ed0afbc36b
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
89 lines
2.4 KiB
Haskell
89 lines
2.4 KiB
Haskell
{- Handles for lock pools.
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-
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- Copyright 2015-2020 Joey Hess <id@joeyh.name>
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-
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- License: BSD-2-clause
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-}
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{-# LANGUAGE CPP #-}
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module Utility.LockPool.LockHandle (
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LockHandle,
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FileLockOps(..),
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dropLock,
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#ifndef mingw32_HOST_OS
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checkSaneLock,
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#endif
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makeLockHandle,
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tryMakeLockHandle,
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) where
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import qualified Utility.LockPool.STM as P
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import Utility.LockPool.STM (LockFile)
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import Utility.DebugLocks
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import Control.Concurrent.STM
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import Control.Monad.Catch
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import Control.Monad.IO.Class (liftIO, MonadIO)
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import Control.Applicative
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import Prelude
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data LockHandle = LockHandle P.LockHandle FileLockOps
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data FileLockOps = FileLockOps
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{ fDropLock :: IO ()
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#ifndef mingw32_HOST_OS
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, fCheckSaneLock :: LockFile -> IO Bool
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#endif
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}
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dropLock :: LockHandle -> IO ()
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dropLock (LockHandle ph _) = P.releaseLock ph
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#ifndef mingw32_HOST_OS
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checkSaneLock :: LockFile -> LockHandle -> IO Bool
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checkSaneLock lockfile (LockHandle _ flo) = fCheckSaneLock flo lockfile
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#endif
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-- Take a lock, by first updating the lock pool, and then taking the file
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-- lock. If taking the file lock fails for any reason, take care to
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-- release the lock in the lock pool.
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makeLockHandle
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:: (MonadIO m, MonadMask m)
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=> P.LockPool
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-> LockFile
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-> (P.LockPool -> LockFile -> STM (P.LockHandle, P.FirstLock))
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-> (LockFile -> P.FirstLock -> m FileLockOps)
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-> m LockHandle
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makeLockHandle pool file pa fa = bracketOnError setup cleanup go
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where
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setup = debugLocks $ liftIO $ atomically (pa pool file)
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cleanup (ph, _) = debugLocks $ liftIO $ P.releaseLock ph
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go (ph, firstlock) = liftIO . mkLockHandle ph =<< fa file firstlock
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tryMakeLockHandle
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:: (MonadIO m, MonadMask m)
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=> P.LockPool
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-> LockFile
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-> (P.LockPool -> LockFile -> STM (Maybe (P.LockHandle, P.FirstLock)))
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-> (LockFile -> P.FirstLock -> m (Maybe FileLockOps))
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-> m (Maybe LockHandle)
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tryMakeLockHandle pool file pa fa = bracketOnError setup cleanup go
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where
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setup = liftIO $ atomically (pa pool file)
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cleanup Nothing = return ()
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cleanup (Just (ph, _)) = liftIO $ P.releaseLock ph
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go Nothing = return Nothing
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go (Just (ph, firstlock)) = do
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mfo <- fa file firstlock
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case mfo of
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Nothing -> do
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liftIO $ cleanup (Just (ph, firstlock))
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return Nothing
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Just fo -> liftIO $ Just <$> mkLockHandle ph fo
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mkLockHandle :: P.LockHandle -> FileLockOps -> IO LockHandle
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mkLockHandle ph fo = do
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atomically $ P.registerCloseLockFile ph (fDropLock fo)
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return $ LockHandle ph fo
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