Add support for configuration exporting and importing

The new config.export variable specifies the alternative file to write the
configuration to as part of the configure meta-operation. For example:

$ b configure: proj/ config.export=proj-config.build

The config.export value "applies" only to the projects on whose root scope it
is specified or if it is a global override (the latter is a bit iffy but we
allow it, for example, to dump everything to stdout). This means that in order
to save a subproject's configuration we will have to use a scope-specific
override (since the default will apply to the outermost amalgamation). For
example:

$ b configure: subproj/ subproj/config.export=.../subproj-config.build

This could be somewhat unnatural but then it will be the amalgamation whose
configuration we normally want to export.

The new config.import variable specifies additional configuration files to be
loaded after the project's default config.build, if any. For example:

$ b create: cfg/,cc config.import=my-config.build

Similar to config.export, the config.import value "applies" only to the
project on whose root scope it is specified or if it is a global override.
This allows the use of the standard override "positioning" machinery (i.e.,
where the override applies) to decide where the extra configuration files are
loaded. The resulting semantics is quite natural and consistent with command
line variable overrides, for example:

$ b   config.import=.../config.build  # outermost amalgamation
$ b ./config.import=.../config.build  # this project
$ b  !config.import=.../config.build  # every project

Both config.export and config.import recognize the special `-` file name as an
instruction to write/read to/from stdout/stdin, respectively. For example:

$ b configure: src-prj/ config.export=- | b configure: dst-prj/ config.import=-

1 files changed, 78 insertions, 30 deletions

diff --git a/libbuild2/config/init.cxx b/libbuild2/config/init.cxx
index 6998017..4e1890a 100644
--- a/libbuild2/config/init.cxx
+++ b/libbuild2/config/init.cxx
@@ -6,6 +6,7 @@
 
 #include <libbuild2/file.hxx>
 #include <libbuild2/rule.hxx>
+#include <libbuild2/lexer.hxx>
 #include <libbuild2/scope.hxx>
 #include <libbuild2/context.hxx>
 #include <libbuild2/filesystem.hxx>  // exists()
@@ -32,9 +33,18 @@ namespace build2
       const string& mname (rs.ctx.current_mname);
       const string& oname (rs.ctx.current_oname);
 
-      // Only create the module if we are configuring or creating. This is a
-      // bit tricky since the build2 core may not yet know if this is the
-      // case. But we know.
+      // While config.import (see below) could theoretically be specified in a
+      // buildfile, config.export is expected to always be specified as a
+      // command line override.
+      //
+      // Note: must be entered during bootstrap since we need it in
+      // configure_execute().
+      //
+      vp.insert<path> ("config.export", true /* ovr */);
+
+      // Only create the module if we are configuring or creating or if it was
+      // forced with config.module (useful if we need to call $config.export()
+      // during other meta-operations).
       //
       if ((                   mname == "configure" || mname == "create") ||
           (mname.empty () && (oname == "configure" || oname == "create")))
@@ -80,42 +90,80 @@ namespace build2
 
       assert (config_hints.empty ()); // We don't known any hints.
 
+      // Note that the config.<name>* variables belong to the module <name>.
+      // So the only "special" variables we can allocate in config.* are
+      // config.config.*, names that have been "gifted" to us by other modules
+      // (like config.version) as well as names that we have reserved to not
+      // be valid module names (build, import, export).
+      //
       auto& vp (rs.ctx.var_pool.rw (rs));
 
-      // Load config.build if one exists (we don't need to worry about
-      // disfigure since we will never be init'ed).
+      auto& c_v (vp.insert<uint64_t> ("config.version", false /*ovr*/));
+      auto& c_i (vp.insert<paths> ("config.import", true /* ovr */));
+
+      // Load config.build if one exists followed by extra files specified in
+      // config.import (we don't need to worry about disfigure since we will
+      // never be init'ed).
       //
-      const variable& c_v (vp.insert<uint64_t> ("config.version", false));
+      auto load_config = [&rs, &c_v] (const path& f, const location& l)
+      {
+        // Check the config version. We assume that old versions cannot
+        // understand new configs and new versions are incompatible with old
+        // configs.
+        //
+        // We extract the value manually instead of loading and then checking
+        // in order to be able to fixup/migrate the file which we may want to
+        // do in the future.
+        //
+
+        // This is tricky for stdin since we cannot reopen it (or put more
+        // than one character back). So what we are going to do is continue
+        // reading after extracting the variable. One side effect of this is
+        // that we won't have the config.version variable entered in the scope
+        // but that is harmless (we could do it manually if necessary).
+        //
+        ifdstream ifs;
+        lexer lex (open_file_or_stdin (f, ifs), f);
+
+        // Assume missing version is 0.
+        //
+        auto p (extract_variable (rs.ctx, lex, c_v));
+        uint64_t v (p.second ? cast<uint64_t> (p.first) : 0);
+
+        if (v != module::version)
+          fail (l) << "incompatible config file " << f <<
+            info << "config file version   " << v
+                   << (p.second ? "" : " (missing)") <<
+            info << "config module version " << module::version <<
+            info << "consider reconfiguring " << project (rs) << '@'
+                   << rs.out_path ();
+
+        source (rs, rs, lex);
+      };
 
       {
         path f (config_file (rs));
 
         if (exists (f))
+          load_config (f, l);
+      }
+
+      if (lookup l = rs[c_i])
+      {
+        // Only load files that were specified on our root scope as well as
+        // global overrides. This way we can use our override "positioning"
+        // machinery (i.e., where the override applies) to decide where the
+        // extra config is loaded. The resulting semantics feels quite natural
+        // and consistent with command line variable overrides:
+        //
+        // b   config.import=.../config.build  # outermost amalgamation
+        // b ./config.import=.../config.build  # this project
+        // b  !config.import=.../config.build  # every project
+        //
+        if (l.belongs (rs) || l.belongs (rs.ctx.global_scope))
         {
-          // Check the config version. We assume that old versions cannot
-          // understand new configs and new versions are incompatible with old
-          // configs.
-          //
-          // We extract the value manually instead of loading and then
-          // checking in order to be able to fixup/migrate the file which we
-          // may want to do in the future.
-          //
-          {
-            // Assume missing version is 0.
-            //
-            auto p (extract_variable (rs.ctx, f, c_v));
-            uint64_t v (p.second ? cast<uint64_t> (p.first) : 0);
-
-            if (v != module::version)
-              fail (l) << "incompatible config file " << f <<
-                info << "config file version   " << v
-                         << (p.second ? "" : " (missing)") <<
-                info << "config module version " << module::version <<
-                info << "consider reconfiguring " << project (rs) << '@'
-                         << out_root;
-          }
-
-          source (rs, rs, f);
+          for (const path& f: cast<paths> (l))
+            load_config (f, location (&f));
         }
       }