Added an unmodified copy of SCons to third_party.

scons-2.0.1/engine/SCons/Taskmaster.py

+#
+# Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 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:
+#
+# The above copyright notice and this permission notice shall be included
+# in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
+# KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
+# WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+__doc__ = """
+Generic Taskmaster module for the SCons build engine.
+
+This module contains the primary interface(s) between a wrapping user
+interface and the SCons build engine.  There are two key classes here:
+
+    Taskmaster
+        This is the main engine for walking the dependency graph and
+        calling things to decide what does or doesn't need to be built.
+
+    Task
+        This is the base class for allowing a wrapping interface to
+        decide what does or doesn't actually need to be done.  The
+        intention is for a wrapping interface to subclass this as
+        appropriate for different types of behavior it may need.
+
+        The canonical example is the SCons native Python interface,
+        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 5134 2010/08/16 23:02:40 bdeegan"
+
+from itertools import chain
+import operator
+import sys
+import traceback
+
+import SCons.Errors
+import SCons.Node
+import SCons.Warnings
+
+StateString = SCons.Node.StateString
+NODE_NO_STATE = SCons.Node.no_state
+NODE_PENDING = SCons.Node.pending
+NODE_EXECUTING = SCons.Node.executing
+NODE_UP_TO_DATE = SCons.Node.up_to_date
+NODE_EXECUTED = SCons.Node.executed
+NODE_FAILED = SCons.Node.failed
+
+
+# A subsystem for recording stats about how different Nodes are handled by
+# the main Taskmaster loop.  There's no external control here (no need for
+# a --debug= option); enable it by changing the value of CollectStats.
+
+CollectStats = None
+
+class Stats(object):
+    """
+    A simple class for holding statistics about the disposition of a
+    Node by the Taskmaster.  If we're collecting statistics, each Node
+    processed by the Taskmaster gets one of these attached, in which case
+    the Taskmaster records its decision each time it processes the Node.
+    (Ideally, that's just once per Node.)
+    """
+    def __init__(self):
+        """
+        Instantiates a Taskmaster.Stats object, initializing all
+        appropriate counters to zero.
+        """
+        self.considered  = 0
+        self.already_handled  = 0
+        self.problem  = 0
+        self.child_failed  = 0
+        self.not_built  = 0
+        self.side_effects  = 0
+        self.build  = 0
+
+StatsNodes = []
+
+fmt = "%(considered)3d "\
+      "%(already_handled)3d " \
+      "%(problem)3d " \
+      "%(child_failed)3d " \
+      "%(not_built)3d " \
+      "%(side_effects)3d " \
+      "%(build)3d "
+
+def dump_stats():
+    for n in sorted(StatsNodes, key=lambda a: str(a)):
+        print (fmt % n.stats.__dict__) + str(n)
+
+
+
+class Task(object):
+    """
+    Default SCons build engine task.
+
+    This controls the interaction of the actual building of node
+    and the rest of the engine.
+
+    This is expected to handle all of the normally-customizable
+    aspects of controlling a build, so any given application
+    *should* be able to do what it wants by sub-classing this
+    class and overriding methods as appropriate.  If an application
+    needs to customze something by sub-classing Taskmaster (or
+    some other build engine class), we should first try to migrate
+    that functionality into this class.
+
+    Note that it's generally a good idea for sub-classes to call
+    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(u'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
+
+        # Let the targets take care of any necessary preparations.
+        # This includes verifying that all of the necessary sources
+        # and dependencies exist, removing the target file(s), etc.
+        #
+        # As of April 2008, the get_executor().prepare() method makes
+        # sure that all of the aggregate sources necessary to build this
+        # Task's target(s) exist in one up-front check.  The individual
+        # target t.prepare() methods check that each target's explicit
+        # or implicit dependencies exists, and also initialize the
+        # .sconsign info.
+        executor = self.targets[0].get_executor()
+        executor.prepare()
+        for t in executor.get_action_targets():
+            t.prepare()
+            for s in t.side_effects:
+                s.prepare()
+
+    def get_target(self):
+        """Fetch the target being built or updated by this task.
+        """
+        return self.node
+
+    def needs_execute(self):
+        # TODO(deprecate):  "return True" is the old default behavior;
+        # change it to NotImplementedError (after running through the
+        # Deprecation Cycle) so the desired behavior is explicitly
+        # determined by which concrete subclass is used.
+        #raise NotImplementedError
+        msg = ('Taskmaster.Task is an abstract base class; instead of\n'
+              '\tusing it directly, '
+              'derive from it and override the abstract methods.')
+        SCons.Warnings.warn(SCons.Warnings.TaskmasterNeedsExecuteWarning, msg)
+        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(u'Task.execute()', self.node))
+
+        try:
+            everything_was_cached = 1
+            for t in self.targets:
+                if t.retrieve_from_cache():
+                    # Call the .built() method without calling the
+                    # .push_to_cache() method, since we just got the
+                    # target from the cache and don't need to push
+                    # it back there.
+                    t.set_state(NODE_EXECUTED)
+                    t.built()
+                else:
+                    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 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.push_to_cache()
+                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, 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().
+        """
+        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:
+                # add disambiguate here to mirror the call on targets above
+                s.disambiguate().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(u'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:
+                    # add disambiguate here to mirror the call on targets in first loop above
+                    s.disambiguate().set_state(NODE_EXECUTING)
+        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(u'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(u'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)
+
+        for p, subtract in parents.items():
+            p.ref_count = p.ref_count - subtract
+            if T: T.write(self.trace_message(u'Task.postprocess()',
+                                             p,
+                                             'adjusted parent ref count'))
+            if p.ref_count == 0:
+                self.tm.candidates.append(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,
+    # etc.  This is done by having the Taskmaster tell us about the
+    # exception, and letting
+
+    def exc_info(self):
+        """
+        Returns info about a recorded exception.
+        """
+        return self.exception
+
+    def exc_clear(self):
+        """
+        Clears any recorded exception.
+
+        This also changes the "exception_raise" attribute to point
+        to the appropriate do-nothing method.
+        """
+        self.exception = (None, None, None)
+        self.exception_raise = self._no_exception_to_raise
+
+    def exception_set(self, exception=None):
+        """
+        Records an exception to be raised at the appropriate time.
+
+        This also changes the "exception_raise" attribute to point
+        to the method that will, in fact
+        """
+        if not exception:
+            exception = sys.exc_info()
+        self.exception = exception
+        self.exception_raise = self._exception_raise
+
+    def _no_exception_to_raise(self):
+        pass
+
+    def _exception_raise(self):
+        """
+        Raises a pending exception that was recorded while getting a
+        Task ready for execution.
+        """
+        exc = self.exc_info()[:]
+        try:
+            exc_type, exc_value, exc_traceback = exc
+        except ValueError:
+            exc_type, exc_value = exc
+            exc_traceback = None
+        raise exc_type, exc_value, exc_traceback
+
+class AlwaysTask(Task):
+    def needs_execute(self):
+        """
+        Always returns True (indicating this Task should always
+        be executed).
+
+        Subclasses that need this behavior (as opposed to the default
+        of only executing Nodes that are out of date w.r.t. their
+        dependencies) can use this as follows:
+
+            class MyTaskSubclass(SCons.Taskmaster.Task):
+                needs_execute = SCons.Taskmaster.Task.execute_always
+        """
+        return True
+
+class OutOfDateTask(Task):
+    def needs_execute(self):
+        """
+        Returns True (indicating this Task should be executed) if this
+        Task's target state indicates it needs executing, which has
+        already been determined by an earlier up-to-date check.
+        """
+        return self.targets[0].get_state() == SCons.Node.executing
+
+
+def find_cycle(stack, visited):
+    if stack[-1] in visited:
+        return None
+    visited.add(stack[-1])
+    for n in stack[-1].waiting_parents:
+        stack.append(n)
+        if stack[0] == stack[-1]:
+            return stack
+        if find_cycle(stack, visited):
+            return stack
+        stack.pop()
+    return None
+
+
+class Taskmaster(object):
+    """
+    The Taskmaster for walking the dependency DAG.
+    """
+
+    def __init__(self, targets=[], tasker=None, order=None, trace=None):
+        self.original_top = targets
+        self.top_targets_left = targets[:]
+        self.top_targets_left.reverse()
+        self.candidates = []
+        if tasker is None:
+            tasker = OutOfDateTask
+        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
+        case a Task will be created for their actual evaluation and
+        potential building.
+
+        Here is where we also allow candidate Nodes to alter the list of
+        Nodes that should be examined.  This is used, for example, when
+        invoking SCons in a source directory.  A source directory Node can
+        return its corresponding build directory Node, essentially saying,
+        "Hey, you really need to build this thing over here instead."
+        """
+        try:
+            return self.candidates.pop()
+        except IndexError:
+            pass
+        try:
+            node = self.top_targets_left.pop()
+        except IndexError:
+            return None
+        self.current_top = node
+        alt, message = node.alter_targets()
+        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)
+        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(u'\n' + self.trace_message('Looking for a node to evaluate'))
+
+        while True:
+            node = self.next_candidate()
+            if node is None:
+                if T: T.write(self.trace_message('No candidate anymore.') + u'\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(self.trace_message(u'    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(self.trace_message(u'       already handled (executed)'))
+                continue
+
+            executor = node.get_executor()
+
+            try:
+                children = executor.get_all_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(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(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(executor.get_all_prerequisites(), children):
+                childstate = child.get_state()
+
+                if T: T.write(self.trace_message(u'       ' + 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(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:
+                for n in executor.get_action_targets():
+                    n.set_state(NODE_FAILED)
+
+                if S: S.child_failed = S.child_failed + 1
+                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(self.trace_message(u'     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 executor.get_action_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(self.trace_message(u'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.
+        """
+        node = self._find_next_ready_node()
+
+        if node is None:
+            return None
+
+        tlist = node.get_executor().get_all_targets()
+
+        task = self.tasker(self, tlist, node in self.original_top, node)
+        try:
+            task.make_ready()
+        except:
+            # We had a problem just trying to get this task ready (like
+            # a child couldn't be linked in to a VariantDir when deciding
+            # whether this node is current).  Arrange to raise the
+            # 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, node_func=lambda n: None):
+        """
+        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(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 len(to_visit):
+                node = to_visit.pop()
+                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
+        # taskmaster 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 not self.pending_children:
+            return
+
+        nclist = [(n, find_cycle([n], set())) for n in self.pending_children]
+
+        genuine_cycles = [
+            node for node,cycle in nclist
+                     if cycle or node.get_state() != NODE_EXECUTED
+        ]
+        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 + "  " + " -> ".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)
+
+# Local Variables:
+# tab-width:4
+# indent-tabs-mode:nil
+# End:
+# vim: set expandtab tabstop=4 shiftwidth=4: