avoid STM deadlock onredundant call to changeStageTo

I couldn't find a way to avoid the deadlock w/o rewriting it to clearly
not have one. I'm not quite sure what was the actual cause of the
deadlock.

This makes me unsure how I now know it clearly doesn't have a
deadlock. But, it was easy to reproduce before (just call it twice in a
row) and doesn't happen now.

CmdLine/Action.hs

@@ -61,14 +61,14 @@ commandAction start = Annex.getState Annex.concurrency >>= \case
   where
 	runconcurrent n = do
 		tv <- Annex.getState Annex.workers
-		workerst <- waitWorkerSlot n (== PerformStage) tv
+		workerst <- waitWorkerSlot n PerformStage tv
 		aid <- liftIO $ async $ snd <$> Annex.run workerst
 			(concurrentjob workerst)
 		liftIO $ atomically $ do
 			pool <- takeTMVar tv
 			let !pool' = addWorkerPool (ActiveWorker aid PerformStage) pool
 			putTMVar tv pool'
-		void $ liftIO $ forkIO $ do
+		void $ liftIO $ forkIO $ debugLocks $ do
 			-- accountCommandAction will usually catch
 			-- exceptions. Just in case, fall back to the
 			-- original workerst.
@@ -122,8 +122,8 @@ commandAction start = Annex.getState Annex.concurrency >>= \case
 -- pool, and return its state.
 --
 -- If the pool is unallocated, it will be allocated to the specified size.
-waitWorkerSlot :: Int -> (WorkerStage -> Bool) -> TMVar (WorkerPool Annex.AnnexState) -> Annex (Annex.AnnexState)
+waitWorkerSlot :: Int -> WorkerStage -> TMVar (WorkerPool Annex.AnnexState) -> Annex Annex.AnnexState
-waitWorkerSlot n wantstage tv =
+waitWorkerSlot n wantstage tv = debugLocks $
 	join $ liftIO $ atomically $ waitWorkerSlot' wantstage tv >>= \case
 		Nothing -> return $ do
 			-- Generate the remote list now, to avoid
@@ -142,26 +142,30 @@ waitWorkerSlot n wantstage tv =
  where
 	findidle st _ [] = (st, WorkerPool [])
 	findidle _ c ((IdleWorker st stage):rest) 
-		| wantstage stage = (st, WorkerPool (c ++ rest))
+		| stage == wantstage = (st, WorkerPool (c ++ rest))
 	findidle st c (w:rest) = findidle st (w:c) rest
 
--- | STM action that waits until there's an idle worker in the worker pool.
+-- | STM action that waits until there's an idle worker in the worker pool,
+-- removes it from the pool, and returns its state.
 --
 -- If the worker pool is not already allocated, returns Nothing.
-waitWorkerSlot' :: (WorkerStage -> Bool) -> TMVar (WorkerPool Annex.AnnexState) -> STM (Maybe (Annex.AnnexState))
+waitWorkerSlot' :: WorkerStage -> TMVar (WorkerPool Annex.AnnexState) -> STM (Maybe (Annex.AnnexState))
 waitWorkerSlot' wantstage tv =
 	takeTMVar tv >>= \case
 		UnallocatedWorkerPool -> do
 			putTMVar tv UnallocatedWorkerPool 
 			return Nothing
 		WorkerPool l -> do
-			(st, pool') <- findidle [] l
+			(st, pool') <- waitWorkerSlot'' wantstage l
 			putTMVar tv pool'
 			return $ Just st
- where
+
+waitWorkerSlot'' :: WorkerStage -> [Worker Annex.AnnexState] -> STM (Annex.AnnexState, WorkerPool Annex.AnnexState)
+waitWorkerSlot'' wantstage = findidle []
+  where
 	findidle _ [] = retry
 	findidle c ((IdleWorker st stage):rest) 
-		| wantstage stage = return (st, WorkerPool (c ++ rest))
+		| stage == wantstage = return (st, WorkerPool (c ++ rest))
 	findidle c (w:rest) = findidle (w:c) rest
 
 {- Waits for all worker threads to finish and merges their AnnexStates
@@ -185,24 +189,25 @@ finishCommandActions = do
  - for each stage. So, an idle worker with the desired stage is found in 
  - the pool (waiting if necessary for one to become idle), and the stages
  - of it and the current thread are swapped.
+ -
+ - Noop if the current thread already has the requested stage, or if the
+ - current thread is not in the worker pool, or if concurrency is not
+ - enabled.
  -}
 changeStageTo :: WorkerStage -> Annex ()
-changeStageTo newstage = do
+changeStageTo newstage = debugLocks $ do
 	mytid <- liftIO myThreadId
 	tv <- Annex.getState Annex.workers
-	liftIO $ atomically $ waitWorkerSlot' (== newstage) tv >>= \case
+	liftIO $ atomically $ do
-		Just idlest -> do
+		pool <- takeTMVar tv
-			pool <- takeTMVar tv
+		case removeThreadIdWorkerPool mytid pool of
-			let restorepool = addWorkerPool (IdleWorker idlest newstage) pool
+			Just ((myaid, oldstage), WorkerPool l)
-			let pool' = case removeThreadIdWorkerPool mytid pool of
+				| oldstage /= newstage -> do
-				Just ((myaid, oldstage), p) ->
+					(idlest, restpool) <- waitWorkerSlot'' newstage l
-					addWorkerPool (IdleWorker idlest oldstage) $
+					let pool' = addWorkerPool (IdleWorker idlest oldstage) $
-						addWorkerPool (ActiveWorker myaid newstage) p
+						addWorkerPool (ActiveWorker myaid newstage) restpool
-				Nothing -> restorepool
+					putTMVar tv pool'
-			putTMVar tv pool'
+			_ -> putTMVar tv pool
-		-- No worker pool is allocated, not running in concurrent
-		-- mode.
-		Nothing -> noop
 
 {- Like commandAction, but without the concurrency. -}
 includeCommandAction :: CommandStart -> CommandCleanup
@@ -306,7 +311,7 @@ allowConcurrentOutput a = do
  -
  - May be called repeatedly by the same thread without blocking. -}
 ensureOnlyActionOn :: Key -> Annex a -> Annex a
-ensureOnlyActionOn k a = 
+ensureOnlyActionOn k a = debugLocks $
 	go =<< Annex.getState Annex.concurrency
   where
 	go NonConcurrent = a