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Differential D11781

dirstate: add a comment about a racy piece of code during updates
ClosedPublic

Authored by marmoute on Nov 24 2021, 6:11 AM.

Details

Reviewers
Alphare
Group Reviewers
hg-reviewers
Commits
rHGc655483ea6e2: dirstate: add a comment about a racy piece of code during updates
Summary

This is a bit that is not really correct but works "fine" in practice. Let us
write the details down so that people stop wondering how that logic might be
correct… It is not.

Diff Detail

Repository
rHG Mercurial
Lint
Automatic diff as part of commit; lint not applicable.
Unit
Automatic diff as part of commit; unit tests not applicable.

Event Timeline

marmoute created this revision.Nov 24 2021, 6:11 AM
Alphare accepted this revision.Nov 24 2021, 8:04 AM
This revision is now accepted and ready to land.Nov 24 2021, 8:04 AM
marmoute edited the summary of this revision. (Show Details)Nov 30 2021, 6:53 PM
marmoute updated this revision to Diff 31223.

Revision Contents
Changeset List

PathPackages
Mmercurial/merge.py (28 lines)

Diff 31243

mercurial/merge.py

atomictemp = ui.configbool(b"experimental", b"update.atomic-file") atomictemp = ui.configbool(b"experimental", b"update.atomic-file")
size = wfctx.write( size = wfctx.write(
fctx(f).data(), fctx(f).data(),
flags, flags,
backgroundclose=True, backgroundclose=True,
atomictemp=atomictemp, atomictemp=atomictemp,
) )
if wantfiledata: 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() s = wfctx.lstat()
mode = s.st_mode mode = s.st_mode
mtime = timestamp.mtime_of(s) mtime = timestamp.mtime_of(s)
# for dirstate.update_file's parentfiledata argument: # for dirstate.update_file's parentfiledata argument:
filedata[f] = (mode, size, mtime) filedata[f] = (mode, size, mtime)
if i == 100: if i == 100:
yield False, (i, f) yield False, (i, f)
i = 0 i = 0