From 5b4e5ce7e5141eac7f0bc81033d7d7676ff0008f Mon Sep 17 00:00:00 2001
From: Joey Hess <joey@kitenet.net>
Date: Mon, 4 Jun 2012 17:32:46 -0400
Subject: deletion

When a new file is annexed, a deletion event occurs when it's moved away
to be replaced by a symlink. Most of the time, there is no problimatic
race, because the same thread runs the add event as the deletion event.
So, once the symlink is in place, the deletion code won't run at all,
due to existing checks that a deleted file is really gone.

But there is a race at startup, as then the inotify thread is running
at the same time as the main thread, which does the initial tree walking
and annexing. It would be possible for the deletion inotify to run
in a perfect race with the addition, and remove the newly added symlink
from the git cache.

To solve this race, added event serialization via a MVar. We putMVar
before running each event, which blocks if an event is already running.
And when an event finishes (or crashes!), we takeMVar to free the lock.

Also, make rm -rf not spew warnings by passing --ignore-unmatch when
deleting directories.
---
 Utility/Inotify.hs | 32 ++++++++++++++++++++++----------
 1 file changed, 22 insertions(+), 10 deletions(-)

(limited to 'Utility/Inotify.hs')

diff --git a/Utility/Inotify.hs b/Utility/Inotify.hs
index 2dcc1ed64..ff3de81b1 100644
--- a/Utility/Inotify.hs
+++ b/Utility/Inotify.hs
@@ -15,6 +15,7 @@ import qualified System.Posix.Files as Files
 import System.Posix.Terminal
 import Control.Concurrent.MVar
 import System.Posix.Signals
+import Control.Exception as E
 
 type Hook = Maybe (FilePath -> IO ())
 
@@ -51,10 +52,13 @@ type Hook = Maybe (FilePath -> IO ())
 watchDir :: INotify -> FilePath -> (FilePath -> Bool) -> Hook -> Hook -> Hook -> Hook -> IO ()
 watchDir i dir ignored add addsymlink del deldir
 	| ignored dir = noop
-	| otherwise = void $ do
-		_ <- addWatch i watchevents dir go
-		mapM walk =<< filter (not . dirCruft) <$>
-			getDirectoryContents dir
+	| otherwise = do
+		mvar <- newEmptyMVar
+		void $ addWatch i watchevents dir $ \event ->
+			serialized mvar (void $ go event)
+		serialized mvar $
+			mapM_ walk =<< filter (not . dirCruft) <$>
+				getDirectoryContents dir
 	where
 		recurse d = watchDir i d ignored add addsymlink del deldir
 
@@ -117,14 +121,22 @@ watchDir i dir ignored add addsymlink del deldir
 
 		filetype t f = catchBoolIO $ t <$> getSymbolicLinkStatus (indir f)
 
+{- Uses an MVar to serialize an action, so that only one thread at a time
+ - runs it. -}
+serialized :: MVar () -> IO () -> IO ()
+serialized mvar a = void $ do
+	putMVar mvar () -- blocks if action already running
+	_ <- E.try a :: IO (Either E.SomeException ())
+	takeMVar mvar -- allow next action to run
+
 {- Pauses the main thread, letting children run until program termination. -}
 waitForTermination :: IO ()
 waitForTermination = do
-	mv <- newEmptyMVar
-	check softwareTermination mv
+	mvar <- newEmptyMVar
+	check softwareTermination mvar
 	whenM (queryTerminal stdInput) $
-		check keyboardSignal mv
-	takeMVar mv
+		check keyboardSignal mvar
+	takeMVar mvar
 	where
-		check sig mv = void $
-			installHandler sig (CatchOnce $ putMVar mv ()) Nothing
+		check sig mvar = void $
+			installHandler sig (CatchOnce $ putMVar mvar ()) Nothing
-- 
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