summaryrefslogtreecommitdiff
path: root/src/engine/SCons/Taskmaster.py
blob: 29e48ac0c261a1ae0c32cfc9b0676c04f8d5053e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
#
# Copyright (c) 2001 - 2019 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.

from __future__ import print_function

import sys

__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 e724ae812eb96f4858a132f5b8c769724744faf6 2019-07-21 00:04:47 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

print_prepare = 0               # set by option --debug=prepare

# 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.attributes.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 customize 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 actual 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.
        """
        global print_prepare
        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()
        if executor is None:
            return
        executor.prepare()
        for t in executor.get_action_targets():
            if print_prepare:
                print("Preparing target %s..."%t)
                for s in t.side_effects:
                    print("...with side-effect %s..."%s)
            t.prepare()
            for s in t.side_effects:
                if print_prepare:
                    print("...Preparing side-effect %s..."%s)
                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:
            cached_targets = []
            for t in self.targets:
                if not t.retrieve_from_cache():
                    break
                cached_targets.append(t)
            if len(cached_targets) < len(self.targets):
                # Remove targets before building. It's possible that we
                # partially retrieved targets from the cache, leaving
                # them in read-only mode. That might cause the command
                # to fail.
                #
                for t in cached_targets:
                    try:
                        t.fs.unlink(t.get_internal_path())
                    except (IOError, OSError):
                        pass
                self.targets[0].build()
            else:
                for t in cached_targets:
                    t.cached = 1
        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 as 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.
        """
        global print_prepare
        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)
                if not t.cached:
                    t.push_to_cache()
                t.built()
                t.visited()
                if (not print_prepare and
                    (not hasattr(self, 'options') or not self.options.debug_includes)):
                    t.release_target_info()
            else:
                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.
        """
        global print_prepare
        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 as 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)
                if (not print_prepare and
                    (not hasattr(self, 'options') or not self.options.debug_includes)):
                    t.release_target_info()

    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
            t.waiting_parents = set()

        for t in targets:
            if t.side_effects is not None:
                for s in t.side_effects:
                    if s.get_state() == NODE_EXECUTING:
                        s.set_state(NODE_NO_STATE)

                    # The side-effects may have been transferred to
                    # NODE_NO_STATE by executed_with{,out}_callbacks, but was
                    # not taken out of the waiting parents/pending children
                    # data structures. Check for that now.
                    if s.get_state() == NODE_NO_STATE and s.waiting_parents:
                        pending_children.discard(s)
                        for p in s.waiting_parents:
                            parents[p] = parents.get(p, 0) + 1
                        s.waiting_parents = set()
                    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  # pylint: disable=unbalanced-tuple-unpacking
            exc_traceback = None

        # raise exc_type(exc_value).with_traceback(exc_traceback)
        if sys.version_info[0] == 2:
            exec("raise exc_type, exc_value, exc_traceback")
        else: #  sys.version_info[0] == 3:
            if isinstance(exc_value, Exception): #hasattr(exc_value, 'with_traceback'):
                # If exc_value is an exception, then just reraise
                exec("raise exc_value.with_traceback(exc_traceback)")
            else:
                # else we'll create an exception using the value and raise that
                exec("raise exc_type(exc_value).with_traceback(exc_traceback)")


        # raise e.__class__, e.__class__(e), sys.exc_info()[2]
        #     exec("raise exc_type(exc_value).with_traceback(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, let's imagine a graph made of
        three nodes (A, B and C) making a cycle. The evaluation starts
        at node A. The Taskmaster first considers 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(SCons.Util.UnicodeType('\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.attributes, 'stats'):
                    node.attributes.stats = Stats()
                    StatsNodes.append(node)
                S = node.attributes.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 as 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

        executor = node.get_executor()
        if executor is None:
            return None

        tlist = executor.get_all_targets()

        task = self.tasker(self, tlist, node in self.original_top, node)
        try:
            task.make_ready()
        except Exception as e :
            # We had a problem just trying to get this task ready (like
            # a child couldn't be linked 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: