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

[William Blevins]

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.)
>
>
>
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>
>
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