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/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