Print better error messages when the dependency graph is broken.

parallel_emerge

 #!/usr/bin/python2.6
 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Program to run emerge in parallel, for significant speedup.
 
 Usage:
  ./parallel_emerge [--board=BOARD] [--workon=PKGS] [--no-workon-deps]
                    [emerge args] package"
@@ skipping 866 matching lines @@
           deps_map[needed_pkg]["provides"].add(bad_pkg)
           deps_map[bad_pkg]["needs"][needed_pkg] = "secret"
 
     def MergeChildren(pkg, merge_type):
       """Merge this package and all packages it provides."""
 
       this_pkg = deps_map[pkg]
       if (this_pkg[merge_type] or pkg not in final_pkgs):
         return
 
+      if pkg not in deps_info:
+        emerge_cmd = "emerge"
+        if self.board:
+          emerge_cmd = "emerge-%s" % self.board
+        emerge_cmd += " -pe =%s %s" % (pkg, " ".join(emerge.cmdline_packages))
+        use_str = os.environ.get("USE")
+        if use_str:
+          emerge_cmd = 'USE="%s" %s' % (use_str, emerge_cmd)
+        print "ERROR: emerge has refused to update %s" % pkg
+        print "Are there impossible-to-satisfy constraints in the dependency"
+        print "graph? To debug the issue, try the following command:"
+        print "  %s" % emerge_cmd
+        sys.exit(1)
+
       # Mark this package as non-optional
       deps_info[pkg]["optional"] = False
       this_pkg[merge_type] = True
       for w in this_pkg["provides"].difference(rebuild_blacklist):
         MergeChildren(w, merge_type)
 
       if this_pkg["action"] == "nomerge":
         this_pkg["action"] = "merge"
 
     def RemotePackageDatabase(binhost_url):
@@ skipping 795 matching lines @@
   # If we already upgraded portage, we don't need to do so again. But we do
   # need to upgrade the rest of the packages. So we'll go ahead and do that.
   if portage_upgrade:
     args = sys.argv[1:] + ["--nomerge=sys-apps/portage"]
     os.execvp(os.path.realpath(sys.argv[0]), args)
 
   print "Done"
 
 if __name__ == "__main__":
   main()