fine-grained locking when annex.pidlock is enabled

This locking has been missing from the beginning of annex.pidlock.
It used to be possble, when two threads are doing conflicting things,
for both to run at the same time despite using locking. Seems likely
that nothing actually had a problem, but it was possible, and this
eliminates that possible source of failure.

Sponsored-by: Dartmouth College's Datalad project

Utility/LockPool/LockHandle.hs

@@ -8,7 +8,7 @@
 {-# LANGUAGE CPP #-}
 
 module Utility.LockPool.LockHandle (
-	LockHandle,
+	LockHandle(..),
 	FileLockOps(..),
 	dropLock,
 #ifndef mingw32_HOST_OS
@@ -86,4 +86,3 @@ mkLockHandle :: P.LockHandle -> FileLockOps -> IO LockHandle
 mkLockHandle ph fo = do
 	atomically $ P.registerCloseLockFile ph (fDropLock fo)
 	return $ LockHandle ph fo
-

Utility/LockPool/PidLock.hs

@@ -10,6 +10,7 @@ module Utility.LockPool.PidLock (
 	LockHandle,
 	waitLock,
 	tryLock,
+	tryLock',
 	checkLocked,
 	getLockStatus,
 	LockStatus(..),
@@ -34,34 +35,72 @@ import Control.Monad.IO.Class
 import Control.Applicative
 import Prelude
 
--- Takes a pid lock, blocking until the lock is available or the timeout.
+-- Does locking using a pid lock, blocking until the lock is available
+-- or the timeout.
+--
+-- There are two levels of locks. A STM lock is used to handle
+-- fine-grained locking amoung threads, locking a specific lockfile,
+-- but only in memory. The pid lock handles locking between processes.
+--
+-- The Seconds is how long to delay if the pid lock is held by another
+-- process.
 waitLock
 	:: (MonadIO m, MonadMask m)
-	=> Seconds
-	-> LockFile
+	=> LockFile
+	-> LockMode
+	-> Seconds
+	-> F.PidLockFile
 	-> (String -> m ())
 	-> m LockHandle
-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 (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 $
+waitLock stmlockfile lockmode timeout pidlockfile displaymessage = do
+	sl@(LockHandle ph _) <- takestmlock
+	pl <- takepidlock
+	-- When the STM lock gets dropped, also drop the pid lock.
+	liftIO $ atomically $
+		P.registerPostReleaseLock ph (dropLock pl)
+	return sl
+  where
+	takestmlock = makeLockHandle P.lockPool stmlockfile
+		(\p f -> P.waitTakeLock p f lockmode)
+		(\_ _ -> pure stmonlyflo)
+	takepidlock = makeLockHandle P.lockPool pidlockfile
+		-- LockShared because multiple threads can share the pid lock;
+		-- it remains locked until all threads using it drop
+		-- their locks.
+		(\p f -> P.waitTakeLock p f LockShared)
+		(\f (P.FirstLock firstlock firstlocksem) -> mkflo
+			<$> 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
+tryLock :: LockFile -> LockMode -> F.PidLockFile -> IO (Maybe LockHandle)
+tryLock stmlockfile lockmode pidlockfile = takestmlock >>= \case
+	Just (sl@(LockHandle ph _)) -> tryLock' pidlockfile >>= \case
+		Just pl -> do
+			liftIO $ atomically $
+				P.registerPostReleaseLock ph (dropLock pl)
+			return (Just sl)
+		Nothing -> do
+			dropLock sl
+			return Nothing
+	Nothing -> return Nothing
+  where
+	takestmlock = tryMakeLockHandle P.lockPool stmlockfile
+		(\p f -> P.tryTakeLock p f lockmode)
+		(\_ _ -> pure (Just stmonlyflo))
+
+tryLock' :: F.PidLockFile -> IO (Maybe LockHandle)
+tryLock' pidlockfile = tryMakeLockHandle P.lockPool pidlockfile
 	(\p f -> P.tryTakeLock p f LockShared)
-	(\f (P.FirstLock firstlock firstlocksem) -> fmap mk
+	(\f (P.FirstLock firstlock firstlocksem) -> fmap mkflo
 		<$> if firstlock
 			then do
 				lh <- F.tryLock f
@@ -85,8 +124,14 @@ getLockStatus file = P.getLockStatus P.lockPool file
 	(StatusLockedBy <$> getProcessID)
 	(F.getLockStatus file)
 
-mk :: F.LockHandle -> FileLockOps
-mk h = FileLockOps
+mkflo :: F.LockHandle -> FileLockOps
+mkflo h = FileLockOps
 	{ fDropLock = F.dropLock h
 	, fCheckSaneLock = \f -> F.checkSaneLock f h
 	}
+		
+stmonlyflo :: FileLockOps
+stmonlyflo = FileLockOps
+	{ fDropLock = return ()
+	, fCheckSaneLock = const (return True)
+	}

Utility/LockPool/STM.hs

@@ -19,6 +19,7 @@ module Utility.LockPool.STM (
 	releaseLock,
 	CloseLockFile,
 	registerCloseLockFile,
+	registerPostReleaseLock,
 ) where
 
 import Utility.Monad
@@ -37,7 +38,7 @@ data LockMode = LockExclusive | LockShared
 
 -- This TMVar is full when the handle is open, and is emptied when it's
 -- closed.
-type LockHandle = TMVar (LockPool, LockFile, CloseLockFile)
+type LockHandle = TMVar (LockPool, LockFile, CloseLockFile, PostReleaseLock)
 
 -- When a shared lock is taken, this will only be true for the first
 -- process, not subsequent processes. The first process should
@@ -54,8 +55,12 @@ type LockCount = Integer
 
 data LockStatus = LockStatus LockMode LockCount FirstLockSem
 
+-- Action that closes the underlying lock file.
 type CloseLockFile = IO ()
 
+-- Action that is run after the LockHandle is released.
+type PostReleaseLock = IO ()
+
 -- This TMVar is normally kept full.
 type LockPool = TMVar (M.Map LockFile LockStatus)
 
@@ -82,7 +87,7 @@ tryTakeLock pool file mode = do
 	m <- takeTMVar pool
 	let success firstlock v = do
 		putTMVar pool (M.insert file v m)
-		tmv <- newTMVar (pool, file, noop)
+		tmv <- newTMVar (pool, file, noop, noop)
 		return (Just (tmv, firstlock))
 	case M.lookup file m of
 		Just (LockStatus mode' n firstlocksem)
@@ -96,14 +101,22 @@ tryTakeLock pool file mode = do
 				return Nothing
 		_ -> do
 			firstlocksem <- newEmptyTMVar
-			success (FirstLock True firstlocksem) $ LockStatus mode 1 firstlocksem
+			success (FirstLock True firstlocksem) $
+				LockStatus mode 1 firstlocksem
 
 -- Call after waitTakeLock or tryTakeLock, to register a CloseLockFile
 -- action to run when releasing the lock.
 registerCloseLockFile :: LockHandle -> CloseLockFile -> STM ()
 registerCloseLockFile h closelockfile = do
-	(p, f, c) <- takeTMVar h
-	putTMVar h (p, f, c >> closelockfile)
+	(p, f, c, r) <- takeTMVar h
+	putTMVar h (p, f, c >> closelockfile, r)
+
+-- Call after waitTakeLock or tryTakeLock, to register a PostReleaseLock
+-- action to run after releasing the lock.
+registerPostReleaseLock :: LockHandle -> PostReleaseLock -> STM ()
+registerPostReleaseLock h postreleaselock = do
+	(p, f, c, r) <- takeTMVar h
+	putTMVar h (p, f, c, r >> postreleaselock)
 
 -- Checks if a lock is being held. If it's held by the current process,
 -- runs the getdefault action; otherwise runs the checker action.
@@ -134,11 +147,12 @@ getLockStatus pool file getdefault checker = do
 --
 -- Note that the lock pool is left empty while the CloseLockFile action
 -- is run, to avoid race with another thread trying to open the same lock
--- file.
+-- file. However, the pool is full again when the PostReleaseLock action
+-- runs.
 releaseLock :: LockHandle -> IO ()
 releaseLock h = go =<< atomically (tryTakeTMVar h)
   where
-	go (Just (pool, file, closelockfile)) = do
+	go (Just (pool, file, closelockfile, postreleaselock)) = do
 		(m, lastuser) <- atomically $ do
 			m <- takeTMVar pool
 			return $ case M.lookup file m of
@@ -147,7 +161,8 @@ releaseLock h = go =<< atomically (tryTakeTMVar h)
 					| otherwise ->
 						(M.insert file (LockStatus mode (pred n) firstlocksem) m, False)
 				Nothing -> (m, True)
-		() <- when lastuser closelockfile
+		when lastuser closelockfile
 		atomically $ putTMVar pool m
+		when lastuser postreleaselock
 	-- The LockHandle was already closed.
 	go Nothing = return ()