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avoid concurrent threads trying to take pid lock at same time

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Seem there are several races that happen when 2 threads run PidLock.tryLock
at the same time. One involves checkSaneLock of the side lock file, which may
be deleted by another process that is dropping the lock, causing checkSaneLock
to fail. And even with the deletion disabled, it can still fail, Probably due
to linkToLock failing when a second thread overwrites the lock file.

The same can happen when 2 processes do, but then one process just fails
to take the lock, which is fine. But with 2 threads, some actions where failing
even though the process as a whole had the pid lock held.

Utility.LockPool.PidLock already maintains a STM lock, and since it uses
LockShared, 2 threads can hold the pidlock at the same time, and when
the first thread drops the lock, it will remain held by the second
thread, and so the pid lock file should not get deleted until the last
thread to hold it drops the lock. Which is the right behavior, and why a
LockShared STM lock is used in the first place.

The problem is that each time it takes the STM lock, it then also calls
PidLock.tryLock. So that was getting called repeatedly and concurrently.

Fixed by noticing when the shared lock is already held, and stop calling
PidLock.tryLock again, just use the pid lock that already exists then.

Also, LockFile.PidLock.tryLock was deleting the pid lock when it failed
to take the lock, which was entirely wrong. It should only drop the side
lock.

Sponsored-by: Dartmouth College's Datalad project
This commit is contained in:
Joey Hess 2021-12-01 15:22:31 -04:00
parent 66b2536ea0
commit ed0afbc36b
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GPG key ID: DB12DB0FF05F8F38
7 changed files with 113 additions and 42 deletions
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8
Utility/LockPool/Posix.hs
View file

@ -35,25 +35,25 @@ import Prelude
lockShared :: Maybe FileMode -> LockFile -> IO LockHandle
lockShared mode file = makeLockHandle P.lockPool file
(\p f -> P.waitTakeLock p f LockShared)
(\f -> mk <$> F.lockShared mode f)
(\f _ -> mk <$> F.lockShared mode f)
-- Takes an exclusive lock, blocking until the lock is available.
lockExclusive :: Maybe FileMode -> LockFile -> IO LockHandle
lockExclusive mode file = makeLockHandle P.lockPool file
(\p f -> P.waitTakeLock p f LockExclusive)
(\f -> mk <$> F.lockExclusive mode f)
(\f _ -> mk <$> F.lockExclusive mode f)
-- Tries to take a shared lock, but does not block.
tryLockShared :: Maybe FileMode -> LockFile -> IO (Maybe LockHandle)
tryLockShared mode file = tryMakeLockHandle P.lockPool file
(\p f -> P.tryTakeLock p f LockShared)
(\f -> fmap mk <$> F.tryLockShared mode f)
(\f _ -> fmap mk <$> F.tryLockShared mode f)
-- Tries to take an exclusive lock, but does not block.
tryLockExclusive :: Maybe FileMode -> LockFile -> IO (Maybe LockHandle)
tryLockExclusive mode file = tryMakeLockHandle P.lockPool file
(\p f -> P.tryTakeLock p f LockExclusive)
(\f -> fmap mk <$> F.tryLockExclusive mode f)
(\f _ -> fmap mk <$> F.tryLockExclusive mode f)
-- Returns Nothing when the file doesn't exist, for cases where
-- that is different from it not being locked.