use a TMVar

SampleMVar won't work; between getting the current value and changing
it, another thread could made a change, which would get lost.

TMVar works well; this update situation is handled by atomic transactions.

Assistant.hs

@@ -53,6 +53,9 @@
  - CommitChan: (STM TChan)
  - 	Commits are indicated by writing to this channel. The pusher reads
  - 	from it.
+ - FailedPushMap (STM TMVar)
+ - 	Failed pushes are indicated by writing to this TMVar. The push
+ - 	retrier blocks until they're available.
  -}
 
 module Assistant where
@@ -89,10 +92,10 @@ startDaemon assistant foreground
 			liftIO $ a $ do
 				changechan <- newChangeChan
 				commitchan <- newCommitChan
-				pushchan <- newFailedPushChan
+				pushmap <- newFailedPushMap
 				_ <- forkIO $ commitThread st changechan commitchan
-				_ <- forkIO $ pushThread st commitchan pushchan
-				_ <- forkIO $ pushRetryThread st pushchan
+				_ <- forkIO $ pushThread st commitchan pushmap
+				_ <- forkIO $ pushRetryThread st pushmap
 				_ <- forkIO $ mergeThread st
 				_ <- forkIO $ daemonStatusThread st dstatus
 				_ <- forkIO $ sanityCheckerThread st dstatus changechan

Assistant/Pushes.hs

@@ -8,30 +8,39 @@
 module Assistant.Pushes where
 
 import Common.Annex
-import Control.Concurrent.SampleVar
 
+import Control.Concurrent.STM
 import Data.Time.Clock
 import qualified Data.Map as M
 
 {- Track the most recent push failure for each remote. -}
 type PushMap = M.Map Remote UTCTime
-type FailedPushes = SampleVar PushMap
+type FailedPushMap = TMVar PushMap
 
-newFailedPushChan :: IO FailedPushChan
-newFailedPushChan = newEmptySampleVar
+{- The TMVar starts empty, and is left empty when there are no
+ - failed pushes. This way we can block until there are some failed pushes.
+ -}
+newFailedPushMap :: IO FailedPushMap
+newFailedPushMap = atomically newEmptyTMVar
 
-{- Gets all failed pushes. Blocks until set. -}
-getFailedPushes :: FailedPushChan -> IO PushMap
-getFailedPushes = readSampleVar
+{- Blocks until there are failed pushes.
+ - Returns Remotes whose pushes failed a given time duration or more ago.
+ - (This may be an empty list.) -}
+getFailedPushesBefore :: FailedPushMap -> NominalDiffTime -> IO [Remote]
+getFailedPushesBefore v duration = do
+	m <- atomically $ readTMVar v
+	now <- getCurrentTime
+	return $ M.keys $ M.filter (not . toorecent now) m
+	where
+		toorecent now time = now `diffUTCTime` time < duration
 
-{- Sets all failed pushes to passed PushMap -}
-setFailedPushes :: FailedPushChan -> PushMap -> IO ()
-setFailedPushes = writeSampleVar
-
-{- Indicates a failure to push to a single remote. -}
-failedPush :: FailedPushChan -> Remote -> IO ()
-failedPush c r = 
-
-{- Indicates that a remote was pushed to successfully. -}
-successfulPush :: FailedPushChan -> Remote -> IO ()
-successfulPush c r = 
+{- Modifies the map. -}
+changeFailedPushMap :: FailedPushMap -> (PushMap -> PushMap) -> IO ()
+changeFailedPushMap v a = atomically $
+	store . a . fromMaybe M.empty =<< tryTakeTMVar v
+	where
+ 		{- tryTakeTMVar empties the TMVar; refill it only if
+		 - the modified map is not itself empty -}
+		store m
+			| m == M.empty = noop
+			| otherwise = putTMVar v $! m

Assistant/Threads/Pusher.hs

@@ -17,27 +17,23 @@ import Utility.ThreadScheduler
 import Utility.Parallel
 
 import Data.Time.Clock
+import qualified Data.Map as M
 
 {- This thread retries pushes that failed before. -}
-pushRetryThread :: ThreadState -> FailedPushChan -> IO ()
-pushRetryThread st pushchan = runEvery (Seconds halfhour) $ do
+pushRetryThread :: ThreadState -> FailedPushMap -> IO ()
+pushRetryThread st pushmap = runEvery (Seconds halfhour) $ do
 	-- We already waited half an hour, now wait until there are failed
 	-- pushes to retry.
-	pushes <- getFailedPushes pushchan
-	-- Check times, to avoid repushing a push that's too new.
-	now <- getCurrentTime
-	let (newpushes, oldpushes) = partition (toorecent now . failedTimeStamp) pushes
-	unless (null newpushes) $
-		refillFailedPushes pushchan newpushes
-	unless (null oldpushes) $
-		pushToRemotes now st pushchan $ map failedRemote oldpushes
+	topush <- getFailedPushesBefore pushmap (fromIntegral halfhour)
+	unless (null topush) $ do
+		now <- getCurrentTime
+		pushToRemotes now st pushmap topush
 	where
 		halfhour = 1800
-		toorecent now time = now `diffUTCTime` time < fromIntegral halfhour
 
 {- This thread pushes git commits out to remotes soon after they are made. -}
-pushThread :: ThreadState -> CommitChan -> FailedPushChan -> IO ()
-pushThread st commitchan pushchan = do
+pushThread :: ThreadState -> CommitChan -> FailedPushMap -> IO ()
+pushThread st commitchan pushmap = do
 	remotes <- runThreadState st $ Command.Sync.syncRemotes []
 	runEvery (Seconds 2) $ do
 		-- We already waited two seconds as a simple rate limiter.
@@ -46,7 +42,7 @@ pushThread st commitchan pushchan = do
 		-- Now see if now's a good time to push.
 		now <- getCurrentTime
 		if shouldPush now commits
-			then pushToRemotes now st pushchan remotes
+			then pushToRemotes now st pushmap remotes
 			else refillCommits commitchan commits
 
 {- Decide if now is a good time to push to remotes.
@@ -65,23 +61,27 @@ shouldPush _now commits
  -
  - Avoids running possibly long-duration commands in the Annex monad, so
  - as not to block other threads. -}
-pushToRemotes :: UTCTime -> ThreadState -> FailedPushChan -> [Remote] -> IO ()
-pushToRemotes now st pushchan remotes = do
+pushToRemotes :: UTCTime -> ThreadState -> FailedPushMap -> [Remote] -> IO ()
+pushToRemotes now st pushmap remotes = do
 	(g, branch) <- runThreadState st $
 		(,) <$> fromRepo id <*> Command.Sync.currentBranch
 	go True branch g remotes
 	where
 		go shouldretry branch g rs = do
 			Command.Sync.updateBranch (Command.Sync.syncBranch branch) g
-			failed <- inParallel (push g branch) rs
-			unless (null failed) $
-				if shouldretry
-					then retry branch g rs
-					else refillFailedPushes pushchan $
-						map (`FailedPush` now) failed
+			(succeeded, failed) <- inParallel (push g branch) rs
+			changeFailedPushMap pushmap $ \m ->
+				M.union (makemap failed) $
+					M.difference m (makemap succeeded)
+			unless (null failed || not shouldretry) $
+				retry branch g failed
+
+		makemap l = M.fromList $ zip l (repeat now)
+
 		push g branch remote =
 			ifM (Command.Sync.pushBranch remote branch g)
 				( exitSuccess, exitFailure)
+
 		retry branch g rs = do
 			runThreadState st $ manualPull branch rs
 			go False branch g rs

Utility/Parallel.hs

@@ -10,11 +10,13 @@ module Utility.Parallel where
 import Common
 
 {- Runs an action in parallel with a set of values.
- - Returns values that caused the action to fail. -}
-inParallel :: (v -> IO ()) -> [v] -> IO [v]
+ - Returns the values partitioned into ones with which the action succeeded,
+ - and ones with which it failed. -}
+inParallel :: (v -> IO ()) -> [v] -> IO ([v], [v])
 inParallel a l = do
 	pids <- mapM (forkProcess . a) l
 	statuses <- mapM (getProcessStatus True False) pids
-	return $ map fst $ filter (failed . snd) $ zip l statuses
+	return $ reduce $ partition (succeeded . snd) $ zip l statuses
 	where
-		failed v = v /= Just (Exited ExitSuccess)
+		succeeded v = v == Just (Exited ExitSuccess)
+		reduce (x,y) = (map fst x, map fst y)