Optimize slot conflict backtracking for #337178.

This borrows logic from commit
fc33f34e3d060b57e03204fb427b01971f8dafff which fixes bug #337178
in the 2.1.9 branch. With this change,
BacktrackingTestCase.testBacktrackingGoodVersionFirst() is solved
with fewer backtracking steps.

diff --git a/pym/_emerge/depgraph.py b/pym/_emerge/depgraph.py
index 82433e8d96f5d07a51a487131ea695555dc89f84..64a25ceb1f4f8812cf97218d96811c12069a8d38 100644 (file)
--- a/pym/_emerge/depgraph.py
+++ b/pym/_emerge/depgraph.py
@@ -899,6 +899,19 @@ class depgraph(object):
                                                                parent_atoms = set()
                                                        backtrack_data.append((to_be_masked, parent_atoms))
 
+                                               # NOTE: Generally, we prefer to mask the higher
+                                               # version since this solves common cases in which a
+                                               # lower version is needed so that all dependencies
+                                               # will be satisfied (bug #337178). However, if
+                                               # existing_node happens to be installed then we
+                                               # mask that since this is a common case that is
+                                               # triggered when --update is not enabled.
+                                               if existing_node.installed:
+                                                       pass
+                                               elif pkg > existing_node:
+                                                       backtrack_data.reverse()
+                                               to_be_masked = backtrack_data[0][0]
+
                                                self._dynamic_config._backtrack_infos["slot conflict"] = backtrack_data
                                                self._dynamic_config._need_restart = True
                                                if "--debug" in self._frozen_config.myopts:
@@ -908,6 +921,7 @@ class depgraph(object):
                                                        msg.append("backtracking due to slot conflict:")
                                                        msg.append("   first package:  %s" % existing_node)
                                                        msg.append("   second package: %s" % pkg)
+                                                       msg.append("  package to mask: %s" % to_be_masked)
                                                        msg.append("      slot: %s" % pkg.slot_atom)
                                                        msg.append("   parents: %s" % \
                                                                [(str(parent), atom) \