Update to SCons 1.2.0.

third_party/scons/scons-local/SCons/Taskmaster.py

 #
 # Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 The SCons Foundation
 #
 # Permission is hereby granted, free of charge, to any person obtaining
 # a copy of this software and associated documentation files (the
 # "Software"), to deal in the Software without restriction, including
 # without limitation the rights to use, copy, modify, merge, publish,
 # distribute, sublicense, and/or sell copies of the Software, and to
 # permit persons to whom the Software is furnished to do so, subject to
 # the following conditions:
@@ skipping 30 matching lines @@
         which has Task subclasses that handle its specific behavior,
         like printing "`foo' is up to date" when a top-level target
         doesn't need to be built, and handling the -c option by removing
         targets as its "build" action.  There is also a separate subclass
         for suppressing this output when the -q option is used.
 
         The Taskmaster instantiates a Task object for each (set of)
         target(s) that it decides need to be evaluated and/or built.
 """
 
-__revision__ = "src/engine/SCons/Taskmaster.py 3603 2008/10/10 05:46:45 scons"
+__revision__ = "src/engine/SCons/Taskmaster.py 3842 2008/12/20 22:59:52 scons"
 
 from itertools import chain
 import operator
 import string
 import sys
 import traceback
 
 import SCons.Errors
 import SCons.Node
 
@@ skipping 69 matching lines @@
     these methods explicitly to update state, etc., rather than
     roll their own interaction with Taskmaster from scratch.
     """
     def __init__(self, tm, targets, top, node):
         self.tm = tm
         self.targets = targets
         self.top = top
         self.node = node
         self.exc_clear()
 
+    def trace_message(self, method, node, description='node'):
+        fmt = '%-20s %s %s\n'
+        return fmt % (method + ':', description, self.tm.trace_node(node))
+
     def display(self, message):
         """
         Hook to allow the calling interface to display a message.
 
         This hook gets called as part of preparing a task for execution
         (that is, a Node to be built).  As part of figuring out what Node
         should be built next, the actually target list may be altered,
         along with a message describing the alteration.  The calling
         interface can subclass Task and provide a concrete implementation
         of this method to see those messages.
         """
         pass
 
     def prepare(self):
         """
         Called just before the task is executed.
 
         This is mainly intended to give the target Nodes a chance to
         unlink underlying files and make all necessary directories before
         the Action is actually called to build the targets.
         """
+        T = self.tm.trace
+        if T: T.write(self.trace_message('Task.prepare()', self.node))
 
         # Now that it's the appropriate time, give the TaskMaster a
         # chance to raise any exceptions it encountered while preparing
         # this task.
         self.exception_raise()
 
         if self.tm.message:
             self.display(self.tm.message)
             self.tm.message = None
 
@@ skipping 30 matching lines @@
         return True
 
     def execute(self):
         """
         Called to execute the task.
 
         This method is called from multiple threads in a parallel build,
         so only do thread safe stuff here.  Do thread unsafe stuff in
         prepare(), executed() or failed().
         """
+        T = self.tm.trace
+        if T: T.write(self.trace_message('Task.execute()', self.node))
 
         try:
             everything_was_cached = 1
             for t in self.targets:
                 if not t.retrieve_from_cache():
                     everything_was_cached = 0
                     break
             if not everything_was_cached:
                 self.targets[0].build()
         except SystemExit:
             exc_value = sys.exc_info()[1]
             raise SCons.Errors.ExplicitExit(self.targets[0], exc_value.code)
         except SCons.Errors.UserError:
             raise
         except SCons.Errors.BuildError:
             raise
-        except:
-            raise SCons.Errors.TaskmasterException(self.targets[0],
-                                                   sys.exc_info())
+        except Exception, e:
+            buildError = SCons.Errors.convert_to_BuildError(e)
+            buildError.node = self.targets[0]
+            buildError.exc_info = sys.exc_info()
+            raise buildError
 
     def executed_without_callbacks(self):
         """
         Called when the task has been successfully executed
         and the Taskmaster instance doesn't want to call
         the Node's callback methods.
         """
+        T = self.tm.trace
+        if T: T.write(self.trace_message('Task.executed_without_callbacks()',
+                                         self.node))
+
         for t in self.targets:
             if t.get_state() == NODE_EXECUTING:
                 for side_effect in t.side_effects:
                     side_effect.set_state(NODE_NO_STATE)
                 t.set_state(NODE_EXECUTED)
 
     def executed_with_callbacks(self):
         """
         Called when the task has been successfully executed and
         the Taskmaster instance wants to call the Node's callback
         methods.
 
         This may have been a do-nothing operation (to preserve build
         order), so we must check the node's state before deciding whether
         it was "built", in which case we call the appropriate Node method.
         In any event, we always call "visited()", which will handle any
         post-visit actions that must take place regardless of whether
         or not the target was an actual built target or a source Node.
         """
+        T = self.tm.trace
+        if T: T.write(self.trace_message('Task.executed_with_callbacks()',
+                                         self.node))
+
         for t in self.targets:
             if t.get_state() == NODE_EXECUTING:
                 for side_effect in t.side_effects:
                     side_effect.set_state(NODE_NO_STATE)
                 t.set_state(NODE_EXECUTED)
                 t.built()
             t.visited()
 
     executed = executed_with_callbacks
 
     def failed(self):
         """
         Default action when a task fails:  stop the build.
+
+        Note: Although this function is normally invoked on nodes in
+        the executing state, it might also be invoked on up-to-date
+        nodes when using Configure().
         """
         self.fail_stop()
 
     def fail_stop(self):
         """
         Explicit stop-the-build failure.
+
+        This sets failure status on the target nodes and all of
+        their dependent parent nodes.
+
+        Note: Although this function is normally invoked on nodes in
+        the executing state, it might also be invoked on up-to-date
+        nodes when using Configure().
         """
-        
+        T = self.tm.trace
+        if T: T.write(self.trace_message('Task.failed_stop()', self.node))
+
         # Invoke will_not_build() to clean-up the pending children
         # list.
-        self.tm.will_not_build(self.targets)
+        self.tm.will_not_build(self.targets, lambda n: n.set_state(NODE_FAILED))
 
         # Tell the taskmaster to not start any new tasks
         self.tm.stop()
 
         # We're stopping because of a build failure, but give the
         # calling Task class a chance to postprocess() the top-level
         # target under which the build failure occurred.
         self.targets = [self.tm.current_top]
         self.top = 1
 
     def fail_continue(self):
         """
         Explicit continue-the-build failure.
 
         This sets failure status on the target nodes and all of
         their dependent parent nodes.
+
+        Note: Although this function is normally invoked on nodes in
+        the executing state, it might also be invoked on up-to-date
+        nodes when using Configure().
         """
-        self.tm.will_not_build(self.targets)
-        
+        T = self.tm.trace
+        if T: T.write(self.trace_message('Task.failed_continue()', self.node))
+
+        self.tm.will_not_build(self.targets, lambda n: n.set_state(NODE_FAILED))
+
     def make_ready_all(self):
         """
         Marks all targets in a task ready for execution.
 
         This is used when the interface needs every target Node to be
         visited--the canonical example being the "scons -c" option.
         """
+        T = self.tm.trace
+        if T: T.write(self.trace_message('Task.make_ready_all()', self.node))
+
         self.out_of_date = self.targets[:]
         for t in self.targets:
             t.disambiguate().set_state(NODE_EXECUTING)
             for s in t.side_effects:
                 s.set_state(NODE_EXECUTING)
 
     def make_ready_current(self):
         """
         Marks all targets in a task ready for execution if any target
         is not current.
 
         This is the default behavior for building only what's necessary.
         """
+        T = self.tm.trace
+        if T: T.write(self.trace_message('Task.make_ready_current()',
+                                         self.node))
+
         self.out_of_date = []
         needs_executing = False
         for t in self.targets:
             try:
                 t.disambiguate().make_ready()
                 is_up_to_date = not t.has_builder() or \
                                 (not t.always_build and t.is_up_to_date())
             except EnvironmentError, e:
                 raise SCons.Errors.BuildError(node=t, errstr=e.strerror, filename=e.filename)
 
             if not is_up_to_date:
                 self.out_of_date.append(t)
                 needs_executing = True
 
         if needs_executing:
             for t in self.targets:
                 t.set_state(NODE_EXECUTING)
                 for s in t.side_effects:
                     s.set_state(NODE_EXECUTING)
-        else:                
+        else:
             for t in self.targets:
                 # We must invoke visited() to ensure that the node
                 # information has been computed before allowing the
                 # parent nodes to execute. (That could occur in a
                 # parallel build...)
                 t.visited()
                 t.set_state(NODE_UP_TO_DATE)
 
     make_ready = make_ready_current
 
     def postprocess(self):
         """
         Post-processes a task after it's been executed.
 
         This examines all the targets just built (or not, we don't care
         if the build was successful, or even if there was no build
         because everything was up-to-date) to see if they have any
         waiting parent Nodes, or Nodes waiting on a common side effect,
         that can be put back on the candidates list.
         """
+        T = self.tm.trace
+        if T: T.write(self.trace_message('Task.postprocess()', self.node))
 
         # We may have built multiple targets, some of which may have
         # common parents waiting for this build.  Count up how many
         # targets each parent was waiting for so we can subtract the
         # values later, and so we *don't* put waiting side-effect Nodes
         # back on the candidates list if the Node is also a waiting
         # parent.
 
         targets = set(self.targets)
 
+        pending_children = self.tm.pending_children
         parents = {}
         for t in targets:
+            # A node can only be in the pending_children set if it has
+            # some waiting_parents.
+            if t.waiting_parents:
+                if T: T.write(self.trace_message('Task.postprocess()',
+                                                 t,
+                                                 'removing'))
+                pending_children.discard(t)
             for p in t.waiting_parents:
                 parents[p] = parents.get(p, 0) + 1
 
         for t in targets:
             for s in t.side_effects:
                 if s.get_state() == NODE_EXECUTING:
                     s.set_state(NODE_NO_STATE)
                     for p in s.waiting_parents:
                         parents[p] = parents.get(p, 0) + 1
                 for p in s.waiting_s_e:
                     if p.ref_count == 0:
                         self.tm.candidates.append(p)
-                        self.tm.pending_children.discard(p)
 
         for p, subtract in parents.items():
             p.ref_count = p.ref_count - subtract
+            if T: T.write(self.trace_message('Task.postprocess()',
+                                             p,
+                                             'adjusted parent ref count'))
             if p.ref_count == 0:
                 self.tm.candidates.append(p)
-                self.tm.pending_children.discard(p)
 
         for t in targets:
             t.postprocess()
 
     # Exception handling subsystem.
     #
     # Exceptions that occur while walking the DAG or examining Nodes
     # must be raised, but must be raised at an appropriate time and in
     # a controlled manner so we can, if necessary, recover gracefully,
     # possibly write out signature information for Nodes we've updated,
@@ skipping 71 matching lines @@
         self.candidates = []
         self.tasker = tasker
         if not order:
             order = lambda l: l
         self.order = order
         self.message = None
         self.trace = trace
         self.next_candidate = self.find_next_candidate
         self.pending_children = set()
 
-
     def find_next_candidate(self):
         """
         Returns the next candidate Node for (potential) evaluation.
 
         The candidate list (really a stack) initially consists of all of
         the top-level (command line) targets provided when the Taskmaster
         was initialized.  While we walk the DAG, visiting Nodes, all the
         children that haven't finished processing get pushed on to the
         candidate list.  Each child can then be popped and examined in
         turn for whether *their* children are all up-to-date, in which
@@ skipping 19 matching lines @@
         if alt:
             self.message = message
             self.candidates.append(node)
             self.candidates.extend(self.order(alt))
             node = self.candidates.pop()
         return node
 
     def no_next_candidate(self):
         """
         Stops Taskmaster processing by not returning a next candidate.
-        
+
         Note that we have to clean-up the Taskmaster candidate list
         because the cycle detection depends on the fact all nodes have
         been processed somehow.
         """
         while self.candidates:
             candidates = self.candidates
             self.candidates = []
-            self.will_not_build(candidates, lambda n: n.state < NODE_UP_TO_DATE)
+            self.will_not_build(candidates)
         return None
 
+    def _validate_pending_children(self):
+        """
+        Validate the content of the pending_children set. Assert if an
+        internal error is found.
+
+        This function is used strictly for debugging the taskmaster by
+        checking that no invariants are violated. It is not used in
+        normal operation.
+
+        The pending_children set is used to detect cycles in the
+        dependency graph. We call a "pending child" a child that is
+        found in the "pending" state when checking the dependencies of
+        its parent node.
+
+        A pending child can occur when the Taskmaster completes a loop
+        through a cycle. For example, lets imagine a graph made of
+        three node (A, B and C) making a cycle. The evaluation starts
+        at node A. The taskmaster first consider whether node A's
+        child B is up-to-date. Then, recursively, node B needs to
+        check whether node C is up-to-date. This leaves us with a
+        dependency graph looking like:
+
+                                      Next candidate \
+                                                      \
+        Node A (Pending) --> Node B(Pending) --> Node C (NoState)
+                ^                                     |
+                |                                     |
+                +-------------------------------------+
+
+        Now, when the Taskmaster examines the Node C's child Node A,
+        it finds that Node A is in the "pending" state. Therefore,
+        Node A is a pending child of node C.
+
+        Pending children indicate that the Taskmaster has potentially
+        loop back through a cycle. We say potentially because it could
+        also occur when a DAG is evaluated in parallel. For example,
+        consider the following graph:
+
+
+        Node A (Pending) --> Node B(Pending) --> Node C (Pending) --> ...
+                |                                     ^
+                |                                     |
+                +----------> Node D (NoState) --------+
+                                  /
+                  Next candidate /
+
+        The Taskmaster first evaluates the nodes A, B, and C and
+        starts building some children of node C. Assuming, that the
+        maximum parallel level has not been reached, the Taskmaster
+        will examine Node D. It will find that Node C is a pending
+        child of Node D.
+
+        In summary, evaluating a graph with a cycle will always
+        involve a pending child at one point. A pending child might
+        indicate either a cycle or a diamond-shaped DAG. Only a
+        fraction of the nodes ends-up being a "pending child" of
+        another node. This keeps the pending_children set small in
+        practice.
+
+        We can differentiate between the two cases if we wait until
+        the end of the build. At this point, all the pending children
+        nodes due to a diamond-shaped DAG will have been properly
+        built (or will have failed to build). But, the pending
+        children involved in a cycle will still be in the pending
+        state.
+
+        The taskmaster removes nodes from the pending_children set as
+        soon as a pending_children node moves out of the pending
+        state. This also helps to keep the pending_children set small.
+        """
+
+        for n in self.pending_children:
+            assert n.state in (NODE_PENDING, NODE_EXECUTING), \
+                (str(n), StateString[n.state])
+            assert len(n.waiting_parents) != 0, (str(n), len(n.waiting_parents))
+            for p in n.waiting_parents:
+                assert p.ref_count > 0, (str(n), str(p), p.ref_count)
+
+
+    def trace_message(self, message):
+        return 'Taskmaster: %s\n' % message
+
+    def trace_node(self, node):
+        return '<%-10s %-3s %s>' % (StateString[node.get_state()],
+                                    node.ref_count,
+                                    repr(str(node)))
+
     def _find_next_ready_node(self):
         """
         Finds the next node that is ready to be built.
 
         This is *the* main guts of the DAG walk.  We loop through the
         list of candidates, looking for something that has no un-built
         children (i.e., that is a leaf Node or has dependencies that are
         all leaf Nodes or up-to-date).  Candidate Nodes are re-scanned
         (both the target Node itself and its sources, which are always
         scanned in the context of a given target) to discover implicit
         dependencies.  A Node that must wait for some children to be
         built will be put back on the candidates list after the children
         have finished building.  A Node that has been put back on the
         candidates list in this way may have itself (or its sources)
         re-scanned, in order to handle generated header files (e.g.) and
         the implicit dependencies therein.
 
         Note that this method does not do any signature calculation or
         up-to-date check itself.  All of that is handled by the Task
         class.  This is purely concerned with the dependency graph walk.
         """
 
         self.ready_exc = None
 
         T = self.trace
-        if T: T.write('\nTaskmaster: Looking for a node to evaluate\n')
+        if T: T.write('\n' + self.trace_message('Looking for a node to evaluate'))
 
         while 1:
             node = self.next_candidate()
             if node is None:
-                if T: T.write('Taskmaster: No candidate anymore.\n\n')
+                if T: T.write(self.trace_message('No candidate anymore.') + '\n')
                 return None
 
             node = node.disambiguate()
             state = node.get_state()
 
+            # For debugging only:
+            #
+            # try:
+            #     self._validate_pending_children()
+            # except:
+            #     self.ready_exc = sys.exc_info()
+            #     return node
+
             if CollectStats:
                 if not hasattr(node, 'stats'):
                     node.stats = Stats()
                     StatsNodes.append(node)
                 S = node.stats
                 S.considered = S.considered + 1
             else:
                 S = None
 
-            if T: T.write('Taskmaster:     Considering node <%-10s %-3s %s> and its children:\n' % 
-                          (StateString[node.get_state()], node.ref_count, repr(str(node))))
+            if T: T.write(self.trace_message('    Considering node %s and its children:' % self.trace_node(node)))
 
             if state == NODE_NO_STATE:
                 # Mark this node as being on the execution stack:
                 node.set_state(NODE_PENDING)
             elif state > NODE_PENDING:
                 # Skip this node if it has already been evaluated:
                 if S: S.already_handled = S.already_handled + 1
-                if T: T.write('Taskmaster:        already handled (executed)\n')
+                if T: T.write(self.trace_message('       already handled (executed)'))
                 continue
 
             try:
                 children = node.children()
             except SystemExit:
                 exc_value = sys.exc_info()[1]
                 e = SCons.Errors.ExplicitExit(node, exc_value.code)
                 self.ready_exc = (SCons.Errors.ExplicitExit, e)
-                if T: T.write('Taskmaster:        SystemExit\n')
+                if T: T.write(self.trace_message('       SystemExit'))
                 return node
             except Exception, e:
                 # We had a problem just trying to figure out the
                 # children (like a child couldn't be linked in to a
                 # VariantDir, or a Scanner threw something).  Arrange to
                 # raise the exception when the Task is "executed."
                 self.ready_exc = sys.exc_info()
                 if S: S.problem = S.problem + 1
-                if T: T.write('Taskmaster:        exception %s while scanning children.\n'%s)
+                if T: T.write(self.trace_message('       exception %s while scanning children.\n' % e))
                 return node
 
             children_not_visited = []
             children_pending = set()
             children_not_ready = []
             children_failed = False
 
             for child in chain(children,node.prerequisites):
                 childstate = child.get_state()
 
-                if T: T.write('Taskmaster:        <%-10s %-3s %s>\n' % 
-                              (StateString[childstate], child.ref_count, repr(str(child))))
+                if T: T.write(self.trace_message('       ' + self.trace_node(child)))
 
                 if childstate == NODE_NO_STATE:
                     children_not_visited.append(child)
                 elif childstate == NODE_PENDING:
                     children_pending.add(child)
                 elif childstate == NODE_FAILED:
                     children_failed = True
 
                 if childstate <= NODE_EXECUTING:
                     children_not_ready.append(child)
 
 
             # These nodes have not even been visited yet.  Add
             # them to the list so that on some next pass we can
             # take a stab at evaluating them (or their children).
             children_not_visited.reverse()
             self.candidates.extend(self.order(children_not_visited))
             #if T and children_not_visited:
-            #    T.write('Taskmaster:      adding to candidates: %s\n' % map(str, children_not_visited))
-            #    T.write('Taskmaster:      candidates now: %s\n' % map(str, self.candidates))
+            #    T.write(self.trace_message('     adding to candidates: %s' % map(str, children_not_visited)))
+            #    T.write(self.trace_message('     candidates now: %s\n' % map(str, self.candidates)))
 
             # Skip this node if any of its children have failed.
             #
             # This catches the case where we're descending a top-level
             # target and one of our children failed while trying to be
             # built by a *previous* descent of an earlier top-level
             # target.
             #
             # It can also occur if a node is reused in multiple
             # targets. One first descends though the one of the
             # target, the next time occurs through the other target.
             #
             # Note that we can only have failed_children if the
             # --keep-going flag was used, because without it the build
             # will stop before diving in the other branch.
             #
             # Note that even if one of the children fails, we still
             # added the other children to the list of candidate nodes
             # to keep on building (--keep-going).
             if children_failed:
                 node.set_state(NODE_FAILED)
 
                 if S: S.child_failed = S.child_failed + 1
-                if T: T.write('Taskmaster:****** <%-10s %-3s %s>\n' % 
-                              (StateString[node.get_state()], node.ref_count, repr(str(node))))
+                if T: T.write(self.trace_message('****** %s\n' % self.trace_node(node)))
                 continue
 
             if children_not_ready:
                 for child in children_not_ready:
                     # We're waiting on one or more derived targets
                     # that have not yet finished building.
                     if S: S.not_built = S.not_built + 1
 
                     # Add this node to the waiting parents lists of
                     # anything we're waiting on, with a reference
                     # count so we can be put back on the list for
                     # re-evaluation when they've all finished.
                     node.ref_count =  node.ref_count + child.add_to_waiting_parents(node)
-                    if T: T.write('Taskmaster:      adjusting ref count: <%-10s %-3s %s>\n' %
-                                  (StateString[node.get_state()], node.ref_count, repr(str(node))))
+                    if T: T.write(self.trace_message('     adjusted ref count: %s, child %s' %
+                                  (self.trace_node(node), repr(str(child)))))
 
+                if T:
+                    for pc in children_pending:
+                        T.write(self.trace_message('       adding %s to the pending children set\n' %
+                                self.trace_node(pc)))
                 self.pending_children = self.pending_children | children_pending
-                
+
                 continue
 
             # Skip this node if it has side-effects that are
             # currently being built:
             wait_side_effects = False
             for se in node.side_effects:
                 if se.get_state() == NODE_EXECUTING:
                     se.add_to_waiting_s_e(node)
                     wait_side_effects = True
 
             if wait_side_effects:
                 if S: S.side_effects = S.side_effects + 1
                 continue
 
             # The default when we've gotten through all of the checks above:
             # this node is ready to be built.
             if S: S.build = S.build + 1
-            if T: T.write('Taskmaster: Evaluating <%-10s %-3s %s>\n' % 
-                          (StateString[node.get_state()], node.ref_count, repr(str(node))))
+            if T: T.write(self.trace_message('Evaluating %s\n' %
+                                             self.trace_node(node)))
+
+            # For debugging only:
+            #
+            # try:
+            #     self._validate_pending_children()
+            # except:
+            #     self.ready_exc = sys.exc_info()
+            #     return node
+
             return node
 
         return None
 
     def next_task(self):
         """
         Returns the next task to be executed.
 
         This simply asks for the next Node to be evaluated, and then wraps
         it in the specific Task subclass with which we were initialized.
@@ skipping 15 matching lines @@
             # exception when the Task is "executed."
             self.ready_exc = sys.exc_info()
 
         if self.ready_exc:
             task.exception_set(self.ready_exc)
 
         self.ready_exc = None
 
         return task
 
-    def will_not_build(self, nodes, mark_fail=lambda n: n.state != NODE_FAILED):
+    def will_not_build(self, nodes, node_func=lambda n: None):
         """
-        Perform clean-up about nodes that will never be built.
+        Perform clean-up about nodes that will never be built. Invokes
+        a user defined function on all of these nodes (including all
+        of their parents).
         """
 
+        T = self.trace
+
         pending_children = self.pending_children
 
-        to_visit = set()
-        for node in nodes:
-            # Set failure state on all of the parents that were dependent
-            # on this failed build.
-            if mark_fail(node):
-                node.set_state(NODE_FAILED)
-                parents = node.waiting_parents
-                to_visit = to_visit | parents
-                pending_children = pending_children - parents
+        to_visit = set(nodes)
+        pending_children = pending_children - to_visit
 
+        if T:
+            for n in nodes:
+                T.write(self.trace_message('       removing node %s from the pending children set\n' %
+                        self.trace_node(n)))
         try:
             while 1:
                 try:
                     node = to_visit.pop()
                 except AttributeError:
                     # Python 1.5.2
                     if len(to_visit):
                         node = to_visit[0]
                         to_visit.remove(node)
                     else:
                         break
-                if mark_fail(node):
-                    node.set_state(NODE_FAILED)
-                    parents = node.waiting_parents
-                    to_visit = to_visit | parents
-                    pending_children = pending_children - parents
+
+                node_func(node)
+
+                # Prune recursion by flushing the waiting children
+                # list immediately.
+                parents = node.waiting_parents
+                node.waiting_parents = set()
+
+                to_visit = to_visit | parents
+                pending_children = pending_children - parents
+
+                for p in parents:
+                    p.ref_count = p.ref_count - 1
+                    if T: T.write(self.trace_message('       removing parent %s from the pending children set\n' %
+                                  self.trace_node(p)))
         except KeyError:
             # The container to_visit has been emptied.
             pass
 
         # We have the stick back the pending_children list into the
         # task master because the python 1.5.2 compatibility does not
         # allow us to use in-place updates
         self.pending_children = pending_children
 
     def stop(self):
         """
         Stops the current build completely.
         """
         self.next_candidate = self.no_next_candidate
 
     def cleanup(self):
         """
         Check for dependency cycles.
         """
-        if self.pending_children:
-            desc = 'Found dependency cycle(s):\n'
-            for node in self.pending_children:
-                cycle = find_cycle([node], set())
-                if cycle:
-                    desc = desc + "  " + string.join(map(str, cycle), " -> ") + "\n"
-                else:
-                    desc = desc + \
-                        "  Internal Error: no cycle found for node %s (%s) in state %s\n" %  \
-                        (node, repr(node), StateString[node.get_state()]) 
+        if not self.pending_children:
+            return
 
-            raise SCons.Errors.UserError, desc
+        # TODO(1.5)
+        #nclist = [ (n, find_cycle([n], set())) for n in self.pending_children ]
+        nclist = map(lambda n: (n, find_cycle([n], set())), self.pending_children)
+
+        # TODO(1.5)
+        #genuine_cycles = [
+        #    node for node, cycle in nclist
+        #             if cycle or node.get_state() != NODE_EXECUTED
+        #]
+        genuine_cycles = filter(lambda t: t[1] or t[0].get_state() != NODE_EXECUTED, nclist)
+        if not genuine_cycles:
+            # All of the "cycles" found were single nodes in EXECUTED state,
+            # which is to say, they really weren't cycles.  Just return.
+            return
+
+        desc = 'Found dependency cycle(s):\n'
+        for node, cycle in nclist:
+            if cycle:
+                desc = desc + "  " + string.join(map(str, cycle), " -> ") + "\n"
+            else:
+                desc = desc + \
+                    "  Internal Error: no cycle found for node %s (%s) in state %s\n" %  \
+                    (node, repr(node), StateString[node.get_state()])
+
+        raise SCons.Errors.UserError, desc