[Scons-users] MD5-timestamp decider overrides csigs from SConsign incorrectly

[William Blevins]

See if this looks like your topic:
http://scons.tigris.org/issues/show_bug.cgi?id=2980

On Fri, Jan 15, 2016 at 1:09 AM, William Blevins <wblevins001 at gmail.com>
wrote:

> This has already been reported I believe; let me lookup the issue.
>
> On Fri, Jan 15, 2016 at 12:44 AM, Sean Houghton <sean.houghton at gmail.com>
> wrote:
>
>> We’ve been having a difficult-to-track-down issue which we believe boils
>> down to the behavior of the MD5-timestamp decider when the list of
>> dependencies has changed between current runs and previous runs.
>>
>>
>>
>> The main Node.changed function calls the decider function for every input
>> to a node in all cases (called out by a comment “Note that we now *always*
>> check every dependency”). The inputs are determined by zip()ing together
>> two lists: a list of current children and a list of previous ninfos. This
>> happens even if the length of the list or the order of dependencies in the
>> list has changed:
>>
>>
>>
>>     diff = len(children) - len(then)
>>
>>     if diff:
>>
>>         # The old and new dependency lists are different lengths.
>>
>>         # This always indicates that the Node must be rebuilt.
>>
>>         # We also extend the old dependency list with enough None
>>
>>         # entries to equal the new dependency list, for the benefit
>>
>>         # of the loop below that updates node information.
>>
>>         then.extend([None] * diff)
>>
>>         if t: Trace(': old %s new %s' % (len(then), len(children)))
>>
>>         result = True
>>
>>
>>
>>     for child, prev_ni in zip(children, then):
>>
>>         if child.changed_since_last_build(self, prev_ni):
>>
>>             if t: Trace(': %s changed' % child)
>>
>>             result = True
>>
>>
>>
>> The implementation of `changed_since_last_build` on FS.File nodes for
>> MD5-timestamp overwrites the csig of a node whenever the timestamp matches:
>>
>>
>>
>>     def changed_timestamp_then_content(self, target, prev_ni):
>>
>>         if not self.changed_timestamp_match(target, prev_ni):
>>
>>             try:
>>
>>                 self.get_ninfo().csig = prev_ni.csig
>>
>>             except AttributeError:
>>
>>                 pass
>>
>>             return False
>>
>>         return self.changed_content(target, prev_ni)
>>
>>
>>
>> If `prev_ni` does not correspond to the current node, the timestamps are
>> checked anyway. If the timestamps happen to match (one second granularity
>> is easily coarse enough for this to be possible), then the node’s current
>> csig is blindly overwritten by the csig pulled from `prev_ni`--even if it
>> belongs to a completely different node!
>>
>>
>>
>> In our setup, which uses CacheDir, this led to bad csigs and a bad
>> “cachedir_bsig”, which led to retrieving the wrong file from the cache
>> directory when the set of dependencies for a node changed.
>>
>>
>>
>> Can someone confirm this a SCons bug? We're switching back to the basic
>> MD5 decider for now. We'll also spend some time and attempt to fix it as
>> soon as possible. (Note that we’re on 2.3.6, but I checked the BitBucket
>> repo and the current version appears to have the same issue.)
>>
>>
>>
>> _______________________________________________
>> Scons-users mailing list
>> Scons-users at scons.org
>> https://pairlist4.pair.net/mailman/listinfo/scons-users
>>
>>
>
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