fix STM deadlock in finishCommandActions

Happened every time, because it was taking the pool TMVar while threads
were still running, and then the thread would try to switch state.

CmdLine/Action.hs

@@ -129,11 +129,12 @@ finishCommandActions = Annex.getState Annex.workers >>= \case
 	Nothing -> noop
 	Just tv -> do
 		Annex.changeState $ \s -> s { Annex.workers = Nothing }
-		pool <- liftIO $ atomically $ takeTMVar tv
-		forM_ (mapMaybe workerAsync $ workerList pool) $ \aid ->
-			liftIO (waitCatch aid) >>= \case
-				Left _ -> noop
-				Right st -> mergeState st
+		sts <- liftIO $ atomically $ do
+			pool <- readTMVar tv
+			if allIdle pool
+				then return (spareVals pool)
+				else retry
+		mapM_ mergeState sts
 
 {- Like commandAction, but without the concurrency. -}
 includeCommandAction :: CommandStart -> CommandCleanup

Types/WorkerPool.hs

@@ -17,7 +17,8 @@ data WorkerPool t = WorkerPool
 	, workerList :: [Worker t]
 	, spareVals :: [t]
 	-- ^ Normally there is one value for each IdleWorker,
-	-- but there can temporarily be fewer.
+	-- but there can temporarily be fewer values, when a thread is
+	-- changing between stages.
 	} 
 	deriving (Show)
 
@@ -130,3 +131,11 @@ deactivateWorker pool aid t = pool
 		| a == aid = IdleWorker st : rest
 		| otherwise = w : go rest
 
+allIdle :: WorkerPool t -> Bool
+allIdle pool = all idle (workerList pool)
+	-- If this does not hold, a thread must be transitioning between
+	-- states, so it's not really idle.
+	&& length (spareVals pool) == length (workerList pool)
+  where
+	idle (IdleWorker _) = True
+	idle (ActiveWorker _ _) = False