
-        load_module ("bin.config", r, b, loc, false, hints);
+        load_module (r, b, "bin.config", loc, false, hints);
 
       // Cache some config values we will be needing below.
       //
@@ -427,13 +427,13 @@ namespace build2
       // Make sure bin.config is loaded.
       //
       if (!cast_false<bool> (b["bin.config.loaded"]))
-        load_module ("bin.config", r, b, loc, false, hints);
+        load_module (r, b, "bin.config", loc, false, hints);
 
       // Enter configuration variables.
       //
       if (first)
       {
-        auto& v (var_pool);
+        auto& v (var_pool.rw (r));
 
         v.insert<process_path> ("bin.rc.path");
         v.insert<process_path> ("bin.ranlib.path");
@@ -528,7 +528,7 @@ namespace build2
         if (ranlib != nullptr)
         {
           r.assign<process_path> ("bin.ranlib.path") = move (ari.ranlib_path);
-          r.assign<string>       ("bin.ranlib.id") = move (ari.ranlib_id);
+          r.assign<string>       ("bin.ranlib.id")   = move (ari.ranlib_id);
           r.assign<string>       ("bin.ranlib.signature") =
             move (ari.ranlib_signature);
           r.assign<string>       ("bin.ranlib.checksum") =
@@ -554,10 +554,10 @@ namespace build2
       // Make sure the bin core and ar.config are loaded.
       //
       if (!cast_false<bool> (b["bin.loaded"]))
-        load_module ("bin", r, b, loc, false, hints);
+        load_module (r, b, "bin", loc, false, hints);
 
       if (!cast_false<bool> (b["bin.ar.config.loaded"]))
-        load_module ("bin.ar.config", r, b, loc, false, hints);
+        load_module (r, b, "bin.ar.config", loc, false, hints);
 
       return true;
     }
@@ -577,13 +577,13 @@ namespace build2
       // Make sure bin.config is loaded.
       //
       if (!cast_false<bool> (b["bin.config.loaded"]))
-        load_module ("bin.config", r, b, loc, false, hints);
+        load_module (r, b, "bin.config", loc, false, hints);
 
       // Enter configuration variables.
       //
       if (first)
       {
-        auto& v (var_pool);
+        auto& v (var_pool.rw (r));
 
         v.insert<process_path> ("bin.ld.path");
         v.insert<path>         ("config.bin.ld", true);
@@ -652,10 +652,10 @@ namespace build2
       // Make sure the bin core and ld.config are loaded.
       //
       if (!cast_false<bool> (b["bin.loaded"]))
-        load_module ("bin", r, b, loc, false, hints);
+        load_module (r, b, "bin", loc, false, hints);
 
       if (!cast_false<bool> (b["bin.ld.config.loaded"]))
-        load_module ("bin.ld.config", r, b, loc, false, hints);
+        load_module (r, b, "bin.ld.config", loc, false, hints);
 
       const string& lid (cast<string> (r["bin.ld.id"]));
 
@@ -666,8 +666,8 @@ namespace build2
       if (lid == "msvc")
       {
         const target_type& pdb (b.derive_target_type<file> ("pdb").first);
-        install_path (pdb, b, dir_path ("bin")); // Goes to install.bin
-        install_mode (pdb, b, "644");            // But not executable.
+        install_path (b, pdb, dir_path ("bin")); // Goes to install.bin
+        install_mode (b, pdb, "644");            // But not executable.
       }
 
       return true;
@@ -688,13 +688,13 @@ namespace build2
       // Make sure bin.config is loaded.
       //
       if (!cast_false<bool> (b["bin.config.loaded"]))
-        load_module ("bin.config", r, b, loc, false, hints);
+        load_module (r, b, "bin.config", loc, false, hints);
 
       // Enter configuration variables.
       //
       if (first)
       {
-        auto& v (var_pool);
+        auto& v (var_pool.rw (r));
 
         v.insert<process_path> ("bin.rc.path");
         v.insert<path>         ("config.bin.rc", true);
@@ -763,10 +763,10 @@ namespace build2
       // Make sure the bin core and rc.config are loaded.
       //
       if (!cast_false<bool> (b["bin.loaded"]))
-        load_module ("bin", r, b, loc, false, hints);
+        load_module (r, b, "bin", loc, false, hints);
 
       if (!cast_false<bool> (b["bin.rc.config.loaded"]))
-        load_module ("bin.rc.config", r, b, loc, false, hints);
+        load_module (r, b, "bin.rc.config", loc, false, hints);
 
       return true;
     }
diff --git a/build2/bin/rule b/build2/bin/rule
index 5031be1..c903990 100644
--- a/build2/bin/rule
+++ b/build2/bin/rule
@@ -20,10 +20,10 @@ namespace build2
       obj_rule () {}
 
       virtual match_result
-      match (action, target&, const string& hint) const override;
+      match (slock&, action, target&, const string& hint) const override;
 
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
     };
 
     class lib_rule: public rule
@@ -32,10 +32,10 @@ namespace build2
       lib_rule () {}
 
       virtual match_result
-      match (action, target&, const string& hint) const override;
+      match (slock&, action, target&, const string& hint) const override;
 
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
 
       static target_state
       perform (action, target&);
diff --git a/build2/bin/rule.cxx b/build2/bin/rule.cxx
index ede2c85..648c8b6 100644
--- a/build2/bin/rule.cxx
+++ b/build2/bin/rule.cxx
@@ -20,7 +20,7 @@ namespace build2
     // obj
     //
     match_result obj_rule::
-    match (action a, target& t, const string&) const
+    match (slock&, action a, target& t, const string&) const
     {
       fail << diag_doing (a, t) << " target group" <<
         info << "explicitly select obje{}, obja{}, or objs{} member";
@@ -29,7 +29,7 @@ namespace build2
     }
 
     recipe obj_rule::
-    apply (action, target&) const {return empty_recipe;}
+    apply (slock&, action, target&) const {return empty_recipe;}
 
     // lib
     //
@@ -46,7 +46,7 @@ namespace build2
                    "insufficient space");
 
     match_result lib_rule::
-    match (action act, target& xt, const string&) const
+    match (slock& ml, action act, target& xt, const string&) const
     {
       lib& t (static_cast<lib&> (xt));
 
@@ -77,7 +77,7 @@ namespace build2
         if (t.a == nullptr)
           t.a = &search<liba> (t.dir, t.out, t.name, nullopt, nullptr);
 
-        match_only (act, *t.a);
+        match_only (ml, act, *t.a);
       }
 
       if (s)
@@ -85,7 +85,7 @@ namespace build2
         if (t.s == nullptr)
           t.s = &search<libs> (t.dir, t.out, t.name, nullopt, nullptr);
 
-        match_only (act, *t.s);
+        match_only (ml, act, *t.s);
       }
 
       t.data (match_data {type}); // Save in the target's auxilary storage.
@@ -102,7 +102,7 @@ namespace build2
     }
 
     recipe lib_rule::
-    apply (action act, target& xt) const
+    apply (slock& ml, action act, target& xt) const
     {
       lib& t (static_cast<lib&> (xt));
 
@@ -115,10 +115,10 @@ namespace build2
       // Now we do full match.
       //
       if (a)
-        build2::match (act, *t.a);
+        build2::match (ml, act, *t.a);
 
       if (s)
-        build2::match (act, *t.s);
+        build2::match (ml, act, *t.s);
 
       return &perform;
     }
diff --git a/build2/c/init.cxx b/build2/c/init.cxx
index f33ba9e..c5b8ac8 100644
--- a/build2/c/init.cxx
+++ b/build2/c/init.cxx
@@ -112,11 +112,11 @@ namespace build2
       // Load cc.core.vars so that we can cache all the cc.* variables.
       //
       if (!cast_false<bool> (rs["cc.core.vars.loaded"]))
-        load_module ("cc.core.vars", rs, rs, loc);
+        load_module (rs, rs, "cc.core.vars", loc);
 
       // Enter all the variables and initialize the module data.
       //
-      auto& v (var_pool);
+      auto& v (var_pool.rw (rs));
 
       cc::config_data d {
         cc::lang::c,
@@ -142,23 +142,23 @@ namespace build2
         v.insert<strings>      ("c.loptions"),
         v.insert<strings>      ("c.libs"),
 
-        v["cc.poptions"],
-        v["cc.coptions"],
-        v["cc.loptions"],
-        v["cc.libs"],
+        v.insert ("cc.poptions"),
+        v.insert ("cc.coptions"),
+        v.insert ("cc.loptions"),
+        v.insert ("cc.libs"),
 
         v.insert<strings>      ("c.export.poptions"),
         v.insert<strings>      ("c.export.coptions"),
         v.insert<strings>      ("c.export.loptions"),
         v.insert<vector<name>> ("c.export.libs"),
 
-        v["cc.export.poptions"],
-        v["cc.export.coptions"],
-        v["cc.export.loptions"],
-        v["cc.export.libs"],
+        v.insert ("cc.export.poptions"),
+        v.insert ("cc.export.coptions"),
+        v.insert ("cc.export.loptions"),
+        v.insert ("cc.export.libs"),
 
-        v["cc.type"],
-        v["cc.system"],
+        v.insert ("cc.type"),
+        v.insert ("cc.system"),
 
         v.insert<string>   ("c.std", variable_visibility::project),
 
@@ -224,7 +224,7 @@ namespace build2
       // Load c.config.
       //
       if (!cast_false<bool> (rs["c.config.loaded"]))
-        load_module ("c.config", rs, rs, loc, false, hints);
+        load_module (rs, rs, "c.config", loc, false, hints);
 
       config_module& cm (*rs.modules.lookup<config_module> ("c.config"));
 
diff --git a/build2/cc/common b/build2/cc/common
index f0476f9..c11e733 100644
--- a/build2/cc/common
+++ b/build2/cc/common
@@ -210,8 +210,8 @@ namespace build2
 
     private:
       file&
-      resolve_library (name,
-                       scope&,
+      resolve_library (scope&,
+                       name,
                        lorder,
                        const dir_paths&,
                        optional<dir_paths>&) const;
diff --git a/build2/cc/common.cxx b/build2/cc/common.cxx
index 6429a38..b941e6e 100644
--- a/build2/cc/common.cxx
+++ b/build2/cc/common.cxx
@@ -4,7 +4,7 @@
 
 #include <build2/cc/common>
 
-#include <build2/file>   // import()
+#include <build2/file>        // import()
 #include <build2/scope>
 #include <build2/context>
 #include <build2/variable>
@@ -287,7 +287,7 @@ namespace build2
             if (sysd == nullptr) find_sysd ();
             if (!lo) find_lo ();
 
-            file& t (resolve_library (n, bs, *lo, *sysd, usrd));
+            file& t (resolve_library (bs, n, *lo, *sysd, usrd));
 
             if (proc_lib)
             {
@@ -386,8 +386,8 @@ namespace build2
     // that's the only way to guarantee it will be up-to-date.
     //
     file& common::
-    resolve_library (name n,
-                     scope& s,
+    resolve_library (scope& s,
+                     name n,
                      lorder lo,
                      const dir_paths& sysd,
                      optional<dir_paths>& usrd) const
@@ -768,7 +768,7 @@ namespace build2
       const char* bl (lt.a != nullptr
                       ? (lt.s != nullptr ? "both" : "static")
                       : "shared");
-      lt.assign ("bin.lib") = bl;
+      lt.assign (var_pool["bin.lib"]) = bl;
 
       target* r (l ? &lt : (p.is_a<liba> () ? static_cast<target*> (a) : s));
 
diff --git a/build2/cc/compile b/build2/cc/compile
index 67897f2..11b3919 100644
--- a/build2/cc/compile
+++ b/build2/cc/compile
@@ -27,10 +27,10 @@ namespace build2
       compile (data&&);
 
       virtual match_result
-      match (action, target&, const string& hint) const override;
+      match (slock&, action, target&, const string& hint) const override;
 
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
 
       target_state
       perform_update (action, target&) const;
@@ -40,10 +40,10 @@ namespace build2
 
     private:
       void
-      append_lib_options (cstrings&, target&, scope&, lorder) const;
+      append_lib_options (scope&, cstrings&, target&, lorder) const;
 
       void
-      hash_lib_options (sha256&, target&, scope&, lorder) const;
+      hash_lib_options (scope&, sha256&, target&, lorder) const;
 
       // Mapping of include prefixes (e.g., foo in <foo/bar>) for auto-
       // generated headers to directories where they will be generated.
@@ -61,10 +61,10 @@ namespace build2
       append_prefixes (prefix_map&, target&, const variable&) const;
 
       void
-      append_lib_prefixes (prefix_map&, target&, scope&, lorder) const;
+      append_lib_prefixes (scope&, prefix_map&, target&, lorder) const;
 
       prefix_map
-      build_prefix_map (target&, scope&, lorder) const;
+      build_prefix_map (scope&, target&, lorder) const;
 
       // Reverse-lookup target type from extension.
       //
@@ -74,7 +74,7 @@ namespace build2
       // Header dependency injection.
       //
       void
-      inject (action, target&, lorder, file&, depdb&) const;
+      inject (slock&, action, target&, lorder, file&, depdb&) const;
 
     private:
       const string rule_id;
diff --git a/build2/cc/compile.cxx b/build2/cc/compile.cxx
index f837765..bc35272 100644
--- a/build2/cc/compile.cxx
+++ b/build2/cc/compile.cxx
@@ -44,7 +44,7 @@ namespace build2
                    "insufficient space");
 
     match_result compile::
-    match (action a, target& t, const string&) const
+    match (slock& ml, action a, target& t, const string&) const
     {
       tracer trace (x, "compile::match");
 
@@ -58,7 +58,8 @@ namespace build2
       // file specified for an obj*{} member overrides the one specified for
       // the group. Also "see through" groups.
       //
-      for (prerequisite_member p: reverse_group_prerequisite_members (a, t))
+      for (prerequisite_member p:
+             reverse_group_prerequisite_members (ml, a, t))
       {
         if (p.is_a (x_src))
         {
@@ -75,7 +76,7 @@ namespace build2
     // (first one is cc.export.*) recursively, prerequisite libraries first.
     //
     void compile::
-    append_lib_options (cstrings& args, target& t, scope& bs, lorder lo) const
+    append_lib_options (scope& bs, cstrings& args, target& t, lorder lo) const
     {
       auto opt = [&args, this] (file& l, const string& t, bool com, bool exp)
       {
@@ -116,7 +117,7 @@ namespace build2
     }
 
     void compile::
-    hash_lib_options (sha256& cs, target& t, scope& bs, lorder lo) const
+    hash_lib_options (scope& bs, sha256& cs, target& t, lorder lo) const
     {
       auto opt = [&cs, this] (file& l, const string& t, bool com, bool exp)
       {
@@ -155,7 +156,7 @@ namespace build2
     // recursively, prerequisite libraries first.
     //
     void compile::
-    append_lib_prefixes (prefix_map& m, target& t, scope& bs, lorder lo) const
+    append_lib_prefixes (scope& bs, prefix_map& m, target& t, lorder lo) const
     {
       auto opt = [&m, this] (file& l, const string& t, bool com, bool exp)
       {
@@ -191,7 +192,7 @@ namespace build2
     }
 
     recipe compile::
-    apply (action a, target& xt) const
+    apply (slock& ml, action a, target& xt) const
     {
       tracer trace (x, "compile::apply");
 
@@ -250,7 +251,7 @@ namespace build2
 
       // Inject dependency on the output directory.
       //
-      fsdir* dir (inject_fsdir (a, t));
+      fsdir* dir (inject_fsdir (ml, a, t));
 
       // Search and match all the existing prerequisites. The injection code
       // takes care of the ones it is adding.
@@ -260,7 +261,7 @@ namespace build2
       //
       optional<dir_paths> usr_lib_dirs; // Extract lazily.
 
-      for (prerequisite_member p: group_prerequisite_members (a, t))
+      for (prerequisite_member p: group_prerequisite_members (ml, a, t))
       {
         // A dependency on a library is there so that we can get its
         // *.export.poptions. In particular, making sure it is executed before
@@ -281,7 +282,7 @@ namespace build2
                 search_library (
                   sys_lib_dirs, usr_lib_dirs, p.prerequisite) == nullptr)
             {
-              match_only (a, p.search ());
+              match_only (ml, a, p.search ());
             }
           }
 
@@ -293,7 +294,7 @@ namespace build2
         if (a.operation () == clean_id && !pt.dir.sub (rs.out_path ()))
           continue;
 
-        build2::match (a, pt);
+        build2::match (ml, a, pt);
         t.prerequisite_targets.push_back (&pt);
       }
 
@@ -351,7 +352,7 @@ namespace build2
 
         // Hash *.export.poptions from prerequisite libraries.
         //
-        hash_lib_options (cs, t, bs, lo);
+        hash_lib_options (bs, cs, t, lo);
 
         // Extra system header dirs (last).
         //
@@ -385,7 +386,7 @@ namespace build2
         if (dd.writing () || dd.mtime () > t.mtime ())
           t.mtime (timestamp_nonexistent);
 
-        inject (a, t, lo, src, dd);
+        inject (ml, a, t, lo, src, dd);
 
         dd.close ();
       }
@@ -525,7 +526,7 @@ namespace build2
     }
 
     auto compile::
-    build_prefix_map (target& t, scope& bs, lorder lo) const -> prefix_map
+    build_prefix_map (scope& bs, target& t, lorder lo) const -> prefix_map
     {
       prefix_map m;
 
@@ -536,7 +537,7 @@ namespace build2
 
       // Then process the include directories from prerequisite libraries.
       //
-      append_lib_prefixes (m, t, bs, lo);
+      append_lib_prefixes (bs, m, t, lo);
 
       return m;
     }
@@ -715,7 +716,12 @@ namespace build2
     }
 
     void compile::
-    inject (action a, target& t, lorder lo, file& src, depdb& dd) const
+    inject (slock& ml,
+            action a,
+            target& t,
+            lorder lo,
+            file& src,
+            depdb& dd) const
     {
       tracer trace (x, "compile::inject");
 
@@ -739,14 +745,14 @@ namespace build2
       const process_path* xc (nullptr);
       cstrings args;
 
-      auto init_args = [&t, lo, &src, &rs, &bs, &xc, &args, this] ()
+      auto init_args = [&ml, &t, lo, &src, &rs, &bs, &xc, &args, this] ()
       {
         xc = &cast<process_path> (rs[x_path]);
         args.push_back (xc->recall_string ());
 
         // Add *.export.poptions from prerequisite libraries.
         //
-        append_lib_options (args, t, bs, lo);
+        append_lib_options (bs, args, t, lo);
 
         append_options (args, t, c_poptions);
         append_options (args, t, x_poptions);
@@ -895,7 +901,7 @@ namespace build2
       // from the depdb cache or from the compiler run. Return whether the
       // extraction process should be restarted.
       //
-      auto add = [&trace, &update, &pm, a, &t, lo, &dd, &bs, this]
+      auto add = [&trace, &ml, &update, &pm, a, &t, lo, &dd, &bs, this]
         (path f, bool cache) -> bool
       {
         // Find or maybe insert the target.
@@ -991,7 +997,7 @@ namespace build2
           // then we would have failed below.
           //
           if (pm.empty ())
-            pm = build_prefix_map (t, bs, lo);
+            pm = build_prefix_map (bs, t, lo);
 
           // First try the whole file. Then just the directory.
           //
@@ -1068,7 +1074,7 @@ namespace build2
 
         // Match to a rule.
         //
-        build2::match (a, *pt);
+        build2::match (ml, a, *pt);
 
         // Update.
         //
@@ -1417,7 +1423,7 @@ namespace build2
 
       // Add *.export.poptions from prerequisite libraries.
       //
-      append_lib_options (args, t, bs, lo);
+      append_lib_options (bs, args, t, lo);
 
       // Extra system header dirs (last).
       //
diff --git a/build2/cc/init.cxx b/build2/cc/init.cxx
index a162daa..ebe7653 100644
--- a/build2/cc/init.cxx
+++ b/build2/cc/init.cxx
@@ -35,7 +35,7 @@ namespace build2
 
       // Enter variables. Note: some overridable, some not.
       //
-      auto& v (var_pool);
+      auto& v (var_pool.rw (r));
 
       v.insert<strings> ("config.cc.poptions", true);
       v.insert<strings> ("config.cc.coptions", true);
@@ -90,10 +90,12 @@ namespace build2
 
       assert (first);
 
+      auto& vp (var_pool.rw (rs));
+
       // Load cc.core.vars.
       //
       if (!cast_false<bool> (rs["cc.core.vars.loaded"]))
-        load_module ("cc.core.vars", rs, rs, loc);
+        load_module (rs, rs, "cc.core.vars", loc);
 
       // Configure.
       //
@@ -169,14 +171,20 @@ namespace build2
         variable_map h;
         if (first)
         {
-          h.assign ("config.bin.target") =
-            cast<target_triplet> (rs["cc.target"]).string ();
+          // Note that these variables have not yet been registered (we don't
+          // yet have the "bin.vars" module).
+          //
+          const variable& t (vp.insert ("config.bin.target"));
+          h.assign (t) = cast<target_triplet> (rs["cc.target"]).string ();
 
           if (auto l = hints["config.bin.pattern"])
-            h.assign ("config.bin.pattern") = cast<string> (l);
+          {
+            const variable& p (vp.insert ("config.bin.pattern"));
+            h.assign (p) = cast<string> (l);
+          }
         }
 
-        load_module ("bin.config", rs, rs, loc, false, h);
+        load_module (rs, rs, "bin.config", loc, false, h);
       }
 
       // Verify bin's target matches ours (we do it even if we loaded it
@@ -205,20 +213,20 @@ namespace build2
         if (cast<string> (l) != "shared")
         {
           if (!cast_false<bool> (rs["bin.ar.config.loaded"]))
-            load_module ("bin.ar.config", rs, rs, loc);
+            load_module (rs, rs, "bin.ar.config", loc);
         }
       }
 
       if (cid == "msvc")
       {
         if (!cast_false<bool> (rs["bin.ld.config.loaded"]))
-          load_module ("bin.ld.config", rs, rs, loc);
+          load_module (rs, rs, "bin.ld.config", loc);
       }
 
       if (tsys == "mingw32")
       {
         if (!cast_false<bool> (rs["bin.rc.config.loaded"]))
-          load_module ("bin.rc.config", rs, rs, loc);
+          load_module (rs, rs, "bin.rc.config", loc);
       }
 
       // Load (optionally) the pkgconfig.config module.
@@ -233,10 +241,13 @@ namespace build2
         // Prepare configuration hints.
         //
         variable_map h;
-        h.assign ("config.pkgconfig.target") =
-          cast<target_triplet> (rs["cc.target"]);
 
-        load_module ("pkgconfig.config", rs, rs, loc, true, h);
+        // Note that this variable has not yet been registered.
+        //
+        const variable& t (vp.insert ("config.pkgconfig.target"));
+        h.assign (t) = cast<target_triplet> (rs["cc.target"]);
+
+        load_module (rs, rs, "pkgconfig.config", loc, true, h);
       }
 
       return true;
@@ -259,12 +270,12 @@ namespace build2
       // Load cc.core.config.
       //
       if (!cast_false<bool> (rs["cc.core.config.loaded"]))
-        load_module ("cc.core.config", rs, rs, loc, false, hints);
+        load_module (rs, rs, "cc.core.config", loc, false, hints);
 
       // Load the bin module.
       //
       if (!cast_false<bool> (rs["bin.loaded"]))
-        load_module ("bin", rs, rs, loc);
+        load_module (rs, rs, "bin", loc);
 
       const string& cid (cast<string> (rs["cc.id"]));
       const string& tsys (cast<string> (rs["cc.target.system"]));
@@ -277,7 +288,7 @@ namespace build2
         if (cast<string> (l) != "shared")
         {
           if (!cast_false<bool> (rs["bin.ar.loaded"]))
-            load_module ("bin.ar", rs, rs, loc);
+            load_module (rs, rs, "bin.ar", loc);
         }
       }
 
@@ -287,7 +298,7 @@ namespace build2
       if (cid == "msvc")
       {
         if (!cast_false<bool> (rs["bin.ld.loaded"]))
-          load_module ("bin.ld", rs, rs, loc);
+          load_module (rs, rs, "bin.ld", loc);
       }
 
       // If our target is MinGW, then we will need the resource compiler
@@ -296,7 +307,7 @@ namespace build2
       if (tsys == "mingw32")
       {
         if (!cast_false<bool> (rs["bin.rc.loaded"]))
-          load_module ("bin.rc", rs, rs, loc);
+          load_module (rs, rs, "bin.rc", loc);
       }
 
       return true;
@@ -308,13 +319,13 @@ namespace build2
     //
     static inline bool
     init_alias (tracer& trace,
+                scope& rs,
+                scope& bs,
                 const char* m,
                 const char* c,
                 const char* c_loaded,
                 const char* cxx,
                 const char* cxx_loaded,
-                scope& rs,
-                scope& bs,
                 const location& loc,
                 const variable_map& hints)
     {
@@ -338,13 +349,13 @@ namespace build2
       //
       if (lc && lp && rs["config.c"])
       {
-        load_module (c, rs, rs, loc, false, hints);
-        load_module (cxx, rs, rs, loc, false, hints);
+        load_module (rs, rs, c, loc, false, hints);
+        load_module (rs, rs, cxx, loc, false, hints);
       }
       else
       {
-        if (lp) load_module (cxx, rs, rs, loc, false, hints);
-        if (lc) load_module (c, rs, rs, loc, false, hints);
+        if (lp) load_module (rs, rs, cxx, loc, false, hints);
+        if (lc) load_module (rs, rs, c, loc, false, hints);
       }
 
       return true;
@@ -360,10 +371,11 @@ namespace build2
                  const variable_map& hints)
     {
       tracer trace ("cc::config_init");
-      return init_alias (trace, "cc.config",
+      return init_alias (trace, rs, bs,
+                         "cc.config",
                          "c.config",   "c.config.loaded",
                          "cxx.config", "cxx.config.loaded",
-                         rs, bs, loc, hints);
+                         loc, hints);
     }
 
     bool
@@ -376,10 +388,11 @@ namespace build2
           const variable_map& hints)
     {
       tracer trace ("cc::init");
-      return init_alias (trace, "cc",
+      return init_alias (trace, rs, bs,
+                         "cc",
                          "c",   "c.loaded",
                          "cxx", "cxx.loaded",
-                         rs, bs, loc, hints);
+                         loc, hints);
     }
   }
 }
diff --git a/build2/cc/install b/build2/cc/install
index 561ed87..ff7af4d 100644
--- a/build2/cc/install
+++ b/build2/cc/install
@@ -25,10 +25,10 @@ namespace build2
       install (data&&, const link&);
 
       virtual target*
-      filter (action, target&, prerequisite_member) const override;
+      filter (slock&, action, target&, prerequisite_member) const override;
 
       virtual match_result
-      match (action, target&, const string&) const override;
+      match (slock&, action, target&, const string&) const override;
 
       virtual void
       install_extra (file&, const install_dir&) const override;
diff --git a/build2/cc/install.cxx b/build2/cc/install.cxx
index 28d7db2..074654b 100644
--- a/build2/cc/install.cxx
+++ b/build2/cc/install.cxx
@@ -23,7 +23,7 @@ namespace build2
     install (data&& d, const link& l): common (move (d)), link_ (l) {}
 
     target* install::
-    filter (action a, target& t, prerequisite_member p) const
+    filter (slock& ml, action a, target& t, prerequisite_member p) const
     {
       if (t.is_a<exe> ())
       {
@@ -53,11 +53,11 @@ namespace build2
           return pt->in (t.weak_scope ()) ? pt : nullptr;
       }
 
-      return file_rule::filter (a, t, p);
+      return file_rule::filter (ml, a, t, p);
     }
 
     match_result install::
-    match (action a, target& t, const string& hint) const
+    match (slock& ml, action a, target& t, const string& hint) const
     {
       // @@ How do we split the hint between the two?
       //
@@ -65,8 +65,8 @@ namespace build2
       // We only want to handle installation if we are also the
       // ones building this target. So first run link's match().
       //
-      match_result r (link_.match (a, t, hint));
-      return r ? install::file_rule::match (a, t, "") : r;
+      match_result r (link_.match (ml, a, t, hint));
+      return r ? install::file_rule::match (ml, a, t, "") : r;
     }
 
     void install::
diff --git a/build2/cc/link b/build2/cc/link
index c7d3c93..5f28204 100644
--- a/build2/cc/link
+++ b/build2/cc/link
@@ -25,10 +25,10 @@ namespace build2
       link (data&&);
 
       virtual match_result
-      match (action, target&, const string& hint) const override;
+      match (slock&, action, target&, const string& hint) const override;
 
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
 
       target_state
       perform_update (action, target&) const;
diff --git a/build2/cc/link.cxx b/build2/cc/link.cxx
index c94eb26..6de294a 100644
--- a/build2/cc/link.cxx
+++ b/build2/cc/link.cxx
@@ -41,7 +41,7 @@ namespace build2
     }
 
     match_result link::
-    match (action a, target& t, const string& hint) const
+    match (slock& ml, action a, target& t, const string& hint) const
     {
       tracer trace (x, "link::match");
 
@@ -64,7 +64,7 @@ namespace build2
       //
       bool seen_x (false), seen_c (false), seen_obj (false), seen_lib (false);
 
-      for (prerequisite_member p: group_prerequisite_members (a, t))
+      for (prerequisite_member p: group_prerequisite_members (ml, a, t))
       {
         if (p.is_a (x_src))
         {
@@ -145,7 +145,7 @@ namespace build2
 
         optional<dir_paths> usr_lib_dirs; // Extract lazily.
 
-        for (prerequisite_member p: group_prerequisite_members (a, t))
+        for (prerequisite_member p: group_prerequisite_members (ml, a, t))
         {
           if (p.is_a<lib> () || p.is_a<liba> () || p.is_a<libs> ())
           {
@@ -159,7 +159,7 @@ namespace build2
             if (pt == nullptr)
             {
               pt = &p.search ();
-              match_only (a, *pt);
+              match_only (ml, a, *pt);
             }
 
             // If the prerequisite came from the lib{} group, then also
@@ -316,7 +316,7 @@ namespace build2
     }
 
     recipe link::
-    apply (action a, target& xt) const
+    apply (slock& ml, action a, target& xt) const
     {
       tracer trace (x, "link::apply");
 
@@ -415,7 +415,7 @@ namespace build2
 
       // Inject dependency on the output directory.
       //
-      inject_fsdir (a, t);
+      inject_fsdir (ml, a, t);
 
       optional<dir_paths> usr_lib_dirs; // Extract lazily.
 
@@ -429,7 +429,7 @@ namespace build2
                               lt == otype::a ? obja::static_type :
                               objs::static_type);
 
-      for (prerequisite_member p: group_prerequisite_members (a, t))
+      for (prerequisite_member p: group_prerequisite_members (ml, a, t))
       {
         target* pt (nullptr);
 
@@ -471,7 +471,7 @@ namespace build2
             pt = &link_member (*l, lo);
           }
 
-          build2::match (a, *pt);
+          build2::match (ml, a, *pt);
           t.prerequisite_targets.push_back (pt);
           continue;
         }
@@ -567,7 +567,7 @@ namespace build2
         //
         bool found (false);
         for (prerequisite_member p1:
-               reverse_group_prerequisite_members (a, *pt))
+               reverse_group_prerequisite_members (ml, a, *pt))
         {
           // Most of the time we will have just a single source so fast-path
           // that case.
@@ -576,7 +576,7 @@ namespace build2
           {
             if (!found)
             {
-              build2::match (a, *pt); // Now p1 should be resolved.
+              build2::match (ml, a, *pt); // Now p1 should be resolved.
 
               // Searching our own prerequisite is ok.
               //
@@ -637,7 +637,7 @@ namespace build2
               ot.prerequisites.emplace_back (p);
           }
 
-          build2::match (a, *pt);
+          build2::match (ml, a, *pt);
         }
 
         t.prerequisite_targets.push_back (pt);
diff --git a/build2/cc/module.cxx b/build2/cc/module.cxx
index 3e20694..c7c7c3c 100644
--- a/build2/cc/module.cxx
+++ b/build2/cc/module.cxx
@@ -279,6 +279,8 @@ namespace build2
         //
         variable_map h;
 
+        // Note that all these variables have already been registered.
+        //
         h.assign ("config.cc.id") = cast<string> (rs[x_id]);
         h.assign ("config.cc.target") = cast<target_triplet> (rs[x_target]);
 
@@ -288,7 +290,7 @@ namespace build2
         if (!ci.bin_pattern.empty ())
           h.assign ("config.bin.pattern") = move (ci.bin_pattern);
 
-        load_module ("cc.core.config", rs, rs, loc, false, h);
+        load_module (rs, rs, "cc.core.config", loc, false, h);
       }
       else
       {
@@ -331,7 +333,7 @@ namespace build2
       // extra bin.* modules we may need.
       //
       if (!cast_false<bool> (rs["cc.core.loaded"]))
-        load_module ("cc.core", rs, rs, loc);
+        load_module (rs, rs, "cc.core", loc);
 
       // Register target types and configure their "installability".
       //
@@ -347,7 +349,7 @@ namespace build2
         for (const target_type* const* ht (x_hdr); *ht != nullptr; ++ht)
         {
           t.insert (**ht);
-          install_path (**ht, rs, dir_path ("include"));
+          install_path (rs, **ht, dir_path ("include"));
         }
       }
 
diff --git a/build2/cli/init.cxx b/build2/cli/init.cxx
index b34711d..2650c1b 100644
--- a/build2/cli/init.cxx
+++ b/build2/cli/init.cxx
@@ -43,7 +43,7 @@ namespace build2
       //
       if (first)
       {
-        auto& v (var_pool);
+        auto& v (var_pool.rw (rs));
 
         // Note: some overridable, some not.
         //
@@ -274,7 +274,7 @@ namespace build2
       //
       if (!cast_false<bool> (bs["cli.config.loaded"]))
       {
-        if (!load_module ("cli.config", rs, bs, l, optional, hints))
+        if (!load_module (rs, bs, "cli.config", l, optional, hints))
           return false;
       }
       else if (!cast_false<bool> (bs["cli.config.configured"]))
diff --git a/build2/cli/rule b/build2/cli/rule
index a46163b..fbacacf 100644
--- a/build2/cli/rule
+++ b/build2/cli/rule
@@ -20,10 +20,10 @@ namespace build2
       compile () {}
 
       virtual match_result
-      match (action, target&, const string& hint) const override;
+      match (slock&, action, target&, const string& hint) const override;
 
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
 
       static target_state
       perform_update (action, target&);
diff --git a/build2/cli/rule.cxx b/build2/cli/rule.cxx
index 706830c..8cef1f5 100644
--- a/build2/cli/rule.cxx
+++ b/build2/cli/rule.cxx
@@ -44,7 +44,7 @@ namespace build2
     }
 
     match_result compile::
-    match (action a, target& xt, const string&) const
+    match (slock& ml, action a, target& xt, const string&) const
     {
       tracer trace ("cli::compile::match");
 
@@ -57,7 +57,7 @@ namespace build2
         // See if we have a .cli source file.
         //
         bool r (false);
-        for (prerequisite_member p: group_prerequisite_members (a, t))
+        for (prerequisite_member p: group_prerequisite_members (ml, a, t))
         {
           if (p.is_a<cli> ())
           {
@@ -125,7 +125,7 @@ namespace build2
         //
         if (g == nullptr || !g->has_prerequisites ())
         {
-          for (prerequisite_member p: group_prerequisite_members (a, t))
+          for (prerequisite_member p: group_prerequisite_members (ml, a, t))
           {
             if (p.is_a<cli> ())
             {
@@ -151,7 +151,7 @@ namespace build2
           // Resolve the group's members. This should link us up to the
           // group.
           //
-          resolve_group_members (a, *g);
+          resolve_group_members (ml, a, *g);
 
           // For ixx{}, verify it is part of the group.
           //
@@ -169,7 +169,7 @@ namespace build2
     }
 
     recipe compile::
-    apply (action a, target& xt) const
+    apply (slock& ml, action a, target& xt) const
     {
       if (cli_cxx* pt = xt.is_a<cli_cxx> ())
       {
@@ -184,11 +184,11 @@ namespace build2
 
         // Inject dependency on the output directory.
         //
-        inject_fsdir (a, t);
+        inject_fsdir (ml, a, t);
 
         // Search and match prerequisite members.
         //
-        search_and_match_prerequisite_members (a, t);
+        search_and_match_prerequisite_members (ml, a, t);
 
         switch (a)
         {
@@ -200,7 +200,7 @@ namespace build2
       else
       {
         cli_cxx& g (*static_cast<cli_cxx*> (xt.group));
-        build2::match (a, g);
+        build2::match (ml, a, g);
         return group_recipe; // Execute the group's recipe.
       }
     }
diff --git a/build2/config/init.cxx b/build2/config/init.cxx
index d6cbe74..1f0b391 100644
--- a/build2/config/init.cxx
+++ b/build2/config/init.cxx
@@ -38,6 +38,8 @@ namespace build2
       rs.meta_operations.insert (configure_id, configure);
       rs.meta_operations.insert (disfigure_id, disfigure);
 
+      auto& vp (var_pool.rw (rs));
+
       // Load config.build if one exists.
       //
       // Note that we have to do this during bootstrap since the order in
@@ -45,7 +47,7 @@ namespace build2
       // possible that some module which needs the configuration will get
       // called first.
       //
-      const variable& c_v (var_pool.insert<uint64_t> ("config.version"));
+      const variable& c_v (vp.insert<uint64_t> ("config.version"));
 
       // Don't load it if we are disfiguring. This is a bit tricky since the
       // build2 core may not yet know it is disfiguring. But we know.
@@ -68,7 +70,7 @@ namespace build2
           {
             // Assume missing version is 0.
             //
-            auto p (extract_variable (f, c_v.name.c_str ()));
+            auto p (extract_variable (f, c_v));
             uint64_t v (p.second ? cast<uint64_t> (p.first) : 0);
 
             if (v != module::version)
@@ -80,7 +82,7 @@ namespace build2
                          << out_root;
           }
 
-          source (f, rs, rs);
+          source (rs, rs, f);
         }
       }
     }
diff --git a/build2/config/operation.cxx b/build2/config/operation.cxx
index f74136c..175338a 100644
--- a/build2/config/operation.cxx
+++ b/build2/config/operation.cxx
@@ -82,7 +82,7 @@ namespace build2
 
         ofs << "config.version = " << module::version << endl;
 
-        if (auto l = root.vars["amalgamation"])
+        if (auto l = root.vars[var_amalgamation])
         {
           const dir_path& d (cast<dir_path> (l));
 
@@ -325,7 +325,7 @@ namespace build2
 
       // Configure subprojects that have been loaded.
       //
-      if (auto l = root.vars["subprojects"])
+      if (auto l = root.vars[var_subprojects])
       {
         for (auto p: cast<subprojects> (l))
         {
@@ -345,13 +345,13 @@ namespace build2
     }
 
     static void
-    configure_match (action, action_targets&)
+    configure_match (ulock&, action, action_targets&)
     {
       // Don't match anything -- see execute ().
     }
 
     static void
-    configure_execute (action a, const action_targets& ts, bool)
+    configure_execute (ulock& ml, action a, const action_targets& ts, bool)
     {
       // Match rules to configure every operation supported by each
       // project. Note that we are not calling operation_pre/post()
@@ -360,6 +360,10 @@ namespace build2
       //
       set<scope*> projects;
 
+      // Relock for shared access.
+      //
+      ml.unlock ();
+
       for (void* v: ts)
       {
         target& t (*static_cast<target*> (v));
@@ -368,6 +372,9 @@ namespace build2
         if (rs == nullptr)
           fail << "out of project target " << t;
 
+        slock sl (*ml.mutex ());
+        model_lock = &sl; // @@ Guard?
+
         for (operations::size_type id (default_id + 1); // Skip default_id
              id < rs->operations.size ();
              ++id)
@@ -379,11 +386,15 @@ namespace build2
           set_current_oif (*oif);
           dependency_count = 0;
 
-          match (action (configure_id, id), t);
+          match (sl, action (configure_id, id), t);
         }
 
+        model_lock = nullptr;
+
         configure_project (a, *rs, projects);
       }
+
+      ml.lock ();
     }
 
     const meta_operation_info configure {
@@ -414,8 +425,9 @@ namespace build2
     }
 
     static void
-    disfigure_load (const path& bf,
+    disfigure_load (ulock&,
                     scope&,
+                    const path& bf,
                     const dir_path&,
                     const dir_path&,
                     const location&)
@@ -425,7 +437,8 @@ namespace build2
     }
 
     static void
-    disfigure_search (scope& root,
+    disfigure_search (ulock&,
+                      scope& root,
                       const target_key&,
                       const location&,
                       action_targets& ts)
@@ -436,7 +449,9 @@ namespace build2
     }
 
     static void
-    disfigure_match (action, action_targets&) {}
+    disfigure_match (ulock&, action, action_targets&)
+    {
+    }
 
     static bool
     disfigure_project (action a, scope& root, set<scope*>& projects)
@@ -457,7 +472,7 @@ namespace build2
       // Disfigure subprojects. Since we don't load buildfiles during
       // disfigure, we do it for all known subprojects.
       //
-      if (auto l = root.vars["subprojects"])
+      if (auto l = root.vars[var_subprojects])
       {
         for (auto p: cast<subprojects> (l))
         {
@@ -475,7 +490,7 @@ namespace build2
           {
             bootstrap_out (nroot);
 
-            value& val (nroot.assign ("src_root"));
+            value& val (nroot.assign (var_src_root));
 
             if (!val)
               val = is_src_root (out_nroot) ? out_nroot : (src_root / pd);
@@ -550,7 +565,7 @@ namespace build2
     }
 
     static void
-    disfigure_execute (action a, const action_targets& ts, bool quiet)
+    disfigure_execute (ulock&, action a, const action_targets& ts, bool quiet)
     {
       tracer trace ("disfigure_execute");
 
diff --git a/build2/config/utility b/build2/config/utility
index 6c18715..e4e463d 100644
--- a/build2/config/utility
+++ b/build2/config/utility
@@ -44,6 +44,8 @@ namespace build2
               bool override = false,
               uint64_t save_flags = 0);
 
+    // Note that the variable is expected to have already been registered.
+    //
     template <typename T>
     inline pair<lookup, bool>
     required (scope& root,
@@ -76,6 +78,8 @@ namespace build2
     pair<lookup, bool>
     omitted (scope& root, const variable&);
 
+    // Note that the variable is expected to have already been registered.
+    //
     inline pair<lookup, bool>
     omitted (scope& root, const string& name)
     {
@@ -91,10 +95,12 @@ namespace build2
     lookup
     optional (scope& root, const variable&);
 
+    // Note that the variable is expected to have already been registered.
+    //
     inline lookup
-    optional (scope& root, const string& var)
+    optional (scope& root, const string& name)
     {
-      return optional (root, var_pool[var]);
+      return optional (root, var_pool[name]);
     }
 
     // Check whether there are any variables specified from the config
diff --git a/build2/config/utility.cxx b/build2/config/utility.cxx
index 7a3fa9c..7db4555 100644
--- a/build2/config/utility.cxx
+++ b/build2/config/utility.cxx
@@ -72,9 +72,10 @@ namespace build2
       // any original values, they will be "visible"; see find_override() for
       // details.
       //
+      const variable& vns (var_pool.rw (r).insert (ns));
       for (scope* s (&r); s != nullptr; s = s->parent_scope ())
       {
-        for (auto p (s->vars.find_namespace (ns));
+        for (auto p (s->vars.find_namespace (vns));
              p.first != p.second;
              ++p.first)
         {
@@ -92,30 +93,30 @@ namespace build2
     }
 
     bool
-    unconfigured (scope& root, const string& ns)
+    unconfigured (scope& rs, const string& ns)
     {
       // Note: not overridable.
       //
-      const variable& var (var_pool.insert<bool> (ns + ".configured"));
+      const variable& var (var_pool.rw (rs).insert<bool> (ns + ".configured"));
 
       if (current_mif->id == configure_id)
-        save_variable (root, var);
+        save_variable (rs, var);
 
-      auto l (root[var]); // Include inherited values.
+      auto l (rs[var]); // Include inherited values.
       return l && !cast<bool> (l);
     }
 
     bool
-    unconfigured (scope& root, const string& ns, bool v)
+    unconfigured (scope& rs, const string& ns, bool v)
     {
       // Note: not overridable.
       //
-      const variable& var (var_pool.insert<bool> (ns + ".configured"));
+      const variable& var (var_pool.rw (rs).insert<bool> (ns + ".configured"));
 
       if (current_mif->id == configure_id)
-        save_variable (root, var);
+        save_variable (rs, var);
 
-      value& x (root.assign (var));
+      value& x (rs.assign (var));
 
       if (x.null || cast<bool> (x) != !v)
       {
diff --git a/build2/context b/build2/context
index 1dd9daf..33b99a0 100644
--- a/build2/context
+++ b/build2/context
@@ -14,7 +14,32 @@
 
 namespace build2
 {
-  class file;
+  // Top-level internal state mutex.
+  //
+  extern shared_mutex model;
+
+  // Thread's shared model lock. NULL in the serial stages.
+  //
+  extern
+#ifdef __cpp_thread_local
+  thread_local
+#else
+  __thread
+#endif
+  slock* model_lock;
+
+  // Cached variables.
+  //
+  extern const variable* var_src_root;
+  extern const variable* var_out_root;
+  extern const variable* var_src_base;
+  extern const variable* var_out_base;
+
+  extern const variable* var_project;
+  extern const variable* var_amalgamation;
+  extern const variable* var_subprojects;
+
+  extern const variable* var_import_target; // import.target
 
   // Current action (meta/operation).
   //
@@ -75,14 +100,14 @@ namespace build2
   // scopes, and variables.
   //
   variable_overrides
-  reset (const strings& cmd_vars);
+  reset (const ulock&, const strings& cmd_vars);
 
   // Return the project name or empty string if unnamed.
   //
   inline const string&
-  project (scope& root)
+  project (const scope& root)
   {
-    auto l (root["project"]);
+    auto l (root[var_project]);
     return l ? cast<string> (l) : empty_string;
   }
 
diff --git a/build2/context.cxx b/build2/context.cxx
index ba6629c..1cf3dd9 100644
--- a/build2/context.cxx
+++ b/build2/context.cxx
@@ -23,6 +23,26 @@ using namespace butl;
 
 namespace build2
 {
+  shared_mutex model;
+
+#ifdef __cpp_thread_local
+  thread_local
+#else
+  __thread
+#endif
+  slock* model_lock;
+
+  const variable* var_src_root;
+  const variable* var_out_root;
+  const variable* var_src_base;
+  const variable* var_out_base;
+
+  const variable* var_project;
+  const variable* var_amalgamation;
+  const variable* var_subprojects;
+
+  const variable* var_import_target;
+
   const string* current_mname;
   const string* current_oname;
 
@@ -37,7 +57,7 @@ namespace build2
   variable_override_cache var_override_cache;
 
   variable_overrides
-  reset (const strings& cmd_vars)
+  reset (const ulock& ml, const strings& cmd_vars)
   {
     tracer trace ("reset");
 
@@ -46,13 +66,15 @@ namespace build2
 
     l6 ([&]{trace << "resetting build state";});
 
+    auto& vp (var_pool.rw (ml));
+
     variable_overrides vos;
 
     var_override_cache.clear ();
 
     targets.clear ();
     scopes.clear ();
-    var_pool.clear ();
+    vp.clear ();
 
     // Reset meta/operation tables. Note that the order should match the id
     // constants in <build2/operation>.
@@ -147,7 +169,7 @@ namespace build2
                              c == '%' ? variable_visibility::project :
                              variable_visibility::normal);
 
-      const variable& var (var_pool[n]);
+      const variable& var (vp.insert (n));
       const char* k (tt == token_type::assign ? ".__override" :
                      tt == token_type::append ? ".__suffix" : ".__prefix");
 
@@ -209,23 +231,23 @@ namespace build2
 
     // Enter builtin variables.
     //
-    {
-      auto& v (var_pool);
-
-      v.insert<dir_path> ("src_root");
-      v.insert<dir_path> ("out_root");
-      v.insert<dir_path> ("src_base");
-      v.insert<dir_path> ("out_base");
+    var_src_root = &vp.insert<dir_path> ("src_root");
+    var_out_root = &vp.insert<dir_path> ("out_root");
+    var_src_base = &vp.insert<dir_path> ("src_base");
+    var_out_base = &vp.insert<dir_path> ("out_base");
 
-      v.insert<string> ("project");
-      v.insert<dir_path> ("amalgamation");
+    // Note that subprojects is not typed since the value requires
+    // pre-processing (see file.cxx).
+    //
+    var_project      = &vp.insert<string>   ("project");
+    var_amalgamation = &vp.insert<dir_path> ("amalgamation");
+    var_subprojects  = &vp.insert           ("subprojects");
 
-      // Not typed since the value requires pre-processing (see file.cxx).
-      //
-      v.insert ("subprojects");
+    var_import_target = &vp.insert<name> ("import.target");
 
-      v.insert<string> ("extension", variable_visibility::target);
-    }
+    // Target extension.
+    //
+    vp.insert<string> ("extension", variable_visibility::target);
 
     gs.assign<dir_path> ("build.work") = work;
     gs.assign<dir_path> ("build.home") = home;
@@ -240,8 +262,8 @@ namespace build2
     // Build system version.
     //
     {
-      gs.assign<uint64_t> ("build.version")      = uint64_t (BUILD2_VERSION);
-      gs.assign<string> ("build.version.string") = BUILD2_VERSION_STR;
+      gs.assign<uint64_t> ("build.version") = uint64_t (BUILD2_VERSION);
+      gs.assign<string>   ("build.version.string") = BUILD2_VERSION_STR;
 
       // AABBCCDD
       //
diff --git a/build2/cxx/init.cxx b/build2/cxx/init.cxx
index 477056c..f76779b 100644
--- a/build2/cxx/init.cxx
+++ b/build2/cxx/init.cxx
@@ -175,11 +175,11 @@ namespace build2
       // Load cc.core.vars so that we can cache all the cc.* variables.
       //
       if (!cast_false<bool> (rs["cc.core.vars.loaded"]))
-        load_module ("cc.core.vars", rs, rs, loc);
+        load_module (rs, rs, "cc.core.vars", loc);
 
       // Enter all the variables and initialize the module data.
       //
-      auto& v (var_pool);
+      auto& v (var_pool.rw (rs));
 
       cc::config_data d {
         cc::lang::cxx,
@@ -294,7 +294,7 @@ namespace build2
       // Load cxx.config.
       //
       if (!cast_false<bool> (rs["cxx.config.loaded"]))
-        load_module ("cxx.config", rs, rs, loc, false, hints);
+        load_module (rs, rs, "cxx.config", loc, false, hints);
 
       config_module& cm (*rs.modules.lookup<config_module> ("cxx.config"));
 
diff --git a/build2/dist/init.cxx b/build2/dist/init.cxx
index 901e9c2..9ee2d46 100644
--- a/build2/dist/init.cxx
+++ b/build2/dist/init.cxx
@@ -23,21 +23,21 @@ namespace build2
     static const rule rule_;
 
     void
-    boot (scope& r, const location&, unique_ptr<module_base>&)
+    boot (scope& rs, const location&, unique_ptr<module_base>&)
     {
       tracer trace ("dist::boot");
 
-      l5 ([&]{trace << "for " << r.out_path ();});
+      l5 ([&]{trace << "for " << rs.out_path ();});
 
       // Register meta-operation.
       //
-      r.meta_operations.insert (dist_id, dist);
+      rs.meta_operations.insert (dist_id, dist);
 
       // Enter module variables. Do it during boot in case they get assigned
       // in bootstrap.build (which is customary for, e.g., dist.package).
       //
       {
-        auto& v (var_pool);
+        auto& v (var_pool.rw (rs));
 
         // Note: some overridable, some not.
         //
@@ -63,7 +63,7 @@ namespace build2
     }
 
     bool
-    init (scope& r,
+    init (scope& rs,
           scope&,
           const location& l,
           unique_ptr<module_base>&,
@@ -79,7 +79,7 @@ namespace build2
         return true;
       }
 
-      const dir_path& out_root (r.out_path ());
+      const dir_path& out_root (rs.out_path ());
       l5 ([&]{trace << "for " << out_root;});
 
       assert (config_hints.empty ()); // We don't known any hints.
@@ -88,8 +88,8 @@ namespace build2
       // to prevent something like insert<target>(dist_id, test_id)
       // taking precedence.
       //
-      r.rules.insert<target> (dist_id, 0, "dist", rule_);
-      r.rules.insert<alias> (dist_id, 0, "dist.alias", rule_);
+      rs.rules.insert<target> (dist_id, 0, "dist", rule_);
+      rs.rules.insert<alias> (dist_id, 0, "dist.alias", rule_);
 
       // Configuration.
       //
@@ -97,22 +97,22 @@ namespace build2
       // must be explicitly specified or we will complain if and when
       // we try to dist.
       //
-      bool s (config::specified (r, "config.dist"));
+      bool s (config::specified (rs, "config.dist"));
 
       // Adjust module priority so that the config.dist.* values are saved at
       // the end of config.build.
       //
       if (s)
-        config::save_module (r, "dist", INT32_MAX);
+        config::save_module (rs, "dist", INT32_MAX);
 
       // dist.root
       //
       {
-        value& v (r.assign ("dist.root"));
+        value& v (rs.assign ("dist.root"));
 
         if (s)
         {
-          if (lookup l = config::optional (r, "config.dist.root"))
+          if (lookup l = config::optional (rs, "config.dist.root"))
             v = cast<dir_path> (l); // Strip abs_dir_path.
         }
       }
@@ -120,11 +120,11 @@ namespace build2
       // dist.cmd
       //
       {
-        value& v (r.assign<process_path> ("dist.cmd"));
+        value& v (rs.assign<process_path> ("dist.cmd"));
 
         if (s)
         {
-          if (lookup l = config::required (r,
+          if (lookup l = config::required (rs,
                                            "config.dist.cmd",
                                            path ("install")).first)
             v = run_search (cast<path> (l), true);
@@ -134,11 +134,11 @@ namespace build2
       // dist.archives
       //
       {
-        value& v (r.assign ("dist.archives"));
+        value& v (rs.assign ("dist.archives"));
 
         if (s)
         {
-          if (lookup l = config::optional (r, "config.dist.archives"))
+          if (lookup l = config::optional (rs, "config.dist.archives"))
             v = *l;
         }
       }
diff --git a/build2/dist/operation.cxx b/build2/dist/operation.cxx
index 1b96282..83a5b01 100644
--- a/build2/dist/operation.cxx
+++ b/build2/dist/operation.cxx
@@ -30,7 +30,7 @@ namespace build2
     }
 
     static void
-    dist_match (action, action_targets&)
+    dist_match (ulock&, action, action_targets&)
     {
       // Don't match anything -- see execute ().
     }
@@ -54,7 +54,7 @@ namespace build2
              const string& ext);
 
     static void
-    dist_execute (action, const action_targets& ts, bool)
+    dist_execute (ulock& ml, action, const action_targets& ts, bool)
     {
       tracer trace ("dist_execute");
 
@@ -97,6 +97,10 @@ namespace build2
       const string& dist_package (cast<string> (l));
       const process_path& dist_cmd (cast<process_path> (rs->vars["dist.cmd"]));
 
+      // Relock for shared access. @@ BOGUS
+      //
+      ml.unlock ();
+
       // Get the list of operations supported by this project. Skip
       // default_id.
       //
@@ -120,6 +124,9 @@ namespace build2
         if (verb >= 6)
           dump (a);
 
+        slock sl (*ml.mutex ());
+        model_lock = &sl; // @@ Guard?
+
         for (void* v: ts)
         {
           target& t (*static_cast<target*> (v));
@@ -131,13 +138,17 @@ namespace build2
 
           l5 ([&]{trace << diag_doing (a, t);});
 
-          match (a, t);
+          match (sl, a, t);
         }
 
+        model_lock = nullptr;
+
         if (verb >= 6)
           dump (a);
       }
 
+      ml.lock ();
+
       // Add buildfiles that are not normally loaded as part of the
       // project, for example, the export stub. They will still be
       // ignored on the next step if the user explicitly marked them
@@ -169,7 +180,7 @@ namespace build2
 
       // The same for subprojects that have been loaded.
       //
-      if (auto l = rs->vars["subprojects"])
+      if (auto l = rs->vars[var_subprojects])
       {
         for (auto p: cast<subprojects> (l))
         {
@@ -247,8 +258,8 @@ namespace build2
 
         action a (perform_id, update_id);
 
-        perform.match (a, files);
-        perform.execute (a, files, true); // Run quiet.
+        perform.match (ml, a, files);
+        perform.execute (ml, a, files, true); // Run quiet.
 
         if (perform.operation_post != nullptr)
           perform.operation_post (update_id);
diff --git a/build2/dist/rule b/build2/dist/rule
index db8e731..61be24f 100644
--- a/build2/dist/rule
+++ b/build2/dist/rule
@@ -22,10 +22,10 @@ namespace build2
       rule () {}
 
       virtual match_result
-      match (action, target&, const string&) const override;
+      match (slock&, action, target&, const string&) const override;
 
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
     };
   }
 }
diff --git a/build2/dist/rule.cxx b/build2/dist/rule.cxx
index 6cbd7df..f5c4018 100644
--- a/build2/dist/rule.cxx
+++ b/build2/dist/rule.cxx
@@ -16,17 +16,17 @@ namespace build2
   namespace dist
   {
     match_result rule::
-    match (action, target&, const string&) const
+    match (slock&, action, target&, const string&) const
     {
       return true; // We always match.
     }
 
     recipe rule::
-    apply (action a, target& t) const
+    apply (slock& ml, action a, target& t) const
     {
       const dir_path& out_root (t.root_scope ().out_path ());
 
-      auto r (group_prerequisite_members (a, t, false));
+      auto r (group_prerequisite_members (ml, a, t, false));
       for (auto i (r.begin ()); i != r.end (); ++i)
       {
         prerequisite_member p (*i);
@@ -48,7 +48,7 @@ namespace build2
         // Don't match targets that are outside of our project.
         //
         if (pt.dir.sub (out_root))
-          build2::match (a, pt);
+          build2::match (ml, a, pt);
       }
 
       return noop_recipe; // We will never be executed.
diff --git a/build2/file b/build2/file
index c8d8f6f..adfd884 100644
--- a/build2/file
+++ b/build2/file
@@ -56,18 +56,18 @@ namespace build2
   // If buildfile is '-', then read from STDIN.
   //
   void
-  source (const path& buildfile, scope& root, scope& base);
+  source (scope& root, scope& base, const path&);
 
   // As above but first check if this buildfile has already been sourced for
   // the base scope. Return false if the file has already been sourced.
   //
   bool
-  source_once (const path& buildfile, scope& root, scope& base);
+  source_once (scope& root, scope& base, const path&);
 
   // As above but checks against the specified scope rather than base.
   //
   bool
-  source_once (const path& buildfile, scope& root, scope& base, scope& once);
+  source_once (scope& root, scope& base, const path&, scope& once);
 
   // Create project's root scope. Only set the src_root variable if the
   // passed src_root value is not empty.
@@ -143,7 +143,7 @@ namespace build2
   // an indication of whether the variable was found.
   //
   pair<value, bool>
-  extract_variable (const path&, const char* var);
+  extract_variable (const path&, const variable&);
 
   // Import has two phases: the first is triggered by the import
   // directive in the buildfile. It will try to find and load the
diff --git a/build2/file.cxx b/build2/file.cxx
index 527f752..9440738 100644
--- a/build2/file.cxx
+++ b/build2/file.cxx
@@ -83,7 +83,7 @@ namespace build2
   }
 
   static void
-  source (const path& bf, scope& root, scope& base, bool boot)
+  source (scope& root, scope& base, const path& bf, bool boot)
   {
     tracer trace ("source");
 
@@ -112,13 +112,13 @@ namespace build2
   }
 
   void
-  source (const path& bf, scope& root, scope& base)
+  source (scope& root, scope& base, const path& bf)
   {
-    source (bf, root, base, false);
+    source (root, base, bf, false);
   }
 
   bool
-  source_once (const path& bf, scope& root, scope& base, scope& once)
+  source_once (scope& root, scope& base, const path& bf, scope& once)
   {
     tracer trace ("source_once");
 
@@ -128,7 +128,7 @@ namespace build2
       return false;
     }
 
-    source (bf, root, base);
+    source (root, base, bf);
     return true;
   }
 
@@ -168,7 +168,7 @@ namespace build2
     // consistent.
     //
     {
-      value& v (rs.assign ("out_root"));
+      value& v (rs.assign (var_out_root));
 
       if (!v)
         v = out_root;
@@ -184,7 +184,7 @@ namespace build2
 
     if (!src_root.empty ())
     {
-      value& v (rs.assign ("src_root"));
+      value& v (rs.assign (var_src_root));
 
       if (!v)
         v = src_root;
@@ -206,7 +206,7 @@ namespace build2
   {
     // The caller must have made sure src_root is set on this scope.
     //
-    value& v (s.assign ("src_root"));
+    value& v (s.assign (var_src_root));
     assert (v);
     const dir_path& d (cast<dir_path> (v));
 
@@ -225,14 +225,14 @@ namespace build2
 
     // Set src/out_base variables.
     //
-    value& ov (s.assign ("out_base"));
+    value& ov (s.assign (var_out_base));
 
     if (!ov)
       ov = out_base;
     else
       assert (cast<dir_path> (ov) == out_base);
 
-    value& sv (s.assign ("src_base"));
+    value& sv (s.assign (var_src_base));
 
     if (!sv)
       sv = src_base;
@@ -309,7 +309,7 @@ namespace build2
     //   prevent multiple sourcing. We handle it here but we still
     //   need something like source_once (once [scope] source).
     //
-    source_once (bf, root, root);
+    source_once (root, root, bf);
   }
 
   // Extract the specified variable value from a buildfile. It is expected to
@@ -317,7 +317,7 @@ namespace build2
   // other than those from the global scope or any variable overrides.
   //
   pair<value, bool>
-  extract_variable (const path& bf, const char* name)
+  extract_variable (const path& bf, const variable& var)
   {
     try
     {
@@ -327,7 +327,7 @@ namespace build2
       token t (lex.next ());
       token_type tt;
 
-      if (t.type != token_type::word || t.value != name ||
+      if (t.type != token_type::word || t.value != var.name ||
           ((tt = lex.next ().type) != token_type::assign &&
            tt != token_type::prepend &&
            tt != token_type::append))
@@ -335,8 +335,6 @@ namespace build2
         return make_pair (value (), false);
       }
 
-      const variable& var (var_pool[t.value]);
-
       parser p;
       temp_scope tmp (*global_scope);
       p.parse_variable (lex, tmp, var, tt);
@@ -382,7 +380,7 @@ namespace build2
         src_root = &fallback_src_root;
       else
       {
-        auto p (extract_variable (f, "src_root"));
+        auto p (extract_variable (f, *var_src_root));
 
         if (!p.second)
           fail << "variable 'src_root' expected as first line in " << f;
@@ -397,10 +395,11 @@ namespace build2
     string name;
     {
       path f (*src_root / bootstrap_file);
-      auto p (extract_variable (f, "project"));
+      auto p (extract_variable (f, *var_project));
 
       if (!p.second)
-        fail << "variable 'project' expected as first line in " << f;
+        fail << "variable '" << var_project->name << "' expected "
+             << "as a first line in " << f;
 
       name = cast<string> (move (p.first));
     }
@@ -513,7 +512,7 @@ namespace build2
       // same root scope multiple time.
       //
       if (root.buildfiles.insert (bf).second)
-        source (bf, root, root, true);
+        source (root, root, bf, true);
       else
         l5 ([&]{trace << "skipping already sourced " << bf;});
 
@@ -532,7 +531,7 @@ namespace build2
     // amalgamated.
     //
     {
-      auto rp (root.vars.insert ("amalgamation")); // Set NULL by default.
+      auto rp (root.vars.insert (*var_amalgamation)); // Set NULL by default.
       value& v (rp.first);
 
       if (v && v.empty ()) // Convert empty to NULL.
@@ -603,8 +602,7 @@ namespace build2
     // NULL value indicates that we found no subprojects.
     //
     {
-      const variable& var (var_pool["subprojects"]);
-      auto rp (root.vars.insert (var)); // Set NULL by default.
+      auto rp (root.vars.insert (*var_subprojects)); // Set NULL by default.
       value& v (rp.first);
 
       if (rp.second)
@@ -725,14 +723,14 @@ namespace build2
     // It should either be NULL or typed (so we assume that the user will
     // never set it to NULL).
     //
-    auto l (root.vars["subprojects"]);
+    auto l (root.vars[var_subprojects]);
     return l.defined () && (l->null || l->type != nullptr);
   }
 
   void
   create_bootstrap_outer (scope& root)
   {
-    auto l (root.vars["amalgamation"]);
+    auto l (root.vars[var_amalgamation]);
 
     if (!l)
       return;
@@ -758,7 +756,7 @@ namespace build2
     {
       bootstrap_out (rs); // #3 happens here, if at all.
 
-      value& v (rs.assign ("src_root"));
+      value& v (rs.assign (var_src_root));
 
       if (!v)
       {
@@ -787,7 +785,7 @@ namespace build2
   scope&
   create_bootstrap_inner (scope& root, const dir_path& out_base)
   {
-    if (auto l = root.vars["subprojects"])
+    if (auto l = root.vars[var_subprojects])
     {
       for (const auto& p: cast<subprojects> (l))
       {
@@ -804,7 +802,7 @@ namespace build2
         {
           bootstrap_out (rs);
 
-          value& v (rs.assign ("src_root"));
+          value& v (rs.assign (var_src_root));
 
           if (!v)
             v = is_src_root (out_root)
@@ -848,7 +846,7 @@ namespace build2
 
       if (s.boot)
       {
-        load_module (n, root, root, s.loc);
+        load_module (root, root, n, s.loc);
         assert (!s.boot);
       }
     }
@@ -858,7 +856,7 @@ namespace build2
     path bf (root.src_path () / root_file);
 
     if (exists (bf))
-      source_once (bf, root, root);
+      source_once (root, root, bf);
   }
 
   names
@@ -903,12 +901,13 @@ namespace build2
       // Note: overridable variable with path auto-completion.
       //
       {
-        const variable& var (var_pool.insert<abs_dir_path> (n, true));
+        const variable& var (
+          var_pool.rw (ibase).insert<abs_dir_path> (n, true));
 
         if (auto l = iroot[var])
         {
           out_root = cast<dir_path> (l);      // Normalized and actualized.
-          config::save_variable (iroot, var); // Mark as part of configuration.
+          config::save_variable (iroot, var); // Mark as part of config.
 
           // Empty config.import.* value means don't look in subprojects or
           // amalgamations and go straight to the rule-specific import (e.g.,
@@ -934,7 +933,7 @@ namespace build2
       {
         auto lookup = [&iroot, &loc] (string name) -> path
         {
-          const variable& var (var_pool.insert<path> (name, true));
+          const variable& var (var_pool.rw (iroot).insert<path> (name, true));
 
           path r;
           if (auto l = iroot[var])
@@ -985,13 +984,13 @@ namespace build2
 
         // First check the amalgamation itself.
         //
-        if (r != &iroot && cast<string> (r->vars["project"]) == proj)
+        if (r != &iroot && cast<string> (r->vars[var_project]) == proj)
         {
           out_root = r->out_path ();
           break;
         }
 
-        if (auto l = r->vars["subprojects"])
+        if (auto l = r->vars[var_subprojects])
         {
           const auto& m (cast<subprojects> (l));
           auto i (m.find (proj));
@@ -1004,7 +1003,7 @@ namespace build2
           }
         }
 
-        if (!r->vars["amalgamation"])
+        if (!r->vars[var_amalgamation])
           break;
       }
 
@@ -1041,7 +1040,7 @@ namespace build2
 
         // Check that the bootstrap process set src_root.
         //
-        if (auto l = root->vars["src_root"])
+        if (auto l = root->vars[*var_src_root])
         {
           const dir_path& p (cast<dir_path> (l));
 
@@ -1061,10 +1060,10 @@ namespace build2
 
       // Now we know this project's name as well as all its subprojects.
       //
-      if (cast<string> (root->vars["project"]) == proj)
+      if (cast<string> (root->vars[var_project]) == proj)
         break;
 
-      if (auto l = root->vars["subprojects"])
+      if (auto l = root->vars[var_subprojects])
       {
         const auto& m (cast<subprojects> (l));
         auto i (m.find (proj));
@@ -1096,13 +1095,13 @@ namespace build2
 
     // "Pass" the imported project's roots to the stub.
     //
-    ts.assign ("out_root") = move (out_root);
-    ts.assign ("src_root") = move (src_root);
+    ts.assign (var_out_root) = move (out_root);
+    ts.assign (var_src_root) = move (src_root);
 
-    // Also pass the target being imported.
+    // Also pass the target being imported in the import.target variable.
     //
     {
-      value& v (ts.assign ("target"));
+      value& v (ts.assign (var_import_target));
 
       if (!target.empty ()) // Otherwise leave NULL.
         v = move (target);
diff --git a/build2/file.ixx b/build2/file.ixx
index 050a3e9..e12654c 100644
--- a/build2/file.ixx
+++ b/build2/file.ixx
@@ -5,8 +5,8 @@
 namespace build2
 {
   inline bool
-  source_once (const path& bf, scope& root, scope& base)
+  source_once (scope& root, scope& base, const path& bf)
   {
-    return source_once (bf, root, base, base);
+    return source_once (root, base, bf, base);
   }
 }
diff --git a/build2/install/init.cxx b/build2/install/init.cxx
index 8c28dad..122eb54 100644
--- a/build2/install/init.cxx
+++ b/build2/install/init.cxx
@@ -52,6 +52,8 @@ namespace build2
 
       if (spec)
       {
+        // Note: overridable.
+        //
         vn = "config.install";
         if (!global)
         {
@@ -59,7 +61,7 @@ namespace build2
           vn += name;
         }
         vn += var;
-        const variable& vr (var_pool.insert<CT> (move (vn), true));
+        const variable& vr (var_pool.rw (r).insert<CT> (move (vn), true));
 
         l = dv != nullptr
           ? config::required (r, vr, *dv, override).first
@@ -71,10 +73,12 @@ namespace build2
       if (global)
         return;
 
+      // Note: not overridable.
+      //
       vn = "install.";
       vn += name;
       vn += var;
-      const variable& vr (var_pool.insert<T> (move (vn))); // Not overridable.
+      const variable& vr (var_pool.rw (r).insert<T> (move (vn)));
 
       value& v (r.assign (vr));
 
@@ -117,7 +121,7 @@ namespace build2
       // This one doesn't have config.* value (only set in a buildfile).
       //
       if (!global)
-        var_pool.insert<bool> (string ("install.") + n + ".subdirs");
+        var_pool.rw (r).insert<bool> (string ("install.") + n + ".subdirs");
     }
 
     static const alias_rule alias_;
@@ -153,8 +157,8 @@ namespace build2
     static const dir_path dir_man1 (dir_path ("man") /= "man1");
 
     bool
-    init (scope& r,
-          scope& b,
+    init (scope& rs,
+          scope& bs,
           const location& l,
           unique_ptr<module_base>&,
           bool first,
@@ -169,7 +173,7 @@ namespace build2
         return true;
       }
 
-      const dir_path& out_root (r.out_path ());
+      const dir_path& out_root (rs.out_path ());
       l5 ([&]{trace << "for " << out_root;});
 
       assert (config_hints.empty ()); // We don't known any hints.
@@ -179,7 +183,7 @@ namespace build2
       // Note that the set_dir() calls below enter some more.
       //
       {
-        auto& v (var_pool);
+        auto& v (var_pool.rw (rs));
 
         // Note: not overridable.
         //
@@ -196,11 +200,11 @@ namespace build2
 
       // Register our alias and file rules.
       //
-      b.rules.insert<alias> (perform_install_id,   "install.alias", alias_);
-      b.rules.insert<alias> (perform_uninstall_id, "uninstall.alias", alias_);
+      bs.rules.insert<alias> (perform_install_id,   "install.alias", alias_);
+      bs.rules.insert<alias> (perform_uninstall_id, "uninstall.alias", alias_);
 
-      b.rules.insert<file> (perform_install_id,   "install.file", file_);
-      b.rules.insert<file> (perform_uninstall_id, "uinstall.file", file_);
+      bs.rules.insert<file> (perform_install_id,   "install.file", file_);
+      bs.rules.insert<file> (perform_uninstall_id, "uinstall.file", file_);
 
       // Configuration.
       //
@@ -211,44 +215,44 @@ namespace build2
       {
         using build2::path;
 
-        bool s (config::specified (r, "config.install"));
+        bool s (config::specified (rs, "config.install"));
 
         // Adjust module priority so that the (numerous) config.install.*
         // values are saved at the end of config.build.
         //
         if (s)
-          config::save_module (r, "install", INT32_MAX);
+          config::save_module (rs, "install", INT32_MAX);
 
-        const string& n (cast<string> (r["project"]));
+        const string& n (project (rs));
 
         // Global config.install.* values.
         //
-        set_dir (s, r, "",          abs_dir_path (), false, "644", "755", cmd);
+        set_dir (s, rs, "",          abs_dir_path (), false, "644", "755", cmd);
 
-        set_dir (s, r, "root",      abs_dir_path ());
+        set_dir (s, rs, "root",      abs_dir_path ());
 
-        set_dir (s, r, "data_root", dir_root);
-        set_dir (s, r, "exec_root", dir_root, false, "755");
+        set_dir (s, rs, "data_root", dir_root);
+        set_dir (s, rs, "exec_root", dir_root, false, "755");
 
-        set_dir (s, r, "sbin",      dir_sbin);
-        set_dir (s, r, "bin",       dir_bin);
-        set_dir (s, r, "lib",       dir_lib);
-        set_dir (s, r, "libexec",   dir_path (dir_libexec) /= n, true);
+        set_dir (s, rs, "sbin",      dir_sbin);
+        set_dir (s, rs, "bin",       dir_bin);
+        set_dir (s, rs, "lib",       dir_lib);
+        set_dir (s, rs, "libexec",   dir_path (dir_libexec) /= n, true);
 
-        set_dir (s, r, "data",      dir_path (dir_data) /= n, true);
-        set_dir (s, r, "include",   dir_include);
+        set_dir (s, rs, "data",      dir_path (dir_data) /= n, true);
+        set_dir (s, rs, "include",   dir_include);
 
-        set_dir (s, r, "doc",       dir_path (dir_doc) /= n, true);
-        set_dir (s, r, "man",       dir_man);
-        set_dir (s, r, "man1",      dir_man1);
+        set_dir (s, rs, "doc",       dir_path (dir_doc) /= n, true);
+        set_dir (s, rs, "man",       dir_man);
+        set_dir (s, rs, "man1",      dir_man1);
       }
 
       // Configure "installability" for built-in target types.
       //
-      install_path<exe>  (b, dir_path ("bin"));  // Install into install.bin.
-      install_path<doc>  (b, dir_path ("doc"));  // Install into install.doc.
-      install_path<man>  (b, dir_path ("man"));  // Install into install.man.
-      install_path<man1> (b, dir_path ("man1")); // Install into install.man1.
+      install_path<exe>  (bs, dir_path ("bin"));  // Install into install.bin.
+      install_path<doc>  (bs, dir_path ("doc"));  // Install into install.doc.
+      install_path<man>  (bs, dir_path ("man"));  // Install into install.man.
+      install_path<man1> (bs, dir_path ("man1")); // Install into install.man1.
 
       return true;
     }
diff --git a/build2/install/rule b/build2/install/rule
index aad5ee4..169c43a 100644
--- a/build2/install/rule
+++ b/build2/install/rule
@@ -22,10 +22,10 @@ namespace build2
       alias_rule () {}
 
       virtual match_result
-      match (action, target&, const string&) const override;
+      match (slock&, action, target&, const string&) const override;
 
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
     };
 
     struct install_dir;
@@ -36,17 +36,17 @@ namespace build2
       file_rule () {}
 
       virtual match_result
-      match (action, target&, const string&) const override;
+      match (slock&, action, target&, const string&) const override;
 
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
 
       // Return NULL if this prerequisite should be ignored and pointer to its
       // target otherwise. The default implementation ignores prerequsites that
       // are outside of this target's project.
       //
       virtual target*
-      filter (action, target&, prerequisite_member) const;
+      filter (slock&, action, target&, prerequisite_member) const;
 
       // Extra installation hooks.
       //
diff --git a/build2/install/rule.cxx b/build2/install/rule.cxx
index a727acc..13c93a7 100644
--- a/build2/install/rule.cxx
+++ b/build2/install/rule.cxx
@@ -39,13 +39,13 @@ namespace build2
     // alias_rule
     //
     match_result alias_rule::
-    match (action, target&, const string&) const
+    match (slock&, action, target&, const string&) const
     {
       return true;
     }
 
     recipe alias_rule::
-    apply (action a, target& t) const
+    apply (slock& ml, action a, target& t) const
     {
       tracer trace ("install::alias_rule::apply");
 
@@ -73,7 +73,7 @@ namespace build2
           continue;
         }
 
-        build2::match (a, pt);
+        build2::match (ml, a, pt);
         t.prerequisite_targets.push_back (&pt);
       }
 
@@ -91,7 +91,7 @@ namespace build2
                    "insufficient space");
 
     match_result file_rule::
-    match (action a, target& t, const string&) const
+    match (slock&, action a, target& t, const string&) const
     {
       // First determine if this target should be installed (called
       // "installable" for short).
@@ -119,14 +119,14 @@ namespace build2
     }
 
     target* file_rule::
-    filter (action, target& t, prerequisite_member p) const
+    filter (slock&, action, target& t, prerequisite_member p) const
     {
       target& pt (p.search ());
       return pt.in (t.root_scope ()) ? &pt : nullptr;
     }
 
     recipe file_rule::
-    apply (action a, target& t) const
+    apply (slock& ml, action a, target& t) const
     {
       match_data md (move (t.data<match_data> ()));
       t.clear_data (); // In case delegated-to rule also uses aux storage.
@@ -149,7 +149,7 @@ namespace build2
       // run standard search_and_match()? Will need an indicator
       // that it was forced (e.g., [install]) for filter() below.
       //
-      auto r (group_prerequisite_members (a, t));
+      auto r (group_prerequisite_members (ml, a, t));
       for (auto i (r.begin ()); i != r.end (); ++i)
       {
         prerequisite_member p (*i);
@@ -162,7 +162,7 @@ namespace build2
 
         // Let a customized rule have its say.
         //
-        target* pt (filter (a, t, p));
+        target* pt (filter (ml, a, t, p));
         if (pt == nullptr)
           continue;
 
@@ -172,7 +172,7 @@ namespace build2
         if (l && cast<path> (l).string () == "false")
           continue;
 
-        build2::match (a, *pt);
+        build2::match (ml, a, *pt);
 
         // If the matched rule returned noop_recipe, then the target
         // state will be set to unchanged as an optimization. Use this
@@ -209,7 +209,7 @@ namespace build2
         // have been found if we signalled that we do not match from
         // match() above.
         //
-        recipe d (match_delegate (a, t, *this).first);
+        recipe d (match_delegate (ml, a, t, *this).first);
 
         // If we have no installable prerequisites, then simply redirect
         // to it.
diff --git a/build2/install/utility b/build2/install/utility
index 713c3e8..239447e 100644
--- a/build2/install/utility
+++ b/build2/install/utility
@@ -17,9 +17,12 @@ namespace build2
     // Set install path, mode for a target type.
     //
     inline void
-    install_path (const target_type& tt, scope& s, dir_path d)
+    install_path (scope& s, const target_type& tt, dir_path d)
     {
-      auto r (s.target_vars[tt]["*"].insert ("install"));
+      auto r (
+        s.target_vars[tt]["*"].insert (
+          var_pool.rw (s).insert ("install")));
+
       if (r.second) // Already set by the user?
         r.first.get () = path_cast<path> (move (d));
     }
@@ -28,13 +31,16 @@ namespace build2
     inline void
     install_path (scope& s, dir_path d)
     {
-      return install_path (T::static_type, s, move (d));
+      return install_path (s, T::static_type, move (d));
     }
 
     inline void
-    install_mode (const target_type& tt, scope& s, string m)
+    install_mode (scope& s, const target_type& tt, string m)
     {
-      auto r (s.target_vars[tt]["*"].insert ("install.mode"));
+      auto r (
+        s.target_vars[tt]["*"].insert (
+          var_pool.rw (s).insert ("install.mode")));
+
       if (r.second) // Already set by the user?
         r.first.get () = move (m);
     }
@@ -43,7 +49,7 @@ namespace build2
     inline void
     install_mode (scope& s, string m)
     {
-      return install_mode (T::static_type, s, move (m));
+      return install_mode (s, T::static_type, move (m));
     }
   }
 }
diff --git a/build2/module b/build2/module
index 7d6439f..7c4672f 100644
--- a/build2/module
+++ b/build2/module
@@ -26,7 +26,9 @@ namespace build2
   };
 
   using module_boot_function =
-    void (scope& root, const location&, unique_ptr<module_base>&);
+    void (scope& root,
+          const location&,
+          unique_ptr<module_base>&);
 
   // Return false if the module configuration (normally based on the default
   // values) was unsuccessful but this is not (yet) an error. One example
@@ -82,7 +84,7 @@ namespace build2
   // Load and boot the specified module.
   //
   void
-  boot_module (const string& name, scope& root, const location&);
+  boot_module (scope& root, const string& name, const location&);
 
   // Load (if not already loaded) and initialize the specified module. Used
   // by the parser but also by some modules to load prerequisite modules.
@@ -96,9 +98,9 @@ namespace build2
   // its tools).
   //
   bool
-  load_module (const string& name,
-               scope& root,
+  load_module (scope& root,
                scope& base,
+               const string& name,
                const location&,
                bool optional = false,
                const variable_map& config_hints = variable_map ());
diff --git a/build2/module.cxx b/build2/module.cxx
index c2f907f..031ae45 100644
--- a/build2/module.cxx
+++ b/build2/module.cxx
@@ -15,7 +15,7 @@ namespace build2
   available_module_map builtin_modules;
 
   void
-  boot_module (const string& name, scope& rs, const location& loc)
+  boot_module (scope& rs, const string& name, const location& loc)
   {
     // First see if this modules has already been loaded for this project.
     //
@@ -50,9 +50,9 @@ namespace build2
   }
 
   bool
-  load_module (const string& name,
-               scope& rs,
+  load_module (scope& rs,
                scope& bs,
+               const string& name,
                const location& loc,
                bool opt,
                const variable_map& hints)
@@ -100,10 +100,13 @@ namespace build2
     bool c (l &&
             i->second.init (rs, bs, loc, i->second.module, f, opt, hints));
 
-    const variable& lv (var_pool.insert<bool> (name + ".loaded",
-                                               variable_visibility::project));
-    const variable& cv (var_pool.insert<bool> (name + ".configured",
-                                               variable_visibility::project));
+    auto& vp (var_pool.rw (rs));
+
+    const variable& lv (vp.insert<bool> (name + ".loaded",
+                                         variable_visibility::project));
+
+    const variable& cv (vp.insert<bool> (name + ".configured",
+                                         variable_visibility::project));
     bs.assign (lv) = l;
     bs.assign (cv) = c;
 
diff --git a/build2/operation b/build2/operation
index 0daf413..d52c748 100644
--- a/build2/operation
+++ b/build2/operation
@@ -198,20 +198,25 @@ namespace build2
     // Meta-operation-specific logic to load the buildfile, search and match
     // the targets, and execute the action on the targets.
     //
-    void (*load) (const path& buildfile,
+    // Note that the model lock is passed locked and is expected to also be
+    // locked on return (but it can be released and re-acquired inside).
+    //
+    void (*load) (ulock&,
                   scope& root,
+                  const path& buildfile,
                   const dir_path& out_base,
                   const dir_path& src_base,
                   const location&);
 
-    void (*search) (scope& root,
+    void (*search) (ulock&,
+                    scope& root,
                     const target_key&,
                     const location&,
                     action_targets&);
 
-    void (*match) (action, action_targets&);
+    void (*match) (ulock&, action, action_targets&);
 
-    void (*execute) (action, const action_targets&, bool quiet);
+    void (*execute) (ulock&, action, const action_targets&, bool quiet);
 
     void (*operation_post) (operation_id); // End of operation batch.
     void (*meta_operation_post) (); // End of meta-operation batch.
@@ -229,8 +234,9 @@ namespace build2
   // scope.
   //
   void
-  load (const path& buildfile,
+  load (ulock&,
         scope& root,
+        const path& buildfile,
         const dir_path& out_base,
         const dir_path& src_base,
         const location&);
@@ -239,17 +245,17 @@ namespace build2
   // that does just that and adds a pointer to the target to the list.
   //
   void
-  search (scope&, const target_key&, const location&, action_targets&);
+  search (ulock&, scope&, const target_key&, const location&, action_targets&);
 
   void
-  match (action, action_targets&);
+  match (ulock&, action, action_targets&);
 
   // Execute the action on the list of targets. This is the default
   // implementation that does just that while issuing appropriate
   // diagnostics (unless quiet).
   //
   void
-  execute (action, const action_targets&, bool quiet);
+  execute (ulock&, action, const action_targets&, bool quiet);
 
   extern const meta_operation_info noop;
   extern const meta_operation_info perform;
diff --git a/build2/operation.cxx b/build2/operation.cxx
index 545bc9e..3871970 100644
--- a/build2/operation.cxx
+++ b/build2/operation.cxx
@@ -44,8 +44,9 @@ namespace build2
   // perform
   //
   void
-  load (const path& bf,
+  load (ulock&,
         scope& root,
+        const path& bf,
         const dir_path& out_base,
         const dir_path& src_base,
         const location&)
@@ -62,11 +63,12 @@ namespace build2
 
     // Load the buildfile unless it has already been loaded.
     //
-    source_once (bf, root, base, root);
+    source_once (root, base, bf, root);
   }
 
   void
-  search (scope&,
+  search (ulock&,
+          scope&,
           const target_key& tk,
           const location& l,
           action_targets& ts)
@@ -81,26 +83,38 @@ namespace build2
   }
 
   void
-  match (action a, action_targets& ts)
+  match (ulock& ml, action a, action_targets& ts)
   {
     tracer trace ("match");
 
     if (verb >= 6)
       dump (a);
 
-    for (void* vt: ts)
+    // Relock for shared access.
+    //
+    ml.unlock ();
+
     {
-      target& t (*static_cast<target*> (vt));
-      l5 ([&]{trace << "matching " << t;});
-      match (a, t);
+      for (void* vt: ts)
+      {
+        target& t (*static_cast<target*> (vt));
+        l5 ([&]{trace << "matching " << t;});
+
+        slock sl (*ml.mutex ());
+        model_lock = &sl; // @@ Guard?
+        match (sl, a, t);
+        model_lock = nullptr;
+      }
     }
 
+    ml.lock ();
+
     if (verb >= 6)
       dump (a);
   }
 
   void
-  execute (action a, const action_targets& ts, bool quiet)
+  execute (ulock& ml, action a, const action_targets& ts, bool quiet)
   {
     tracer trace ("execute");
 
@@ -109,13 +123,21 @@ namespace build2
     //
     vector<reference_wrapper<target>> psp;
 
-    auto body = [a, quiet, &psp, &trace] (void* v)
+    auto body = [&ml, a, quiet, &psp, &trace] (void* v)
     {
       target& t (*static_cast<target*> (v));
 
       l5 ([&]{trace << diag_doing (a, t);});
 
-      switch (execute (a, t))
+      target_state ts;
+      {
+        slock sl (*ml.mutex ());
+        model_lock = &sl; // @@ Guard?
+        ts = execute (a, t);
+        model_lock = nullptr;
+      }
+
+      switch (ts)
       {
       case target_state::unchanged:
         {
@@ -135,11 +157,17 @@ namespace build2
       }
     };
 
+    // Relock for shared access.
+    //
+    ml.unlock ();
+
     if (current_mode == execution_mode::first)
       for (void* v: ts) body (v);
     else
       for (void* v: reverse_iterate (ts)) body (v);
 
+    ml.lock ();
+
     // We should have executed every target that we matched.
     //
     assert (dependency_count == 0);
diff --git a/build2/parser b/build2/parser
index f91bbb6..e589948 100644
--- a/build2/parser
+++ b/build2/parser
@@ -521,7 +521,6 @@ namespace build2
 
   protected:
     bool pre_parse_ = false;
-
     bool boot_;
 
     const path* path_; // Current path.
diff --git a/build2/parser.cxx b/build2/parser.cxx
index ec9d9e4..fe12b98 100644
--- a/build2/parser.cxx
+++ b/build2/parser.cxx
@@ -457,7 +457,8 @@ namespace build2
             type att (tt);
 
             const variable& var (
-              var_pool.insert (parse_variable_name (move (pns), ploc)));
+              var_pool.rw (*scope_).insert (
+                parse_variable_name (move (pns), ploc)));
 
             // Apply variable attributes.
             //
@@ -725,7 +726,8 @@ namespace build2
       if (tt == type::assign || tt == type::prepend || tt == type::append)
       {
         const variable& var (
-          var_pool.insert (parse_variable_name (move (ns), nloc)));
+          var_pool.rw (*scope_).insert (
+            parse_variable_name (move (ns), nloc)));
 
         // Apply variable attributes.
         //
@@ -1049,9 +1051,10 @@ namespace build2
       // Is this the 'foo=...' case?
       //
       size_t p (t.value.find ('='));
+      auto& vp (var_pool.rw (*scope_));
 
       if (p != string::npos)
-        var = &var_pool.insert (split (p));
+        var = &vp.insert (split (p));
       //
       // This could still be the 'foo =...' case.
       //
@@ -1066,7 +1069,7 @@ namespace build2
             (v[p = 0] == '=' ||
              (n > 1 && v[0] == '+' && v[p = 1] == '=')))
         {
-          var = &var_pool[t.value];
+          var = &vp.insert (move (t.value));
           next (t, tt); // Get the peeked token.
           split (p);    // Returned name should be empty.
         }
@@ -1235,9 +1238,9 @@ namespace build2
         assert (v.empty ()); // Module versioning not yet implemented.
 
         if (boot_)
-          boot_module (n, *root_, l);
+          boot_module (*root_, n, l);
         else
-          load_module (n, *root_, *scope_, l, optional);
+          load_module (*root_, *scope_, n, l, optional);
       }
     }
 
@@ -1503,8 +1506,12 @@ namespace build2
 
     value& lhs (
       kind == type::assign
-      ? target_ != nullptr ? target_->assign (var) : scope_->assign (var)
-      : target_ != nullptr ? target_->append (var) : scope_->append (var));
+      ? (target_ != nullptr
+         ? target_->assign (var)
+         :  scope_->assign (var))
+      : (target_ != nullptr
+         ? target_->append (var)
+         :  scope_->append (var)));
 
     apply_value_attributes (&var, lhs, move (rhs), kind);
   }
@@ -3491,7 +3498,7 @@ namespace build2
 
     // Lookup.
     //
-    const auto& var (var_pool.insert (move (name)));
+    const auto& var (var_pool.rw (*scope_).insert (move (name)));
     return target_ != nullptr ? (*target_)[var] : (*scope_)[var];
 
     // Undefined/NULL namespace variables are not allowed.
diff --git a/build2/pkgconfig/init.cxx b/build2/pkgconfig/init.cxx
index ff386bb..352e845 100644
--- a/build2/pkgconfig/init.cxx
+++ b/build2/pkgconfig/init.cxx
@@ -39,7 +39,7 @@ namespace build2
       //
       // config.pkgconfig.target is a hint.
       //
-      auto& vp (var_pool);
+      auto& vp (var_pool.rw (rs));
 
       const variable& c_x (vp.insert<path> ("config.pkgconfig", true));
       const variable& x_path (vp.insert<process_path> ("pkgconfig.path"));
@@ -142,7 +142,7 @@ namespace build2
       //
       if (!cast_false<bool> (rs["pkgconfig.config.loaded"]))
       {
-        if (!load_module ("pkgconfig.config", rs, rs, loc, optional, hints))
+        if (!load_module (rs, rs, "pkgconfig.config", loc, optional, hints))
           return false;
       }
       else if (!cast_false<bool> (rs["pkgconfig.config.configured"]))
diff --git a/build2/rule b/build2/rule
index d01df6b..cc98bfa 100644
--- a/build2/rule
+++ b/build2/rule
@@ -37,10 +37,10 @@ namespace build2
   {
   public:
     virtual match_result
-    match (action, target&, const string& hint) const = 0;
+    match (slock&, action, target&, const string& hint) const = 0;
 
     virtual recipe
-    apply (action, target&) const = 0;
+    apply (slock&, action, target&) const = 0;
   };
 
   // Fallback rule that only matches if the file exists.
@@ -51,10 +51,10 @@ namespace build2
     file_rule () {}
 
     virtual match_result
-    match (action, target&, const string& hint) const override;
+    match (slock&, action, target&, const string&) const override;
 
     virtual recipe
-    apply (action, target&) const override;
+    apply (slock&, action, target&) const override;
 
     static const file_rule instance;
   };
@@ -65,10 +65,10 @@ namespace build2
     alias_rule () {}
 
     virtual match_result
-    match (action, target&, const string& hint) const override;
+    match (slock&, action, target&, const string&) const override;
 
     virtual recipe
-    apply (action, target&) const override;
+    apply (slock&, action, target&) const override;
 
     static const alias_rule instance;
   };
@@ -79,10 +79,10 @@ namespace build2
     fsdir_rule () {}
 
     virtual match_result
-    match (action, target&, const string& hint) const override;
+    match (slock&, action, target&, const string&) const override;
 
     virtual recipe
-    apply (action, target&) const override;
+    apply (slock&, action, target&) const override;
 
     static target_state
     perform_update (action, target&);
@@ -101,10 +101,13 @@ namespace build2
     fallback_rule () {}
 
     virtual match_result
-    match (action, target&, const string&) const override {return true;}
+    match (slock&, action, target&, const string&) const override
+    {
+      return true;
+    }
 
     virtual recipe
-    apply (action, target&) const override {return noop_recipe;}
+    apply (slock&, action, target&) const override {return noop_recipe;}
 
     static const fallback_rule instance;
   };
diff --git a/build2/rule.cxx b/build2/rule.cxx
index 42d5eb8..88c7941 100644
--- a/build2/rule.cxx
+++ b/build2/rule.cxx
@@ -26,7 +26,7 @@ namespace build2
   // use it as a guide to implement your own, normal, rules.
   //
   match_result file_rule::
-  match (action a, target& t, const string&) const
+  match (slock&, action a, target& t, const string&) const
   {
     tracer trace ("file_rule::match");
 
@@ -79,7 +79,7 @@ namespace build2
   }
 
   recipe file_rule::
-  apply (action a, target& t) const
+  apply (slock& ml, action a, target& t) const
   {
     // Update triggers the update of this target's prerequisites so it would
     // seem natural that we should also trigger their cleanup. However, this
@@ -99,7 +99,7 @@ namespace build2
 
     // Search and match all the prerequisites.
     //
-    search_and_match_prerequisites (a, t);
+    search_and_match_prerequisites (ml, a, t);
 
     // Note that we used to provide perform_update() which checked that this
     // target is not older than any of its prerequisites. However, later we
@@ -115,20 +115,20 @@ namespace build2
   // alias_rule
   //
   match_result alias_rule::
-  match (action, target&, const string&) const
+  match (slock&, action, target&, const string&) const
   {
     return true;
   }
 
   recipe alias_rule::
-  apply (action a, target& t) const
+  apply (slock& ml, action a, target& t) const
   {
     // Inject dependency on our directory (note: not parent) so that it is
     // automatically created on update and removed on clean.
     //
-    inject_fsdir (a, t, false);
+    inject_fsdir (ml, a, t, false);
 
-    search_and_match_prerequisites (a, t);
+    search_and_match_prerequisites (ml, a, t);
     return default_recipe;
   }
 
@@ -137,22 +137,22 @@ namespace build2
   // fsdir_rule
   //
   match_result fsdir_rule::
-  match (action, target&, const string&) const
+  match (slock&, action, target&, const string&) const
   {
     return true;
   }
 
   recipe fsdir_rule::
-  apply (action a, target& t) const
+  apply (slock& ml, action a, target& t) const
   {
     // Inject dependency on the parent directory. Note that we don't do it for
     // clean since we shouldn't (and can't possibly, since it's our parent) be
     // removing it.
     //
     if (a.operation () != clean_id)
-      inject_fsdir (a, t);
+      inject_fsdir (ml, a, t);
 
-    search_and_match_prerequisites (a, t);
+    search_and_match_prerequisites (ml, a, t);
 
     switch (a)
     {
diff --git a/build2/scope b/build2/scope
index eff60b0..16ddfbb 100644
--- a/build2/scope
+++ b/build2/scope
@@ -94,9 +94,17 @@ namespace build2
     }
 
     lookup
+    operator[] (const variable* var) const // For cached variables.
+    {
+      assert (var != nullptr);
+      return operator[] (*var);
+    }
+
+    lookup
     operator[] (const string& name) const
     {
-      return operator[] (var_pool[name]);
+      const variable* var (var_pool.find (name));
+      return var != nullptr ? operator[] (*var) : lookup ();
     }
 
     // As above, but include target type/pattern-specific variables.
@@ -108,23 +116,11 @@ namespace build2
     }
 
     lookup
-    find (const string& var, const target_key& tk) const
-    {
-      return find (var_pool[var], tk);
-    }
-
-    lookup
     find (const variable& var, const target_type& tt, const string& tn) const
     {
       return find (var, &tt, &tn).first;
     }
 
-    lookup
-    find (const string& var, const target_type& tt, const string& tn) const
-    {
-      return find (var_pool[var], tt, tn);
-    }
-
     pair<lookup, size_t>
     find (const variable& var,
           const target_type* tt = nullptr,
@@ -149,24 +145,35 @@ namespace build2
                    pair<lookup, size_t> original,
                    bool target = false) const;
 
-    // Return a value suitable for assignment (or append if you only
-    // want to append to the value from this scope). If the variable
-    // does not exist in this scope's map, then a new one with the
-    // NULL value is added and returned. Otherwise the existing value
-    // is returned.
+    // Return a value suitable for assignment (or append if you only want to
+    // append to the value from this scope). If the value does not exist in
+    // this scope's map, then a new one with the NULL value is added and
+    // returned. Otherwise the existing value is returned.
     //
     value&
     assign (const variable& var) {return vars.assign (var);}
 
     value&
-    assign (const string& name) {return vars.assign (name);}
+    assign (const variable* var) // For cached variables.
+    {
+      assert (var != nullptr);
+      return vars.assign (*var);
+    }
+
+    value&
+    assign (string name)
+    {
+      return assign (variable_pool::instance.insert (move (name)));
+    }
 
-    // Unlike the two above, assign a typed non-overridable variable with
-    // normal visibility.
+    // Assign a typed non-overridable variable with normal visibility.
     //
     template <typename T>
     value&
-    assign (string name) {return vars.assign<T> (move (name));}
+    assign (string name)
+    {
+      return vars.assign (variable_pool::instance.insert<T> (move (name)));
+    }
 
     // Return a value suitable for appending. If the variable does not
     // exist in this scope's map, then outer scopes are searched for
@@ -177,9 +184,6 @@ namespace build2
     value&
     append (const variable&);
 
-    value&
-    append (const string& name) {return append (var_pool[name]);}
-
     // Target type/pattern-specific variables.
     //
     variable_type_map target_vars;
@@ -278,7 +282,10 @@ namespace build2
     }
   };
 
-  // Note that the scope map is only for paths from the out tree.
+  // Scope map.
+  //
+  // Protected by the model mutex. Note that the scope map is only for paths
+  // from the out tree.
   //
   using scope_map_base = butl::dir_path_map<scope>;
 
diff --git a/build2/target b/build2/target
index 339ff90..0dec5d4 100644
--- a/build2/target
+++ b/build2/target
@@ -331,9 +331,17 @@ namespace build2
     }
 
     lookup
+    operator[] (const variable* var) const // For cached variables.
+    {
+      assert (var != nullptr);
+      return operator[] (*var);
+    }
+
+    lookup
     operator[] (const string& name) const
     {
-      return operator[] (var_pool[name]);
+      const variable* var (var_pool.find (name));
+      return var != nullptr ? operator[] (*var) : lookup ();
     }
 
     // As above but also return the depth at which the value is found. The
@@ -354,9 +362,6 @@ namespace build2
         : base_scope ().find_override (var, move (p), true);
     }
 
-    pair<lookup, size_t>
-    find (const string& name) const {return find (var_pool[name]);}
-
     // If target_only is true, then only look in target and its target group
     // without continuing in scopes.
     //
@@ -368,27 +373,11 @@ namespace build2
     value&
     assign (const variable& var) {return vars.assign (var);}
 
-    value&
-    assign (const string& name) {return vars.assign (name);}
-
-    // Unlike the two above, assign a typed non-overridable variable with
-    // normal visibility.
-    //
-    template <typename T>
-    value&
-    assign (string name) {return vars.assign<T> (move (name)).first.get ();}
-
     // Return a value suitable for appending. See class scope for details.
     //
     value&
     append (const variable&);
 
-    value&
-    append (const string& name)
-    {
-      return append (var_pool[name]);
-    }
-
     // A target that is not (yet) entered as part of a real dependency
     // declaration (for example, that is entered as part of a target-specific
     // variable assignment) is called implied.
@@ -810,17 +799,17 @@ namespace build2
 
   template <typename T>
   inline prerequisite_members_range<T>
-  prerequisite_members (action a, T&& x, bool members = true)
+  prerequisite_members (slock& ml, action a, T&& x, bool members = true)
   {
-    return prerequisite_members_range<T> (a, forward<T> (x), members);
+    return prerequisite_members_range<T> (ml, a, forward<T> (x), members);
   }
 
   template <typename T>
   class prerequisite_members_range
   {
   public:
-    prerequisite_members_range (action a, T&& r, bool m)
-        : a_ (a), members_ (m), r_ (forward<T> (r)), e_ (r_.end ()) {}
+    prerequisite_members_range (slock& l, action a, T&& r, bool m)
+        : l_ (l), a_ (a), members_ (m), r_ (forward<T> (r)), e_ (r_.end ()) {}
 
     using base_iterator = decltype (declval<T> ().begin ());
 
@@ -925,6 +914,7 @@ namespace build2
     end () const {return iterator (this, e_);}
 
   private:
+    slock& l_;
     action a_;
     bool members_; // Go into group members by default?
     T r_;
@@ -934,35 +924,38 @@ namespace build2
   // prerequisite_members(t.prerequisites)
   //
   inline auto
-  prerequisite_members (action a, target& t, bool members = true)
+  prerequisite_members (slock& ml, action a, target& t, bool members = true)
   {
-    return prerequisite_members (a, t.prerequisites, members);
+    return prerequisite_members (ml, a, t.prerequisites, members);
   }
 
   // prerequisite_members(reverse_iterate(t.prerequisites))
   //
   inline auto
-  reverse_prerequisite_members (action a, target& t, bool members = true)
+  reverse_prerequisite_members (
+    slock& ml, action a, target& t, bool members = true)
   {
     return prerequisite_members (
-      a, reverse_iterate (t.prerequisites), members);
+      ml, a, reverse_iterate (t.prerequisites), members);
   }
 
   // prerequisite_members(group_prerequisites (t))
   //
   inline auto
-  group_prerequisite_members (action a, target& t, bool members = true)
+  group_prerequisite_members (
+    slock& ml, action a, target& t, bool members = true)
   {
-    return prerequisite_members (a, group_prerequisites (t), members);
+    return prerequisite_members (ml, a, group_prerequisites (t), members);
   }
 
   // prerequisite_members(reverse_iterate (group_prerequisites (t)))
   //
   inline auto
-  reverse_group_prerequisite_members (action a, target& t, bool members = true)
+  reverse_group_prerequisite_members (
+    slock& ml, action a, target& t, bool members = true)
   {
     return prerequisite_members (
-      a, reverse_iterate (group_prerequisites (t)), members);
+      ml, a, reverse_iterate (group_prerequisites (t)), members);
   }
 
   // A target with an unspecified extension is considered equal to the one
diff --git a/build2/target.cxx b/build2/target.cxx
index 64b376c..81d613f 100644
--- a/build2/target.cxx
+++ b/build2/target.cxx
@@ -613,8 +613,9 @@ namespace build2
       out_base.normalize ();
 
       // In our world modifications to the scope structure during search &
-      // match should be "pure" in the sense that they should not affect any
-      // existing targets that have already been searched & matched.
+      // match should be "pure append" in the sense that they should not
+      // affect any existing targets that have already been searched &
+      // matched.
       //
       // A straightforward way to enforce this is to not allow any existing
       // targets to be inside any newly created scopes (except, perhaps for
@@ -625,6 +626,10 @@ namespace build2
       // not a subdirectory of out_base. So for now we just assume that this
       // is so. And so it is.
 
+      // Relock for exclusive access.
+      //
+      rlock rl (model_lock);
+
       pair<scope&, scope*> sp (switch_scope (*s.root_scope (), out_base));
 
       if (sp.second != nullptr) // Ignore scopes out of any project.
@@ -638,7 +643,7 @@ namespace build2
         {
           l5 ([&]{trace << "loading buildfile " << bf << " for " << pk;});
 
-          if (source_once (bf, root, base, root))
+          if (source_once (root, base, bf, root))
           {
             // If we loaded the buildfile, examine the target again.
             //
diff --git a/build2/target.ixx b/build2/target.ixx
index e2ac111..f3417aa 100644
--- a/build2/target.ixx
+++ b/build2/target.ixx
@@ -23,7 +23,7 @@ namespace build2
   // prerequisite_members
   //
   group_view
-  resolve_group_members (action, target&); // <build2/algorithm>
+  resolve_group_members (slock&, action, target&); // <build2/algorithm>
 
   template <typename T>
   inline auto prerequisite_members_range<T>::iterator::
@@ -78,7 +78,7 @@ namespace build2
     {
       // Otherwise assume it is a normal group.
       //
-      g_ = resolve_group_members (r_->a_, search (*i_));
+      g_ = resolve_group_members (r_->l_, r_->a_, search (*i_));
 
       if (g_.members == nullptr) // Members are not know.
       {
diff --git a/build2/target.txx b/build2/target.txx
index b72af7d..5519126 100644
--- a/build2/target.txx
+++ b/build2/target.txx
@@ -18,7 +18,7 @@ namespace build2
     //
     do
     {
-      g_ = resolve_group_members (r_->a_, search (*i_));
+      g_ = resolve_group_members (r_->l_, r_->a_, search (*i_));
       assert (g_.members != nullptr); // Group could not be resolved.
 
       if (g_.count != 0) // Skip empty see through groups.
@@ -45,7 +45,7 @@ namespace build2
   {
     // Include target type/pattern-specific variables.
     //
-    if (auto l = s.find (var, tk))
+    if (auto l = s.find (var_pool[var], tk))
     {
       // Help the user here and strip leading '.' from the extension.
       //
diff --git a/build2/test/init.cxx b/build2/test/init.cxx
index 94e4073..fd8db99 100644
--- a/build2/test/init.cxx
+++ b/build2/test/init.cxx
@@ -36,7 +36,7 @@ namespace build2
       // Enter module variables. Do it during boot in case they get assigned
       // in bootstrap.build.
       //
-      auto& vp (var_pool);
+      auto& vp (var_pool.rw (rs));
 
       // Tests to execute.
       //
diff --git a/build2/test/rule b/build2/test/rule
index 20ad905..0e208ff 100644
--- a/build2/test/rule
+++ b/build2/test/rule
@@ -21,7 +21,7 @@ namespace build2
     {
     public:
       virtual match_result
-      match (action, target&, const string&) const override;
+      match (slock&, action, target&, const string&) const override;
 
       target_state
       perform_script (action, target&) const;
@@ -31,7 +31,7 @@ namespace build2
     {
     public:
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
 
       static target_state
       perform_test (action, target&);
@@ -41,7 +41,7 @@ namespace build2
     {
     public:
       virtual recipe
-      apply (action, target&) const override;
+      apply (slock&, action, target&) const override;
 
       target_state
       perform_test (action, target&) const;
diff --git a/build2/test/rule.cxx b/build2/test/rule.cxx
index 1f27979..17821a4 100644
--- a/build2/test/rule.cxx
+++ b/build2/test/rule.cxx
@@ -35,7 +35,7 @@ namespace build2
                    "insufficient space");
 
     match_result rule_common::
-    match (action a, target& t, const string&) const
+    match (slock& ml, action a, target& t, const string&) const
     {
       // The (admittedly twisted) logic of this rule tries to achieve the
       // following: If the target is testable, then we want both first update
@@ -63,7 +63,7 @@ namespace build2
         // If we have any prerequisites of the test{} type, then this is the
         // testscript case.
         //
-        for (prerequisite_member p: group_prerequisite_members (a, t))
+        for (prerequisite_member p: group_prerequisite_members (ml, a, t))
         {
           if (p.is_a<testscript> ())
           {
@@ -91,7 +91,7 @@ namespace build2
 
           // Use lookup depths to figure out who "overrides" whom.
           //
-          auto p (t.find ("test"));
+          auto p (t.find (var_pool["test"]));
           const name* n (cast_null<name> (p.first));
 
           // Note that test can be set to an "override" target.
@@ -102,7 +102,7 @@ namespace build2
           {
             auto test = [&t, &p] (const char* var)
             {
-              return t.find (var).second < p.second;
+              return t.find (var_pool[var]).second < p.second;
             };
 
             md.test =
@@ -155,7 +155,7 @@ namespace build2
     }
 
     recipe alias_rule::
-    apply (action a, target& t) const
+    apply (slock& ml, action a, target& t) const
     {
       match_data md (move (t.data<match_data> ()));
       t.clear_data (); // In case delegated-to rule also uses aux storage.
@@ -171,7 +171,7 @@ namespace build2
       // standard alias rule.
       //
       if (a.operation () == update_id)
-        return match_delegate (a, t, *this).first;
+        return match_delegate (ml, a, t, *this).first;
 
       // For the test operation we have to implement our own search and match
       // because we need to ignore prerequisites that are outside of our
@@ -181,7 +181,7 @@ namespace build2
       // not ours seems right. Note that we still want to make sure they are
       // up to date (via the above delegate) since our tests might use them.
       //
-      search_and_match_prerequisites (a, t, t.root_scope ());
+      search_and_match_prerequisites (ml, a, t, t.root_scope ());
 
       // If not a test then also redirect to the alias rule.
       //
@@ -191,7 +191,7 @@ namespace build2
     }
 
     recipe rule::
-    apply (action a, target& t) const
+    apply (slock& ml, action a, target& t) const
     {
       tracer trace ("test::rule::apply");
 
@@ -208,11 +208,11 @@ namespace build2
       if (md.script)
       {
         if (a.operation () == update_id)
-          return match_delegate (a, t, *this).first;
+          return match_delegate (ml, a, t, *this).first;
 
         // Collect all the testscript targets in prerequisite_targets.
         //
-        for (prerequisite_member p: group_prerequisite_members (a, t))
+        for (prerequisite_member p: group_prerequisite_members (ml, a, t))
         {
           if (p.is_a<testscript> ())
             t.prerequisite_targets.push_back (&p.search ());
@@ -229,10 +229,10 @@ namespace build2
         // We should have either arguments or input/roundtrip. Again, use
         // lookup depth to figure out who takes precedence.
         //
-        auto ip (t.find ("test.input"));
-        auto op (t.find ("test.output"));
-        auto rp (t.find ("test.roundtrip"));
-        auto ap (t.find ("test.arguments"));
+        auto ip (t.find (var_pool["test.input"]));
+        auto op (t.find (var_pool["test.output"]));
+        auto rp (t.find (var_pool["test.roundtrip"]));
+        auto ap (t.find (var_pool["test.arguments"]));
 
         auto test = [&t] (pair<lookup, size_t>& x, const char* xn,
                           pair<lookup, size_t>& y, const char* yn)
@@ -286,7 +286,7 @@ namespace build2
           //
           if (it != nullptr)
           {
-            build2::match (a, *it);
+            build2::match (ml, a, *it);
 
             if (it->state () == target_state::unchanged)
             {
@@ -299,7 +299,7 @@ namespace build2
           {
             if (in != on)
             {
-              build2::match (a, *ot);
+              build2::match (ml, a, *ot);
 
               if (ot->state () == target_state::unchanged)
               {
@@ -315,7 +315,7 @@ namespace build2
           // been found if we signalled that we do not match from match()
           // above.
           //
-          recipe d (match_delegate (a, t, *this).first);
+          recipe d (match_delegate (ml, a, t, *this).first);
 
           // If we have no input/output that needs updating, then simply
           // redirect to it.
diff --git a/build2/types b/build2/types
index f8abef2..1e7a347 100644
--- a/build2/types
+++ b/build2/types
@@ -9,7 +9,6 @@
 #include <tuple>
 #include <vector>
 #include <string>
-#include <future>
 #include <memory>           // unique_ptr, shared_ptr
 #include <utility>          // pair, move()
 #include <cstddef>          // size_t, nullptr_t
@@ -19,6 +18,11 @@
 #include <functional>       // function, reference_wrapper
 #include <initializer_list>
 
+#include <mutex>
+#include <future>
+#include <butl/ft/shared_mutex>
+#include <shared_mutex>
+
 #include <ios>           // ios_base::failure
 #include <exception>     // exception
 #include <stdexcept>     // logic_error, invalid_argument, runtime_error
@@ -69,10 +73,56 @@ namespace build2
   using std::istream;
   using std::ostream;
 
+  // Concurrency.
+  //
   using std::future;
 
-  // Exceptions. While <exception> is included, there is no using for
-  // std::exception -- use qualified.
+#ifdef __cpp_lib_shared_mutex
+  using shared_mutex = std::shared_mutex;
+#else
+  using shared_mutex = std::shared_timed_mutex;
+#endif
+
+  using slock = std::shared_lock<shared_mutex>;
+  using ulock = std::unique_lock<shared_mutex>;
+
+  // Re-lock shared to exclusive for the lifetime or rlock.
+  //
+  struct rlock
+  {
+    explicit
+    rlock (slock* sl)
+        : sl_ (sl)
+    {
+      if (sl_ != nullptr)
+      {
+        sl_->unlock ();
+        ul_ = ulock (*sl_->mutex ());
+      }
+    }
+
+    ~rlock ()
+    {
+      if (sl_ != nullptr)
+      {
+        ul_.unlock ();
+        sl_->lock ();
+      }
+    }
+
+    // Can be treated as const ulock.
+    //
+    operator const ulock& () const {return ul_;}
+
+  private:
+    slock* sl_;
+    ulock ul_;
+  };
+
+  // Exceptions.
+  //
+  // While <exception> is included, there is no using for std::exception --
+  // use qualified.
   //
   using std::logic_error;
   using std::invalid_argument;
diff --git a/build2/variable b/build2/variable
index 2206fe0..4377d7b 100644
--- a/build2/variable
+++ b/build2/variable
@@ -755,36 +755,32 @@ namespace build2
     static const value_type_ex value_type;
   };
 
-  // variable_pool
+  // Variable pool.
+  //
+  // Protected by the model mutex.
   //
   class variable_pool
   {
   public:
-    // Find existing or insert new. Find bias.
+    // Find existing or insert new.
     //
     const variable&
-    operator[] (const string& name);
-
-    // Find existing or insert new. Insert bias.
-    //
-    const variable&
-    insert (string name);
+    operator[] (const string& name) const;
 
     // Return NULL if there is no variable with this name.
     //
     const variable*
-    find (const string& name);
+    find (const string& name) const;
 
-    // Insert or override.
+    // Find existing or insert new (untyped, non-overridable, normal
+    // visibility).
     //
-    template <typename T>
     const variable&
-    insert (string name)
-    {
-      return insert (
-        move (name), &value_traits<T>::value_type, nullptr, nullptr);
-    }
+    insert (string name);
 
+    // Insert or override (type/visibility). Note that by default the
+    // variable is not overridable.
+    //
     const variable&
     insert (string name, variable_visibility v)
     {
@@ -805,9 +801,16 @@ namespace build2
 
     template <typename T>
     const variable&
+    insert (string name)
+    {
+      return insert (move (name), &value_traits<T>::value_type);
+    }
+
+    template <typename T>
+    const variable&
     insert (string name, variable_visibility v)
     {
-      return insert (move (name), &value_traits<T>::value_type, &v, nullptr);
+      return insert (move (name), &value_traits<T>::value_type, &v);
     }
 
     template <typename T>
@@ -829,12 +832,29 @@ namespace build2
     void
     clear () {map_.clear ();}
 
+    // Proof of lock for RW access.
+    //
+    variable_pool&
+    rw (const ulock&) const {return const_cast<variable_pool&> (*this);}
+
+    variable_pool&
+    rw (scope&) const {return const_cast<variable_pool&> (*this);}
+
   private:
+    // Classes that can access bypassing the lock.
+    //
+    friend class scope;
+
+    static variable_pool instance;
+
     const variable&
     insert (string name,
             const build2::value_type*,
-            const variable_visibility*,
-            const bool* overridable);
+            const variable_visibility* = nullptr,
+            const bool* overridable = nullptr);
+
+  public:
+    static const variable_pool& cinstance; // For var_pool initialization.
 
   private:
     using key = butl::map_key<string>;
@@ -860,7 +880,7 @@ namespace build2
     map map_;
   };
 
-  extern variable_pool var_pool;
+  extern const variable_pool& var_pool;
 }
 
 // variable_map
@@ -929,9 +949,17 @@ namespace build2
     }
 
     lookup
+    operator[] (const variable* var) const // For cached variables.
+    {
+      assert (var != nullptr);
+      return operator[] (*var);
+    }
+
+    lookup
     operator[] (const string& name) const
     {
-      return operator[] (var_pool[name]);
+      const variable* var (var_pool.find (name));
+      return var != nullptr ? operator[] (*var) : lookup ();
     }
 
     // If typed is false, leave the value untyped even if the variable is.
@@ -947,18 +975,10 @@ namespace build2
     value&
     assign (const variable& var) {return insert (var).first;}
 
-    value&
-    assign (const string& name) {return insert (name).first;}
-
-    // Unlike the two above, assign a typed, non-overridable variable with
-    // normal visibility.
+    // Note that the variable is expected to have already been registered.
     //
-    template <typename T>
     value&
-    assign (string name)
-    {
-      return assign (var_pool.insert<T> (move (name)));
-    }
+    assign (const string& name) {return insert (var_pool[name]).first;}
 
     // As above but also return an indication of whether the new value (which
     // will be NULL) was actually inserted. Similar to find(), if typed is
@@ -967,16 +987,10 @@ namespace build2
     pair<reference_wrapper<value>, bool>
     insert (const variable&, bool typed = true);
 
-    pair<reference_wrapper<value>, bool>
-    insert (const string& name, bool typed = true)
-    {
-      return insert (var_pool[name], typed);
-    }
-
     pair<const_iterator, const_iterator>
-    find_namespace (const string& ns) const
+    find_namespace (const variable& ns) const
     {
-      auto r (m_.find_prefix (var_pool[ns]));
+      auto r (m_.find_prefix (ns));
       return make_pair (const_iterator (r.first), const_iterator (r.second));
     }
 
diff --git a/build2/variable.cxx b/build2/variable.cxx
index 48ea8aa..ec72fdc 100644
--- a/build2/variable.cxx
+++ b/build2/variable.cxx
@@ -906,6 +906,23 @@ namespace build2
   // variable_pool
   //
   const variable& variable_pool::
+  insert (string n)
+  {
+    // We are not overriding anything so skip the insert_() checks.
+    //
+    auto p (
+      insert (
+        variable {move (n), nullptr, nullptr, variable_visibility::normal}));
+
+    const variable& r (p.first->second);
+
+    if (r.override != nullptr)
+      fail << "variable " << r.name << " cannot be overridden";
+
+    return r;
+  }
+
+  const variable& variable_pool::
   insert (string n,
           const build2::value_type* t,
           const variable_visibility* v,
@@ -943,16 +960,18 @@ namespace build2
       }
 
       // Check overridability (all overrides, if any, should already have
-      // been enetered (see context.cxx:reset()).
+      // been entered (see context.cxx:reset()).
       //
-      if (o != nullptr && r.override != nullptr && !*o)
+      if (r.override != nullptr && (o == nullptr || !*o))
         fail << "variable " << r.name << " cannot be overridden";
     }
 
     return r;
   }
 
-  variable_pool var_pool;
+  variable_pool variable_pool::instance;
+  const variable_pool& variable_pool::cinstance = variable_pool::instance;
+  const variable_pool& var_pool = variable_pool::cinstance;
 
   // variable_map
   //
diff --git a/build2/variable.ixx b/build2/variable.ixx
index bb97750..816f4bf 100644
--- a/build2/variable.ixx
+++ b/build2/variable.ixx
@@ -622,33 +622,19 @@ namespace build2
 
   // variable_pool
   //
-  inline const variable* variable_pool::
-  find (const string& n)
-  {
-    auto i (map_.find (&n));
-    return i != map_.end () ? &i->second : nullptr;
-  }
-
   inline const variable& variable_pool::
-  insert (string n)
+  operator[] (const string& n) const
   {
-    // We are not overriding anything so skip the custom insert() checks.
-    //
-    auto p (
-      insert (
-        variable {move (n), nullptr, nullptr, variable_visibility::normal}));
-
-    return p.first->second;
+    const variable* r (find (n));
+    assert (r != nullptr);
+    return *r;
   }
 
-
-  inline const variable& variable_pool::
-  operator[] (const string& n)
+  inline const variable* variable_pool::
+  find (const string& n) const
   {
-    if (const variable* v = find (n))
-      return *v;
-    else
-      return insert (n);
+    auto i (map_.find (&n));
+    return i != map_.end () ? &i->second : nullptr;
   }
 
   // variable_map::iterator_adapter
diff --git a/unit-tests/function/buildfile b/unit-tests/function/buildfile
index 5c326b8..27c73d1 100644
--- a/unit-tests/function/buildfile
+++ b/unit-tests/function/buildfile
@@ -4,6 +4,8 @@
 
 #@@ Temporary until we get utility library support.
 #
+if ($cxx.target.class != "windows")
+  cxx.libs += -lpthread
 import libs = libbutl%lib{butl}
 src = token lexer diagnostics utility variable name b-options types-parsers \
 context scope parser target operation rule prerequisite file module function \
@@ -11,6 +13,7 @@ functions-builtin functions-path functions-process-path functions-string \
 functions-target-triplet algorithm search dump filesystem \
 config/{utility init operation}
 
+
 exe{driver}: cxx{driver} ../../build2/cxx{$src} $libs test{call syntax}
 
 include ../../build2/
diff --git a/unit-tests/function/driver.cxx b/unit-tests/function/driver.cxx
index 2f605b0..4fbbdb6 100644
--- a/unit-tests/function/driver.cxx
+++ b/unit-tests/function/driver.cxx
@@ -26,7 +26,9 @@ namespace build2
   main (int, char* argv[])
   {
     init (argv[0], 1);  // Fake build system driver, default verbosity.
-    reset (strings ()); // No command line variables.
+
+    ulock ml (model);
+    reset (ml, strings ()); // No command line variables.
 
     function_family f ("dummy");
 
diff --git a/unit-tests/test/script/parser/driver.cxx b/unit-tests/test/script/parser/driver.cxx
index f641dc4..1e959c1 100644
--- a/unit-tests/test/script/parser/driver.cxx
+++ b/unit-tests/test/script/parser/driver.cxx
@@ -142,9 +142,10 @@ namespace build2
       {
         tracer trace ("main");
 
-        init (argv[0], 1);  // Fake build system driver, default verbosity.
-        sched.startup (1);  // Serial execution.
-        reset (strings ()); // No command line variables.
+        init (argv[0], 1);      // Fake build system driver, default verbosity.
+        ulock ml (model);
+        sched.startup (1);      // Serial execution.
+        reset (ml, strings ()); // No command line variables.
 
         bool scope (false);
         bool id (false);
@@ -191,7 +192,7 @@ namespace build2
 
           value& v (
             tt.assign (
-              var_pool.insert<target_triplet> (
+              var_pool.rw (ml).insert<target_triplet> (
                 "test.target", variable_visibility::project)));
 
           v = cast<target_triplet> ((*global_scope)["build.host"]);