Fix unexpected 'unable to satisfy dependency' error in pkg_configure() by turning collect_order_dependent() into collect_dependent()

19 files changed, 276 insertions, 166 deletions

diff --git a/bpkg/pkg-build-collect.cxx b/bpkg/pkg-build-collect.cxx
index d251a77..c0a592f 100644
--- a/bpkg/pkg-build-collect.cxx
+++ b/bpkg/pkg-build-collect.cxx
@@ -1675,9 +1675,9 @@ namespace bpkg
                               << bp.available_name_version_db () << " ("
                               << c->value << ')';});
 
-                // Note that in contrast to collect_order_dependents(), here
-                // we also save both unsatisfied constraints and the
-                // dependency chain, for the sake of the diagnostics.
+                // Note that in contrast to collect_dependents(), here we also
+                // save both unsatisfied constraints and the dependency chain,
+                // for the sake of the diagnostics.
                 //
                 vector<unsatisfied_constraint> ucs {
                   unsatisfied_constraint {
@@ -5806,7 +5806,7 @@ namespace bpkg
               // then it doesn't need to be reconfigured either since nothing
               // changes for its config clauses. Otherwise, the
               // build_package::adjust_reconfigure flag will be added normally
-              // by collect_order_dependents().
+              // by collect_dependents().
               //
               collect_existing_dependent (o,
                                           ed,
@@ -7091,50 +7091,49 @@ namespace bpkg
     return order (db, name, chain, fdb, reorder);
   }
 
-  void build_packages::
-  collect_order_dependents (const repointed_dependents& rpt_depts,
-                            unsatisfied_dependents& unsatisfied_depts)
+  set<package_key> build_packages::
+  collect_dependents (const repointed_dependents& rpt_depts,
+                      unsatisfied_dependents& unsatisfied_depts)
   {
+    set<package_key> r;
+
+    // First, cache the packages in the map since we will be adding new
+    // entries to the map while collecting dependents of the initial package
+    // set, recursively.
+    //
     // Note: the pointer is stable (points to a value in std::map).
     //
-    set<const build_package*> visited_deps;
+    vector<build_package*> deps;
 
-    // For each package on the list we want to insert all its dependents
-    // before it so that they get configured after the package on which they
-    // depend is configured (remember, our build order is reverse, with the
-    // last package being built first). This applies to both packages that are
-    // already on the list as well as the ones that we add, recursively.
-    //
-    for (auto i (begin ()); i != end (); ++i)
+    for (auto& p: map_)
     {
-      const build_package& p (*i);
+      build_package& d (p.second.package);
 
       // Prune if this is not a configured package being up/down-graded
       // or reconfigured.
       //
-      assert (p.action);
-
-      // Dropped package may have no dependents.
-      //
-      if (*p.action != build_package::drop && p.reconfigure ())
-        collect_order_dependents (i,
-                                  rpt_depts,
-                                  unsatisfied_depts,
-                                  visited_deps);
+      if (d.action && *d.action != build_package::drop && d.reconfigure ())
+        deps.push_back (&d);
     }
+
+    // Note: the pointer is stable (see above for details).
+    //
+    set<const build_package*> visited_deps;
+
+    for (build_package* p: deps)
+      collect_dependents (*p, rpt_depts, unsatisfied_depts, visited_deps, r);
+
+    return r;
   }
 
   void build_packages::
-  collect_order_dependents (iterator pos,
-                            const repointed_dependents& rpt_depts,
-                            unsatisfied_dependents& unsatisfied_depts,
-                            set<const build_package*>& visited_deps)
+  collect_dependents (build_package& p,
+                      const repointed_dependents& rpt_depts,
+                      unsatisfied_dependents& unsatisfied_depts,
+                      set<const build_package*>& visited_deps,
+                      set<package_key>& r)
   {
-    tracer trace ("collect_order_dependents");
-
-    assert (pos != end ());
-
-    build_package& p (*pos);
+    tracer trace ("collect_dependents");
 
     // Bail out if the dependency has already been visited and add it to the
     // visited set otherwise.
@@ -7167,14 +7166,8 @@ namespace bpkg
         // dependent. But first "prune" if the dependent is being dropped or
         // this is a replaced prerequisite of the repointed dependent.
         //
-        // Note that the package drops are not ordered at this stage, since to
-        // order them properly all the package reconfigurations must be
-        // determined.
-        //
-        // Also note that the repointed dependents are always collected and
-        // have all their collected prerequisites ordered (including new and
-        // old ones). See collect_build_prerequisites() and order() for
-        // details.
+        // Note that the repointed dependents are always collected (see
+        // collect_build_prerequisites() for details).
         //
         bool check (ud != 0 && dc);
 
@@ -7187,30 +7180,27 @@ namespace bpkg
           if (dp.action && *dp.action == build_package::drop)
             continue;
 
-          if (i->second.position != end ())
-          {
-            repointed_dependents::const_iterator j (
-              rpt_depts.find (package_key {ddb, dn}));
+          repointed_dependents::const_iterator j (
+            rpt_depts.find (package_key {ddb, dn}));
 
-            if (j != rpt_depts.end ())
-            {
-              const map<package_key, bool>& prereqs_flags (j->second);
+          if (j != rpt_depts.end ())
+          {
+            const map<package_key, bool>& prereqs_flags (j->second);
 
-              auto k (prereqs_flags.find (package_key {pdb, n}));
+            auto k (prereqs_flags.find (package_key {pdb, n}));
 
-              if (k != prereqs_flags.end () && !k->second)
-                continue;
-            }
-
-            // There is one tricky aspect: the dependent could be in the
-            // process of being reconfigured or up/downgraded as well. In this
-            // case all we need to do is detect this situation and skip the
-            // test since all the (new) constraints of this package have been
-            // satisfied in collect_build().
-            //
-            if (check)
-              check = !dp.dependencies;
+            if (k != prereqs_flags.end () && !k->second)
+              continue;
           }
+
+          // There is one tricky aspect: the dependent could be in the process
+          // of being reconfigured or up/downgraded as well. In this case all
+          // we need to do is detect this situation and skip the test since
+          // all the (new) constraints of this package have been satisfied in
+          // collect_build().
+          //
+          if (check)
+            check = !dp.dependencies;
         }
 
         if (check)
@@ -7272,57 +7262,32 @@ namespace bpkg
             build_package::adjust_reconfigure};
         };
 
-        // We can have three cases here: the package is already on the list,
-        // the package is in the map (but not on the list) and it is in
-        // neither.
-        //
-        // If the existing entry is pre-entered, is an adjustment, or is a
-        // build that is not supposed to be built (not in the list), then we
-        // merge it into the new adjustment entry. Otherwise (is a build in
-        // the list), we just add the reconfigure adjustment flag to it.
+        // If the existing entry is pre-entered or is an adjustment, then we
+        // merge it into the new adjustment entry. Otherwise (is a build), we
+        // just add the reconfigure adjustment flag to it, unless it is
+        // already being reconfigured. In the later case we don't add the
+        // dependent to the resulting set since we neither add a new entry to
+        // the map nor modify an existing one.
         //
+        bool add (true);
         if (i != map_.end ())
         {
           build_package& dp (i->second.package);
-          iterator& dpos (i->second.position);
 
-          if (!dp.action                         || // Pre-entered.
-              *dp.action != build_package::build || // Non-build.
-              dpos == end ())                       // Build not in the list.
+          if (!dp.action ||                       // Pre-entered.
+              *dp.action != build_package::build) // Adjustment.
           {
             build_package bp (adjustment ());
             bp.merge (move (dp));
             dp = move (bp);
           }
-          else                                       // Build in the list.
-            dp.flags |= build_package::adjust_reconfigure;
-
-          // It may happen that the dependent is already in the list but is
-          // not properly ordered against its dependencies that get into the
-          // list via another dependency path. Thus, we check if the dependent
-          // is to the right of its dependency and, if that's the case,
-          // reinsert it in front of the dependency.
-          //
-          if (dpos != end ())
+          else                                    // Build.
           {
-            for (auto i (pos); i != end (); ++i)
-            {
-              if (i == dpos)
-              {
-                erase (dpos);
-                dpos = insert (pos, dp);
-
-                // Remove the moved dependent from the visited dependencies
-                // set, if present, so its own dependents can be reordered to
-                // the left of this dependent.
-                //
-                visited_deps.erase (&dp);
-                break;
-              }
-            }
+            if (!dp.reconfigure ())
+              dp.flags |= build_package::adjust_reconfigure;
+            else
+              add = false;
           }
-          else
-            dpos = insert (pos, dp);
         }
         else
         {
@@ -7330,10 +7295,13 @@ namespace bpkg
           //
           i = map_.emplace (package_key {ddb, dn},
                             data_type {end (), adjustment ()}).first;
-
-          i->second.position = insert (pos, i->second.package);
         }
 
+        if (add)
+          r.insert (i->first);
+
+        build_package& dp (i->second.package);
+
         // Add this dependent's constraint, if present, to the dependency's
         // constraints list for completeness, while suppressing duplicates.
         //
@@ -7344,7 +7312,7 @@ namespace bpkg
           constraint_type c (move (*dc),
                              ddb,
                              move (dn),
-                             i->second.package.selected->version,
+                             dp.selected->version,
                              true /* selected_dependent */);
 
           if (find_if (p.constraints.begin (), p.constraints.end (),
@@ -7358,16 +7326,13 @@ namespace bpkg
           }
         }
 
-        // Recursively collect our own dependents inserting them before us.
+        // Recursively collect our own dependents.
         //
         // Note that we cannot end up with an infinite recursion for
-        // configured packages due to a dependency cycle (see order() for
-        // details).
+        // configured packages due to a dependency cycle since we "prune" for
+        // visited dependencies (also see order() for details).
         //
-        collect_order_dependents (i->second.position,
-                                  rpt_depts,
-                                  unsatisfied_depts,
-                                  visited_deps);
+        collect_dependents (dp, rpt_depts, unsatisfied_depts, visited_deps, r);
       }
     }
   }
diff --git a/bpkg/pkg-build-collect.hxx b/bpkg/pkg-build-collect.hxx
index 74a48a0..86878f0 100644
--- a/bpkg/pkg-build-collect.hxx
+++ b/bpkg/pkg-build-collect.hxx
@@ -1571,6 +1571,27 @@ namespace bpkg
                              const function<add_priv_cfg_function>&,
                              postponed_configuration* = nullptr);
 
+    // If a configured package is being up/down-graded or reconfigured then
+    // that means all its configured dependents could be affected and we have
+    // to reconfigure them. This function examines every such a package that
+    // is already in the map and collects all its configured dependents. We
+    // also need to make sure the dependents are ok with the up/downgrade. If
+    // some dependency constraints are not satisfied, then cache them and
+    // proceed further as if no problematic constraints are imposed (see
+    // unsatisfied_dependents for details). Return the set of the collected
+    // dependents.
+    //
+    // Should we reconfigure just the direct depends or also include indirect,
+    // recursively? Consider this plausible scenario as an example: We are
+    // upgrading a package to a version that provides an additional API. When
+    // its direct dependent gets reconfigured, it notices this new API and
+    // exposes its own extra functionality that is based on it. Now it would
+    // make sense to let its own dependents (which would be our original
+    // package's indirect ones) to also notice this.
+    //
+    std::set<package_key>
+    collect_dependents (const repointed_dependents&, unsatisfied_dependents&);
+
     // Order the previously-collected package with the specified name and
     // configuration returning its position.
     //
@@ -1584,27 +1605,6 @@ namespace bpkg
            const function<find_database_function>&,
            bool reorder = true);
 
-    // If a configured package is being up/down-graded then that means all its
-    // dependents could be affected and we have to reconfigure them. This
-    // function examines every package that is already on the list and
-    // collects and orders all its dependents. We also need to make sure the
-    // dependents are ok with the up/downgrade. If some dependency constraints
-    // are not satisfied, then cache them and proceed further as if no
-    // problematic constraints are imposed (see unsatisfied_dependents for
-    // details).
-    //
-    // Should we reconfigure just the direct depends or also include indirect,
-    // recursively? Consider this plauisible scenario as an example: We are
-    // upgrading a package to a version that provides an additional API. When
-    // its direct dependent gets reconfigured, it notices this new API and
-    // exposes its own extra functionality that is based on it. Now it would
-    // make sense to let its own dependents (which would be our original
-    // package's indirect ones) to also notice this.
-    //
-    void
-    collect_order_dependents (const repointed_dependents&,
-                              unsatisfied_dependents&);
-
     void
     clear ();
 
@@ -1744,6 +1744,20 @@ namespace bpkg
                                   unsatisfied_dependents&,
                                   bool add_required_by);
 
+    // Skip the dependents collection for the specified dependency if that has
+    // already been done.
+    //
+    // Note that if this function has already been called for this dependency,
+    // then all its dependents are already in the map and their dependency
+    // constraints have been checked.
+    //
+    void
+    collect_dependents (build_package&,
+                        const repointed_dependents&,
+                        unsatisfied_dependents&,
+                        std::set<const build_package*>& visited_deps,
+                        std::set<package_key>& result);
+
     struct package_ref
     {
       database& db;
@@ -1761,26 +1775,6 @@ namespace bpkg
            const function<find_database_function>&,
            bool reorder);
 
-    // Skip the dependents collection/ordering for the specified dependency if
-    // that has already been done.
-    //
-    // Note that if this function has already been called for this dependency,
-    // then all its dependents are already in the map and the dependency
-    // constraints have been checked for them. Also they are in the list and
-    // are ordered to the left of this dependency, unless this dependency has
-    // been moved to the left itself since the previous visit. Such a move can
-    // only happen if this dependency is a dependent of some other dependency
-    // whose dependents have been collected/ordered since that previous visit.
-    // This function tracks such moves and just removes the moved dependencies
-    // from the visited set, so their dependents can be properly reordered
-    // after the move.
-    //
-    void
-    collect_order_dependents (iterator,
-                              const repointed_dependents&,
-                              unsatisfied_dependents&,
-                              std::set<const build_package*>& visited_deps);
-
   private:
     struct data_type
     {
diff --git a/bpkg/pkg-build.cxx b/bpkg/pkg-build.cxx
index ede26bf..5844397 100644
--- a/bpkg/pkg-build.cxx
+++ b/bpkg/pkg-build.cxx
@@ -4543,6 +4543,9 @@ namespace bpkg
           continue;
         }
 
+        set<package_key> depts (
+          pkgs.collect_dependents (rpt_depts, unsatisfied_depts));
+
         // Now that we have collected all the package versions that we need to
         // build, arrange them in the "dependency order", that is, with every
         // package on the list only possibly depending on the ones after
@@ -4554,9 +4557,10 @@ namespace bpkg
         //
         // The order of dependency upgrades/downgrades/drops is not really
         // deterministic. We, however, do upgrades/downgrades before hold_pkgs
-        // so that they appear (e.g., on the plan) last. We handle drops
-        // later, though, after collecting/ordering dependents when all the
-        // package reconfigurations are determined.
+        // so that they appear (e.g., on the plan) after the packages being
+        // built to hold. We handle drops last, though, so that the unused
+        // packages are likely get purged before the package fetches, so that
+        // the disk space they occupy can be reused.
         //
         for (const dep& d: deps)
         {
@@ -4576,6 +4580,9 @@ namespace bpkg
                       find_prereq_database,
                       false /* reorder */);
 
+        // Order the existing dependents which have participated in
+        // negotiation of the configuration of their dependencies.
+        //
         for (const postponed_configuration& cfg: postponed_cfgs)
         {
           for (const auto& d: cfg.dependents)
@@ -4588,6 +4595,14 @@ namespace bpkg
           }
         }
 
+        // Order the existing dependents whose dependencies are being
+        // up/down-graded or reconfigured.
+        //
+        for (const package_key& p: depts)
+          pkgs.order (p.db, p.name, find_prereq_database, false /* reorder */);
+
+        // Order the re-collected packages (deviated dependents, etc).
+        //
         for (build_package* p: postponed_recs)
         {
           assert (p->recursive_collection);
@@ -4595,12 +4610,6 @@ namespace bpkg
           pkgs.order (p->db, p->name (), find_prereq_database);
         }
 
-        // Collect and order all the dependents that we will need to
-        // reconfigure because of the up/down-grades of packages that are now
-        // on the list.
-        //
-        pkgs.collect_order_dependents (rpt_depts, unsatisfied_depts);
-
         // Make sure all the packages that we need to unhold are on the list.
         //
         for (const dependency_package& p: dep_pkgs)
@@ -6463,7 +6472,7 @@ namespace bpkg
 
         // In the simulation mode unconstrain all the unsatisfactory
         // dependencies, if any, while configuring the dependent (see
-        // build_packages::collect_order_dependents() for details).
+        // build_packages::collect_dependents() for details).
         //
         // Note: must be called at most once.
         //
diff --git a/tests/common/satisfy/libbar-2.1.0.tar.gz b/tests/common/satisfy/libbar-2.1.0.tar.gz
new file mode 100644
index 0000000..0ff50c5
--- /dev/null
+++ b/tests/common/satisfy/libbar-2.1.0.tar.gz
diff --git a/tests/common/satisfy/libbax-1.0.0.tar.gz b/tests/common/satisfy/libbax-1.0.0.tar.gz
new file mode 100644
index 0000000..7f4dfd0
--- /dev/null
+++ b/tests/common/satisfy/libbax-1.0.0.tar.gz
diff --git a/tests/common/satisfy/libbax-2.0.0.tar.gz b/tests/common/satisfy/libbax-2.0.0.tar.gz
new file mode 100644
index 0000000..f130da0
--- /dev/null
+++ b/tests/common/satisfy/libbax-2.0.0.tar.gz
diff --git a/tests/common/satisfy/libbox-1.0.0.tar.gz b/tests/common/satisfy/libbox-1.0.0.tar.gz
new file mode 100644
index 0000000..90227f7
--- /dev/null
+++ b/tests/common/satisfy/libbox-1.0.0.tar.gz
diff --git a/tests/common/satisfy/libbox-2.0.0.tar.gz b/tests/common/satisfy/libbox-2.0.0.tar.gz
new file mode 100644
index 0000000..15d457f
--- /dev/null
+++ b/tests/common/satisfy/libbox-2.0.0.tar.gz
diff --git a/tests/common/satisfy/libfoo-2.0.0.tar.gz b/tests/common/satisfy/libfoo-2.0.0.tar.gz
new file mode 100644
index 0000000..fd8eeb3
--- /dev/null
+++ b/tests/common/satisfy/libfoo-2.0.0.tar.gz
diff --git a/tests/common/satisfy/t4f/libbar-1.2.0.tar.gz b/tests/common/satisfy/t4f/libbar-1.2.0.tar.gz
new file mode 120000
index 0000000..b4a7773
--- /dev/null
+++ b/tests/common/satisfy/t4f/libbar-1.2.0.tar.gz
@@ -0,0 +1 @@
+../libbar-1.2.0.tar.gz
\ No newline at end of file
diff --git a/tests/common/satisfy/t4f/libbar-2.1.0.tar.gz b/tests/common/satisfy/t4f/libbar-2.1.0.tar.gz
new file mode 120000
index 0000000..0df079a
--- /dev/null
+++ b/tests/common/satisfy/t4f/libbar-2.1.0.tar.gz
@@ -0,0 +1 @@
+../libbar-2.1.0.tar.gz
\ No newline at end of file
diff --git a/tests/common/satisfy/t4f/libbax-1.0.0.tar.gz b/tests/common/satisfy/t4f/libbax-1.0.0.tar.gz
new file mode 120000
index 0000000..137b938
--- /dev/null
+++ b/tests/common/satisfy/t4f/libbax-1.0.0.tar.gz
@@ -0,0 +1 @@
+../libbax-1.0.0.tar.gz
\ No newline at end of file
diff --git a/tests/common/satisfy/t4f/libbax-2.0.0.tar.gz b/tests/common/satisfy/t4f/libbax-2.0.0.tar.gz
new file mode 120000
index 0000000..465832f
--- /dev/null
+++ b/tests/common/satisfy/t4f/libbax-2.0.0.tar.gz
@@ -0,0 +1 @@
+../libbax-2.0.0.tar.gz
\ No newline at end of file
diff --git a/tests/common/satisfy/t4f/libbox-1.0.0.tar.gz b/tests/common/satisfy/t4f/libbox-1.0.0.tar.gz
new file mode 120000
index 0000000..3bc2a0c
--- /dev/null
+++ b/tests/common/satisfy/t4f/libbox-1.0.0.tar.gz
@@ -0,0 +1 @@
+../libbox-1.0.0.tar.gz
\ No newline at end of file
diff --git a/tests/common/satisfy/t4f/libbox-2.0.0.tar.gz b/tests/common/satisfy/t4f/libbox-2.0.0.tar.gz
new file mode 120000
index 0000000..1ec50e6
--- /dev/null
+++ b/tests/common/satisfy/t4f/libbox-2.0.0.tar.gz
@@ -0,0 +1 @@
+../libbox-2.0.0.tar.gz
\ No newline at end of file
diff --git a/tests/common/satisfy/t4f/libfoo-2.0.0.tar.gz b/tests/common/satisfy/t4f/libfoo-2.0.0.tar.gz
new file mode 120000
index 0000000..406696d
--- /dev/null
+++ b/tests/common/satisfy/t4f/libfoo-2.0.0.tar.gz
@@ -0,0 +1 @@
+../libfoo-2.0.0.tar.gz
\ No newline at end of file
diff --git a/tests/common/satisfy/t4f/repositories.manifest b/tests/common/satisfy/t4f/repositories.manifest
new file mode 100644
index 0000000..4cc7c0a
--- /dev/null
+++ b/tests/common/satisfy/t4f/repositories.manifest
@@ -0,0 +1,4 @@
+: 1
+:
+location: ../t4a
+role: complement
diff --git a/tests/pkg-build.testscript b/tests/pkg-build.testscript
index 3cb3f03..4766e2a 100644
--- a/tests/pkg-build.testscript
+++ b/tests/pkg-build.testscript
@@ -106,6 +106,16 @@
 # |   |-- libfoo-1.1.0+1.tar.gz
 # |   `-- repositories.manifest
 # |
+# |-- t4f
+# |   |-- libfoo-2.0.0.tar.gz   -> libbar == 1.2.0
+# |   |-- libbar-1.2.0.tar.gz
+# |   |-- libbar-2.1.0.tar.gz   -> libbox
+# |   |-- libbox-1.0.0.tar.gz   -> libbax
+# |   |-- libbox-2.0.0.tar.gz   -> libbax == 1.0.0
+# |   |-- libbax-1.0.0.tar.gz
+# |   |-- libbax-2.0.0.tar.gz
+# |   `-- repositories.manifest
+# |
 # |-- t5
 # |   |-- libbar-1.2.0.tar.gz
 # |   |-- libbox-1.2.0.tar.gz
@@ -486,6 +496,7 @@ posix = ($cxx.target.class != 'windows')
   cp -r $src/t4c  $out/t4c  && $rep_create $out/t4c  &$out/t4c/packages.manifest
   cp -r $src/t4d  $out/t4d  && $rep_create $out/t4d  &$out/t4d/packages.manifest
   cp -r $src/t4e  $out/t4e  && $rep_create $out/t4e  &$out/t4e/packages.manifest
+  cp -r $src/t4f  $out/t4f  && $rep_create $out/t4f  &$out/t4f/packages.manifest
   cp -r $src/t5   $out/t5   && $rep_create $out/t5   &$out/t5/packages.manifest
   cp -r $src/t6   $out/t6   && $rep_create $out/t6   &$out/t6/packages.manifest
   cp -r $src/t7a  $out/t7a  && $rep_create $out/t7a  &$out/t7a/packages.manifest
@@ -3849,10 +3860,9 @@ test.arguments += --sys-no-query
 
     : adjust-merge-build
     :
-    : Test that the registered in the map but not ordered package build
-    : (libfoo) is properly merged into the reconfigure adjustment as a
-    : dependent of the reconfigured dependency (see collect_order_dependents()
-    : for more details).
+    : Test that the registered in the map package build (libfoo) is properly
+    : merged into the reconfigure adjustment as a dependent of the
+    : reconfigured dependency (see collect_dependents() for more details).
     :
     {
       $clone_root_cfg;
@@ -3886,6 +3896,113 @@ test.arguments += --sys-no-query
 
       $pkg_drop libbaz libbar libfoo
     }
+
+    : reconfiguration
+    :
+    {
+      +$clone_root_cfg
+      +$rep_add $rep/t4f && $rep_fetch
+
+      : re-order
+      :
+      : This test reproduced a failure of the collect_order_dependents()
+      : function (now turned into collect_dependents()) to properly order
+      : dependents of a being upgraded package, if the current version of this
+      : package is a dependent of a being reconfigured dependency. The now
+      : fixed bug ended up with the 'unable to satisfy dependency' failure of
+      : the subsequent pkg_configure() function call for the described case.
+      :
+      {
+        $clone_cfg;
+
+        $* libbar libbox/1.0.0 ?libbax/1.0.0 2>!;
+
+        $pkg_status -ar >>EOO;
+          libbax configured !1.0.0 available 2.0.0
+          !libbox configured !1.0.0 available 2.0.0
+            libbax configured !1.0.0 available 2.0.0
+          !libbar configured 2.1.0
+            !libbox configured !1.0.0 available 2.0.0
+              libbax configured !1.0.0 available 2.0.0
+          EOO
+
+        $* libfoo libbax 2>>EOE;
+          warning: package libfoo dependency on (libbar == 1.2.0) is forcing downgrade of libbar/2.1.0 to 1.2.0
+          disfigured libbar/2.1.0
+          disfigured libbox/1.0.0
+          disfigured libbax/1.0.0
+          fetched libbax/2.0.0
+          unpacked libbax/2.0.0
+          fetched libbar/1.2.0
+          unpacked libbar/1.2.0
+          fetched libfoo/2.0.0
+          unpacked libfoo/2.0.0
+          configured libbax/2.0.0
+          configured libbox/1.0.0
+          configured libbar/1.2.0
+          configured libfoo/2.0.0
+          EOE
+
+        $pkg_status -ar >>EOO;
+          !libbax configured 2.0.0
+          !libbox configured !1.0.0 available 2.0.0
+            !libbax configured 2.0.0
+          !libbar configured 1.2.0 available 2.1.0
+          !libfoo configured 2.0.0
+            !libbar configured 1.2.0 available 2.1.0
+          EOO
+
+        $pkg_drop libbax libbox libbar libfoo
+      }
+
+      : re-order-unsatisfied
+      :
+      : Similar to the above but the dependent of the mentioned package is
+      : unsatisfied with its dependencies.
+      :
+      {
+        $clone_cfg;
+
+        $* libbar libbox ?libbax/1.0.0 2>!;
+
+        $pkg_status -ar >>EOO;
+          libbax configured !1.0.0 available 2.0.0
+          !libbox configured 2.0.0
+            libbax configured !1.0.0 available 2.0.0
+          !libbar configured 2.1.0
+            !libbox configured 2.0.0
+              libbax configured !1.0.0 available 2.0.0
+          EOO
+
+        $* libfoo libbax/2.0.0 --verbose 5 2>>~%EOE% != 0;
+          %.*
+          trace: pkg_build: refine package collection/plan execution from scratch
+          trace: collect_build: add libfoo/2.0.0
+          trace: collect_build: add libbax/2.0.0
+          trace: collect_build_prerequisites: begin libfoo/2.0.0
+          trace: collect_build: add libbar/1.2.0
+          warning: package libfoo dependency on (libbar == 1.2.0) is forcing downgrade of libbar/2.1.0 to 1.2.0
+          trace: collect_build_prerequisites: no cfg-clause for dependency libbar/1.2.0 of dependent libfoo/2.0.0
+          trace: collect_build_prerequisites: begin libbar/1.2.0
+          trace: collect_build_prerequisites: end libbar/1.2.0
+          trace: collect_build_prerequisites: end libfoo/2.0.0
+          trace: collect_build_prerequisites: skip unsatisfied existing dependent libbox of dependency libbax/2.0.0 due to constraint (libbax == 1.0.0)
+          trace: collect_build_prerequisites: begin libbax/2.0.0
+          trace: collect_build_prerequisites: end libbax/2.0.0
+          trace: collect_dependents: postpone failure for existing dependent libbox unsatisfied with dependency libbax/2.0.0 (== 1.0.0)
+          trace: execute_plan: simulate: yes
+          %.*
+          error: unable to upgrade package libbax/1.0.0 to 2.0.0
+            info: because configured package libbox/2.0.0 depends on (libbax == 1.0.0)
+            info: consider re-trying with --upgrade|-u potentially combined with --recursive|-r
+            info: or explicitly request up/downgrade of package libbox
+            info: or explicitly specify package libbax version to manually satisfy these constraints
+          %.*
+          EOE
+
+        $pkg_drop libbox libbar
+      }
+    }
   }
 
   : upgrade
@@ -3913,6 +4030,19 @@ test.arguments += --sys-no-query
        configured libbar/0.0.2
        EOE
 
+    +$pkg_status -ar >>EOO
+      libfix configured 0.0.1 available 0.0.3
+      !libfoo configured !0.0.1 available 1.0.0
+        libfix configured 0.0.1 available 0.0.3
+      !libbaz configured !0.0.2 available 0.1.0 0.0.4 0.0.3
+        !libfoo configured !0.0.1 available 1.0.0
+          libfix configured 0.0.1 available 0.0.3
+      !libbar configured !0.0.2 available 1.0.0 0.0.3
+        !libbaz configured !0.0.2 available 0.1.0 0.0.4 0.0.3
+          !libfoo configured !0.0.1 available 1.0.0
+            libfix configured 0.0.1 available 0.0.3
+      EOO
+
     clone_cfg = [cmdline] cp --no-cleanup -r ../cfg ./ &cfg/***
 
     : immediate
diff --git a/tests/pkg-build/t4f b/tests/pkg-build/t4f
new file mode 120000
index 0000000..00f2c86
--- /dev/null
+++ b/tests/pkg-build/t4f
@@ -0,0 +1 @@
+../common/satisfy/t4f
\ No newline at end of file