assistant: Detect stale git lock files at startup time, and remove them.

Extends the index.lock handling to other git lock files. I surveyed
all lock files used by git, and found more than I expected. All are
handled the same in git; it leaves them open while doing the operation,
possibly writing the new file content to the lock file, and then closes
them when done.

The gc.pid file is excluded because it won't affect the normal operation
of the assistant, and waiting for a gc to finish on startup wouldn't be
good.

All threads except the webapp thread wait on the new startup sanity checker
thread to complete, so they won't try to do things with git that fail
due to stale lock files. The webapp thread mostly avoids doing that kind of
thing itself. A few configurators might fail on lock files, but only if the
user is explicitly trying to run them. The webapp needs to start
immediately when the user has opened it, even if there are stale lock
files.

Arranging for the threads to wait on the startup sanity checker was a bit
of a bear. Have to get all the NotificationHandles set up before the
startup sanity checker runs, or they won't see its signal. Perhaps
the NotificationBroadcaster is not the best interface to have used for
this. Oh well, it works.

This commit was sponsored by Michael Jakl

Assistant/Threads/SanityChecker.hs

@@ -1,11 +1,12 @@
 {- git-annex assistant sanity checker
  -
- - Copyright 2012 Joey Hess <joey@kitenet.net>
+ - Copyright 2012, 2013 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
 module Assistant.Threads.SanityChecker (
+	sanityCheckerStartupThread,
 	sanityCheckerDailyThread,
 	sanityCheckerHourlyThread
 ) where
@@ -20,10 +21,20 @@ import Utility.ThreadScheduler
 import qualified Assistant.Threads.Watcher as Watcher
 import Utility.LogFile
 import Utility.Batch
+import Utility.NotificationBroadcaster
 import Config
+import qualified Git
+import qualified Utility.Lsof as Lsof
 
 import Data.Time.Clock.POSIX
 
+{- This thread runs once at startup, and most other threads wait for it
+ - to finish. (However, the webapp thread does not, to prevent the UI
+ - being nonresponsive.) -}
+sanityCheckerStartupThread :: NamedThread
+sanityCheckerStartupThread = namedThreadUnchecked "SanityCheckerStartup" $
+	startupCheck
+
 {- This thread wakes up hourly for inxepensive frequent sanity checks. -}
 sanityCheckerHourlyThread :: NamedThread
 sanityCheckerHourlyThread = namedThread "SanityCheckerHourly" $ forever $ do
@@ -69,6 +80,14 @@ waitForNextCheck = do
 			oneDay - truncate (now - lastcheck)
 		| otherwise = oneDay
 
+startupCheck :: Assistant ()
+startupCheck = do
+	checkStaleGitLocks
+
+	{- Notify other threads that the startup sanity check is done. -}
+	status <- getDaemonStatus
+	liftIO $ sendNotification $ startupSanityCheckNotifier status
+
 {- It's important to stay out of the Annex monad as much as possible while
  - running potentially expensive parts of this check, since remaining in it
  - will block the watcher. -}
@@ -128,6 +147,46 @@ checkLogSize n = do
   where
 	filesize f = fromIntegral . fileSize <$> liftIO (getFileStatus f)
 
+{- Detect when a git lock file exists and has no git process currently
+ - writing to it. This strongly suggests it is a stale lock file.
+ -
+ - However, this could be on a network filesystem. Which is not very safe
+ - anyway (the assistant relies on being able to check when files have
+ - no writers to know when to commit them). Just in case, when the lock
+ - file appears stale, we delay for one minute, and check its size. If
+ - the size changed, delay for another minute, and so on. This will at
+ - least work to detect is another machine is writing out a new index
+ - file, since git does so by writing the new content to index.lock.
+ -}
+checkStaleGitLocks :: Assistant ()
+checkStaleGitLocks = do
+	lockfiles <- filter (not . isInfixOf "gc.pid") 
+		. filter (".lock" `isSuffixOf`)
+		<$> (liftIO . dirContentsRecursiveSkipping (annexDir `isInfixOf`)
+			=<< liftAnnex (fromRepo Git.localGitDir))
+	checkStaleLocks lockfiles
+checkStaleLocks :: [FilePath] -> Assistant ()
+checkStaleLocks lockfiles = go =<< getsizes
+  where
+  	getsize lf = catchMaybeIO $ 
+		(\s -> (lf, fileSize s)) <$> getFileStatus lf
+  	getsizes = liftIO $ catMaybes <$> mapM getsize lockfiles
+	go [] = return ()
+	go l = ifM (liftIO $ null <$> Lsof.query ("--" : map fst l))
+		( do
+			waitforit "to check stale git lock file"
+			l' <- getsizes
+			if l' == l
+				then liftIO $ mapM_ nukeFile (map fst l)
+				else go l'
+		, do
+			waitforit "for git lock file writer"
+			go =<< getsizes
+		)
+	waitforit why = do
+		notice ["Waiting for 60 seconds", why]
+		liftIO $ threadDelaySeconds $ Seconds 60
+
 oneMegabyte :: Int
 oneMegabyte = 1000000
 
@@ -136,3 +195,4 @@ oneHour = 60 * 60
 
 oneDay :: Int
 oneDay = 24 * oneHour
+