In the topological sort for merge order, try to avoid selecting root nodes

whenever possible. This helps ensure that the maximimum possible number of
soft dependencies  have been removed from the graph before their parent nodes
have been selected. This is especially important when those dependencies are
going to be rebuilt by revdep-rebuild or `emerge -e system` after the CHOST
has been changed (like when building a stage3 from a stage2). With this patch,
`emerge -e system` properly rebuilds dev-lang/python before sys-apps/file,
which helps to avoid a potential build failure. 

svn path=/main/trunk/; revision=7728

1 files changed, 34 insertions, 2 deletions

diff --git a/pym/emerge/__init__.py b/pym/emerge/__init__.py
index 8354da39f..2550da4c4 100644
--- a/pym/emerge/__init__.py
+++ b/pym/emerge/__init__.py
@@ -2246,6 +2246,26 @@ class depgraph(object):
 		# failed to select any nodes.  It is reset whenever nodes are
 		# successfully selected.
 		prefer_asap = True
+
+		# By default, try to avoid selecting root nodes whenever possible. This
+		# helps ensure that the maximimum possible number of soft dependencies
+		# have been removed from the graph before their parent nodes have
+		# selected. This is especially important when those dependencies are
+		# going to be rebuilt by revdep-rebuild or `emerge -e system` after the
+		# CHOST has been changed (like when building a stage3 from a stage2).
+		accept_root_node = False
+
+		# State of prefer_asap and accept_root_node flags for successive
+		# iterations that loosen the criteria for node selection.
+		#
+		# iteration   prefer_asap   accept_root_node
+		# 1           True          False
+		# 2           False         False
+		# 3           False         True
+		#
+		# If no nodes are selected on the 4th iteration, it is due to
+		# unresolved blockers or circular dependencies.
+
 		while not mygraph.empty():
 			selected_nodes = None
 			if prefer_asap and asap_nodes:
@@ -2280,7 +2300,8 @@ class depgraph(object):
 								# found a non-root node
 								selected_nodes = [node]
 								break
-						if not selected_nodes:
+						if not selected_nodes and \
+							(accept_root_node or ignore_priority is None):
 							# settle for a root node
 							selected_nodes = [nodes[0]]
 			if not selected_nodes:
@@ -2313,6 +2334,9 @@ class depgraph(object):
 					for ignore_priority in xrange(DepPriority.SOFT,
 						DepPriority.MEDIUM_SOFT + 1):
 						for node in nodes:
+							if not accept_root_node and \
+								not mygraph.parent_nodes(node):
+								continue
 							selected_nodes = set()
 							if gather_deps(ignore_priority,
 								mergeable_nodes, selected_nodes, node):
@@ -2328,6 +2352,12 @@ class depgraph(object):
 						prefer_asap = False
 						continue
 
+					if not selected_nodes and not accept_root_node:
+						# Maybe there are only root nodes left, so accept them
+						# for the next iteration.
+						accept_root_node = True
+						continue
+
 					if selected_nodes and ignore_priority > DepPriority.SOFT:
 						# Try to merge ignored medium deps as soon as possible.
 						for node in selected_nodes:
@@ -2408,8 +2438,10 @@ class depgraph(object):
 				sys.exit(1)
 
 			# At this point, we've succeeded in selecting one or more nodes, so
-			# it's now safe to reset the prefer_asap to it's default state.
+			# it's now safe to reset the prefer_asap and accept_root_node flags
+			# to their default states.
 			prefer_asap = True
+			accept_root_node = False
 
 			for node in selected_nodes:
 				if node[-1] != "nomerge":