clean up side lock files when we're done with them

There's a potential race, but it's detected and just results in the other
process failing to take the side lock, so possibly retrying one second
later on. The race window is quite narrow so the extra delay is minor.

Left the side lock files mode 666 because an interruption can leave a side
lock file created by another user for a shared repository. When this
happens, the non-owning user can't delete it (+t) but can still lock it,
and so the code falls back to acting as it did before this commit.

Utility/LockFile/PidLock.hs

@@ -39,9 +39,9 @@ import System.Directory
 
 type LockFile = FilePath
 
-data LockHandle = LockHandle FilePath FileStatus SideLockHandle
+data LockHandle = LockHandle LockFile FileStatus SideLockHandle
 
-type SideLockHandle = Maybe Posix.LockHandle
+type SideLockHandle = Maybe (LockFile, Posix.LockHandle)
 
 data PidLock = PidLock
 	{ lockingPid :: ProcessID
@@ -58,16 +58,21 @@ readPidLock :: LockFile -> IO (Maybe PidLock)
 readPidLock lockfile = (readish =<<) <$> catchMaybeIO (readFile lockfile)
 
 -- To avoid races when taking over a stale pid lock, a side lock is used.
--- This is a regular posix exclusive lock. The side lock is put in
--- /dev/shm. This will work on most any Linux system, even if its whole
--- root filesystem doesn't support posix locks.
+-- This is a regular posix exclusive lock.
 trySideLock :: LockFile -> (SideLockHandle -> IO a) -> IO a
 trySideLock lockfile a = do
 	sidelock <- sideLockFile lockfile
 	mlck <- catchDefaultIO Nothing $ 
 		withUmask nullFileMode $
 			Posix.tryLockExclusive (Just mode) sidelock
-	a mlck
+	-- Check the lock we just took, in case we opened a side lock file
+	-- belonging to another process that will have since deleted it.
+	case mlck of
+		Nothing -> a Nothing
+		Just lck -> ifM (Posix.checkSaneLock sidelock lck)
+			( a (Just (sidelock, lck))
+			, a Nothing
+			)
   where
 	-- Let all users write to the lock file in /dev/shm or /tmp,
 	-- so that other users can reuse it to take the lock.
@@ -76,6 +81,19 @@ trySideLock lockfile a = do
 	-- delete a stale lock.
 	mode = combineModes (readModes ++ writeModes)
 
+dropSideLock :: SideLockHandle -> IO ()
+dropSideLock Nothing = return ()
+dropSideLock (Just (f, h)) = do
+	-- Delete the file first, to ensure that any process that is trying
+	-- to take the side lock will only succeed once the file is
+	-- deleted, and so will be able to immediately see that it's taken
+	-- a stale lock.
+	_ <- tryIO $ removeFile f 
+	Posix.dropLock h
+
+-- The side lock is put in /dev/shm. This will work on most any
+-- Linux system, even if its whole root filesystem doesn't support posix
+-- locks. /tmp is used as a fallback.
 sideLockFile :: LockFile -> IO LockFile
 sideLockFile lockfile = do
 	f <- absPath lockfile
@@ -102,8 +120,7 @@ tryLock lockfile = trySideLock lockfile $ \sidelock -> do
 	hClose h
 	st <- getFileStatus tmp
 	let failedlock = do
-		nukeFile tmp
-		maybe noop Posix.dropLock sidelock
+		dropLock $ LockHandle tmp st sidelock
 		return Nothing
 	let tooklock = return $ Just $ LockHandle lockfile st sidelock
 	ifM (linkToLock sidelock tmp lockfile)
@@ -176,7 +193,7 @@ checkInsaneLustre dest = do
 		_ -> do
 			-- Try to clean up the extra copy we made
 			-- that has the same name. Egads.
-			tryIO $ removeFile dest
+			_ <- tryIO $ removeFile dest
 			return True
 
 -- | Waits as necessary to take a lock.
@@ -199,9 +216,7 @@ dropLock :: LockHandle -> IO ()
 dropLock (LockHandle lockfile _ sidelock) = do
 	-- Drop side lock first, at which point the pid lock will be
 	-- considered stale.
-	-- The side lock file cannot be deleted because another process may
-	-- have it open and be waiting to lock it.
-	maybe noop Posix.dropLock sidelock
+	dropSideLock sidelock
 	nukeFile lockfile
 
 getLockStatus :: LockFile -> IO LockStatus