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narrow the race where a file gets modified before update-index

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Check just before running update-index if the worktree file's content is
still the same, don't update it when it's been modified. This narrows
the race window a lot, from possibly minutes or hours, to seconds or
less.

(Use replaceFile so that the worktree update happens atomically,
allowing the InodeCache of the new worktree file to itself be gathered
w/o any other race.)

This doesn't eliminate the race; it can still occur in the window before
update-index runs. When annex.queue is large, a lot of files will be
statted by the checks, and so the window may still be large enough to be a
problem.

When only a few files are being processed, the window is as small as it
is in the race where a modification gets overwritten by git-annex when
it updates the worktree. Or maybe as small as whatever race git
checkout/pull/merge may have when the worktree gets modified during it.
Still, I've kept a todo about this race.

This commit was supported by the NSF-funded DataLad project.
This commit is contained in:
Joey Hess 2018-08-16 15:15:20 -04:00
parent 6a445dc086
commit 82a239675f
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GPG key ID: DB12DB0FF05F8F38
5 changed files with 52 additions and 29 deletions
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19
Annex/Content.hs
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@ -593,12 +593,13 @@ populatePointerFile restage k obj f = go =<< liftIO (isPointerFile f)
go (Just k') | k == k' = do
destmode <- liftIO $ catchMaybeIO $ fileMode <$> getFileStatus f
liftIO $ nukeFile f
ifM (linkOrCopy k obj f destmode)
( do
thawContent f
restagePointerFile restage f
, liftIO $ writePointerFile f k destmode
)
ic <- replaceFile f $ \tmp -> do
ifM (linkOrCopy k obj tmp destmode)
( thawContent tmp
, liftIO $ writePointerFile tmp k destmode
)
withTSDelta (liftIO . genInodeCache tmp)
maybe noop (restagePointerFile restage f) ic
go _ = return ()
data LinkAnnexResult = LinkAnnexOk | LinkAnnexFailed | LinkAnnexNoop
@ -839,8 +840,10 @@ removeAnnex (ContentRemovalLock key) = withObjectLoc key remove removedirect
mode <- liftIO $ catchMaybeIO $ fileMode <$> getFileStatus file
secureErase file
liftIO $ nukeFile file
liftIO $ writePointerFile file key mode
restagePointerFile (Restage True) file
ic <- replaceFile file $ \tmp -> do
liftIO $ writePointerFile tmp key mode
withTSDelta (liftIO . genInodeCache tmp)
maybe noop (restagePointerFile (Restage True) file) ic
-- Modified file, so leave it alone.
-- If it was a hard link to the annex object,
-- that object might have been frozen as part of the