From 9785222714d65ded2274723c8b0a210c6152ea36 Mon Sep 17 00:00:00 2001
From: Joey Hess <joeyh@joeyh.name>
Date: Thu, 25 May 2017 16:02:17 -0400
Subject: Fix transfer log file locking problem when running concurrent
 transfers.

orElse is great, but was not the right thing to use here because
waitTakeLock could retry for other reasons than the lock being held,
which made tryTakeLock fail when it shouldn't.

Instead, move the code to tryTakeLock and implement waitTakeLock using
tryTakeLock and retry.

(Also, in runTransfer, when checkSaneLock fails, dropLock to avoid leaking a
lock handle.)

This commit was supported by the NSF-funded DataLad project.
---
 Utility/LockPool/STM.hs | 30 +++++++++++++++---------------
 1 file changed, 15 insertions(+), 15 deletions(-)

(limited to 'Utility')

diff --git a/Utility/LockPool/STM.hs b/Utility/LockPool/STM.hs
index d1ee0dbaf..5cb7b8834 100644
--- a/Utility/LockPool/STM.hs
+++ b/Utility/LockPool/STM.hs
@@ -49,9 +49,9 @@ type LockPool = TMVar (M.Map LockFile LockStatus)
 
 -- A shared global variable for the lockPool. Avoids callers needing to
 -- maintain state for this implementation detail.
+{-# NOINLINE lockPool #-}
 lockPool :: LockPool
 lockPool = unsafePerformIO (newTMVarIO M.empty)
-{-# NOINLINE lockPool #-}
 
 -- Updates the LockPool, blocking as necessary if another thread is holding
 -- a conflicting lock.
@@ -62,23 +62,23 @@ lockPool = unsafePerformIO (newTMVarIO M.empty)
 -- 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 pool file mode = do
-	m <- takeTMVar pool
-	v <- case M.lookup file m of
-		Just (LockStatus mode' n closelockfile)
-			| mode == LockShared && mode' == LockShared ->
-				return $ LockStatus mode (succ n) closelockfile
-			| n > 0 -> retry -- wait for lock
-		_ -> return $ LockStatus mode 1 noop
-	putTMVar pool (M.insert file v m)
-	newTMVar (pool, file)
+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 pool file mode =
-	(Just <$> waitTakeLock pool file mode)
-		`orElse`
-	return Nothing
+tryTakeLock pool file mode = do
+	m <- takeTMVar pool
+	let success v = do
+		putTMVar pool (M.insert file v m)
+		Just <$> newTMVar (pool, file)
+	case M.lookup file m of
+		Just (LockStatus mode' n closelockfile)
+			| mode == LockShared && mode' == LockShared ->
+				success $ LockStatus mode (succ n) closelockfile
+			| n > 0 -> do
+				putTMVar pool m
+				return Nothing
+		_ -> success $ LockStatus mode 1 noop
 
 -- Call after waitTakeLock or tryTakeLock, to register a CloseLockFile
 -- action to run when releasing the lock.
-- 
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