avoid concurrent threads trying to take pid lock at same time

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

Utility/LockPool/LockHandle.hs

@@ -52,33 +52,33 @@ makeLockHandle
 	:: (MonadIO m, MonadMask m)
 	=> P.LockPool
 	-> LockFile
-	-> (P.LockPool -> LockFile -> STM P.LockHandle)
-	-> (LockFile -> m FileLockOps)
+	-> (P.LockPool -> LockFile -> STM (P.LockHandle, P.FirstLock))
+	-> (LockFile -> P.FirstLock -> m FileLockOps)
 	-> m LockHandle
 makeLockHandle pool file pa fa = bracketOnError setup cleanup go
   where
 	setup = debugLocks $ liftIO $ atomically (pa pool file)
-	cleanup ph = debugLocks $ liftIO $ P.releaseLock ph
-	go ph = liftIO . mkLockHandle ph =<< fa file
+	cleanup (ph, _) = debugLocks $ liftIO $ P.releaseLock ph
+	go (ph, firstlock) = liftIO . mkLockHandle ph =<< fa file firstlock
 
 tryMakeLockHandle
 	:: (MonadIO m, MonadMask m)
 	=> P.LockPool
 	-> LockFile
-	-> (P.LockPool -> LockFile -> STM (Maybe P.LockHandle))
-	-> (LockFile -> m (Maybe FileLockOps))
+	-> (P.LockPool -> LockFile -> STM (Maybe (P.LockHandle, P.FirstLock)))
+	-> (LockFile -> P.FirstLock -> m (Maybe FileLockOps))
 	-> m (Maybe LockHandle)
 tryMakeLockHandle pool file pa fa = bracketOnError setup cleanup go
   where
 	setup = liftIO $ atomically (pa pool file)
 	cleanup Nothing = return ()
-	cleanup (Just ph) = liftIO $ P.releaseLock ph
+	cleanup (Just (ph, _)) = liftIO $ P.releaseLock ph
 	go Nothing = return Nothing
-	go (Just ph) = do
-		mfo <- fa file
+	go (Just (ph, firstlock)) = do
+		mfo <- fa file firstlock
 		case mfo of
 			Nothing -> do
-				liftIO $ cleanup (Just ph)
+				liftIO $ cleanup (Just (ph, firstlock))
 				return Nothing
 			Just fo -> liftIO $ Just <$> mkLockHandle ph fo
 
@@ -86,4 +86,4 @@ mkLockHandle :: P.LockHandle -> FileLockOps -> IO LockHandle
 mkLockHandle ph fo = do
 	atomically $ P.registerCloseLockFile ph (fDropLock fo)
 	return $ LockHandle ph fo
-	
+

Utility/LockPool/PidLock.hs

@@ -1,6 +1,6 @@
 {- Pid locks, using lock pools.
  -
- - Copyright 2015-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2021 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
@@ -26,7 +26,9 @@ import Utility.ThreadScheduler
 
 import System.IO
 import System.Posix
+import Control.Concurrent.STM
 import Data.Maybe
+import Control.Monad
 import Control.Monad.Catch
 import Control.Monad.IO.Class
 import Control.Applicative
@@ -43,13 +45,35 @@ waitLock timeout file displaymessage = makeLockHandle P.lockPool file
 	-- LockShared for STM lock, because a pid lock can be the top-level
 	-- lock with various other STM level locks gated behind it.
 	(\p f -> P.waitTakeLock p f LockShared)
-	(\f -> mk <$> F.waitLock timeout f displaymessage)
+	(\f (P.FirstLock firstlock firstlocksem) -> mk 
+		<$> if firstlock
+			then F.waitLock timeout f displaymessage $
+				void . atomically . tryPutTMVar firstlocksem . P.FirstLockSemWaited
+			else liftIO (atomically $ readTMVar firstlocksem) >>= \case
+				P.FirstLockSemWaited True -> F.alreadyLocked f
+				P.FirstLockSemTried True -> F.alreadyLocked f
+				P.FirstLockSemWaited False -> F.waitedLock timeout f displaymessage
+				P.FirstLockSemTried False -> F.waitLock timeout f displaymessage $
+					void . atomically . tryPutTMVar firstlocksem . P.FirstLockSemWaited
+	)
 
 -- Tries to take a pid lock, but does not block.
 tryLock :: LockFile -> IO (Maybe LockHandle)
 tryLock file = tryMakeLockHandle P.lockPool file
 	(\p f -> P.tryTakeLock p f LockShared)
-	(\f -> fmap mk <$> F.tryLock f)
+	(\f (P.FirstLock firstlock firstlocksem) -> fmap mk
+		<$> if firstlock
+			then do
+				lh <- F.tryLock f
+				void $ atomically $ tryPutTMVar firstlocksem 
+					(P.FirstLockSemTried (isJust lh))
+				return lh
+			else liftIO (atomically $ readTMVar firstlocksem) >>= \case
+					P.FirstLockSemWaited True -> Just <$> F.alreadyLocked f
+					P.FirstLockSemTried True -> Just <$> F.alreadyLocked f
+					P.FirstLockSemWaited False -> return Nothing
+					P.FirstLockSemTried False -> return Nothing
+	)
 
 checkLocked :: LockFile -> IO (Maybe Bool)
 checkLocked file = P.getLockStatus P.lockPool file

Utility/LockPool/Posix.hs

@@ -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.

Utility/LockPool/STM.hs

@@ -1,6 +1,6 @@
 {- STM implementation of lock pools.
  -
- - Copyright 2015-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2021 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
@@ -11,6 +11,8 @@ module Utility.LockPool.STM (
 	LockFile,
 	LockMode(..),
 	LockHandle,
+	FirstLock(..),
+	FirstLockSemVal(..),
 	waitTakeLock,
 	tryTakeLock,
 	getLockStatus,
@@ -27,8 +29,6 @@ import qualified Data.Map.Strict as M
 import Control.Concurrent.STM
 import Control.Exception
 import Control.Monad
-import Control.Applicative
-import Prelude
 
 type LockFile = RawFilePath
 
@@ -39,9 +39,20 @@ data LockMode = LockExclusive | LockShared
 -- closed.
 type LockHandle = TMVar (LockPool, LockFile, CloseLockFile)
 
+-- When a shared lock is taken, this will only be true for the first
+-- process, not subsequent processes. The first process should
+-- fill the FirstLockSem after doing any IO actions to finish lock setup
+-- and subsequent processes can block on that getting filled to know
+-- when the lock is fully set up.
+data FirstLock = FirstLock Bool FirstLockSem
+
+type FirstLockSem = TMVar FirstLockSemVal
+
+data FirstLockSemVal = FirstLockSemWaited Bool | FirstLockSemTried Bool
+
 type LockCount = Integer
 
-data LockStatus = LockStatus LockMode LockCount
+data LockStatus = LockStatus LockMode LockCount FirstLockSem
 
 type CloseLockFile = IO ()
 
@@ -62,24 +73,30 @@ lockPool = unsafePerformIO (newTMVarIO M.empty)
 -- the same shared lock should not be blocked on the exclusive lock.
 -- Keeping the whole Map in a TMVar accomplishes this, at the expense of
 -- sometimes retrying after unrelated changes in the map.
-waitTakeLock :: LockPool -> LockFile -> LockMode -> STM LockHandle
+waitTakeLock :: LockPool -> LockFile -> LockMode -> STM (LockHandle, FirstLock)
 waitTakeLock pool file mode = maybe retry return =<< tryTakeLock pool file mode
 
 -- Avoids blocking if another thread is holding a conflicting lock.
-tryTakeLock :: LockPool -> LockFile -> LockMode -> STM (Maybe LockHandle)
+tryTakeLock :: LockPool -> LockFile -> LockMode -> STM (Maybe (LockHandle, FirstLock))
 tryTakeLock pool file mode = do
 	m <- takeTMVar pool
-	let success v = do
+	let success firstlock v = do
 		putTMVar pool (M.insert file v m)
-		Just <$> newTMVar (pool, file, noop)
+		tmv <- newTMVar (pool, file, noop)
+		return (Just (tmv, firstlock))
 	case M.lookup file m of
-		Just (LockStatus mode' n)
-			| mode == LockShared && mode' == LockShared ->
-				success $ LockStatus mode (succ n)
+		Just (LockStatus mode' n firstlocksem)
+			| mode == LockShared && mode' == LockShared -> do
+				fl@(FirstLock _ firstlocksem') <- if n == 0
+					then FirstLock True <$> newEmptyTMVar
+					else pure (FirstLock False firstlocksem)
+				success fl $ LockStatus mode (succ n) firstlocksem'
 			| n > 0 -> do
 				putTMVar pool m
 				return Nothing
-		_ -> success $ LockStatus mode 1
+		_ -> do
+			firstlocksem <- newEmptyTMVar
+			success (FirstLock True firstlocksem) $ LockStatus mode 1 firstlocksem
 
 -- Call after waitTakeLock or tryTakeLock, to register a CloseLockFile
 -- action to run when releasing the lock.
@@ -101,7 +118,7 @@ getLockStatus pool file getdefault checker = do
 	v <- atomically $ do
 		m <- takeTMVar pool
 		let threadlocked = case M.lookup file m of
-			Just (LockStatus _ n) | n > 0 -> True
+			Just (LockStatus _ n _) | n > 0 -> True
 			_ -> False
 		if threadlocked
 			then do
@@ -125,10 +142,10 @@ releaseLock h = go =<< atomically (tryTakeTMVar h)
 		(m, lastuser) <- atomically $ do
 			m <- takeTMVar pool
 			return $ case M.lookup file m of
-				Just (LockStatus mode n)
+				Just (LockStatus mode n firstlocksem)
 					| n == 1 -> (M.delete file m, True)
 					| otherwise ->
-						(M.insert file (LockStatus mode (pred n)) m, False)
+						(M.insert file (LockStatus mode (pred n) firstlocksem) m, False)
 				Nothing -> (m, True)
 		() <- when lastuser closelockfile
 		atomically $ putTMVar pool m