[Updated] D11781: dirstate: add a comment about a racy piece of code during updates
marmoute (Pierre-Yves David)
phabricator at mercurial-scm.org
Tue Nov 30 23:53:41 UTC 2021
marmoute edited the summary of this revision.
marmoute updated this revision to Diff 31223.
REPOSITORY
rHG Mercurial
CHANGES SINCE LAST UPDATE
https://phab.mercurial-scm.org/D11781?vs=31103&id=31223
BRANCH
default
CHANGES SINCE LAST ACTION
https://phab.mercurial-scm.org/D11781/new/
REVISION DETAIL
https://phab.mercurial-scm.org/D11781
AFFECTED FILES
mercurial/merge.py
CHANGE DETAILS
diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -1404,6 +1404,34 @@
atomictemp=atomictemp,
)
if wantfiledata:
+ # XXX note that there is a race window between the time we
+ # write the clean data into the file and we stats it. So another
+ # writing process meddling with the file content right after we
+ # wrote it could cause bad stat data to be gathered.
+ #
+ # They are 2 data we gather here
+ # - the mode:
+ # That we actually just wrote, we should not need to read
+ # it from disk, (except not all mode might have survived
+ # the disk round-trip, which is another issue: we should
+ # not depends on this)
+ # - the mtime,
+ # On system that support nanosecond precision, the mtime
+ # could be accurate enough to tell the two writes appart.
+ # However gathering it in a racy way make the mtime we
+ # gather "unreliable".
+ #
+ # (note: we get the size from the data we write, which is sane)
+ #
+ # So in theory the data returned here are fully racy, but in
+ # practice "it works mostly fine".
+ #
+ # Do not be surprised if you end up reading this while looking
+ # for the causes of some buggy status. Feel free to improve
+ # this in the future, but we cannot simply stop gathering
+ # information. Otherwise `hg status` call made after a large `hg
+ # update` runs would have to redo a similar amount of work to
+ # restore and compare all files content.
s = wfctx.lstat()
mode = s.st_mode
mtime = timestamp.mtime_of(s)
To: marmoute, #hg-reviewers, Alphare
Cc: mercurial-patches
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