optimisation

Avoid an unncessary STM transaction. This will happen when the worker
pool is not completely full of the new stage, which is the common case.

In the uncommon case, this adds only a tiny bit of overhead for the
extra traversal of the worker pool. And the thread is going to block
for some time anyway.

Annex/Concurrent.hs

@@ -100,8 +100,8 @@ enteringStage newstage a = Annex.getState Annex.workers >>= \case
  - ActiveWorker with an IdleWorker.
  -
  - Must avoid a deadlock if all worker threads end up here at the same
- - time, or if there are no suitable IdleWorkers left. So we first
- - replace our ActiveWorker with an IdleWorker in the pool, to allow
+ - time, or if there are no suitable IdleWorkers left. So if necessary
+ - we first replace our ActiveWorker with an IdleWorker in the pool, to allow
  - some other thread to use it, before waiting for a suitable IdleWorker
  - for us to use.
  -
@@ -112,17 +112,26 @@ enteringStage newstage a = Annex.getState Annex.workers >>= \case
  -}
 changeStageTo :: ThreadId -> TMVar (WorkerPool AnnexState) -> WorkerStage -> Annex (Maybe WorkerStage)
 changeStageTo mytid tv newstage = liftIO $
-	replaceidle >>= maybe (return Nothing) waitidle
+	replaceidle >>= maybe
+		(return Nothing)
+		(either waitidle (return . Just))
   where
 	replaceidle = atomically $ do
 		pool <- takeTMVar tv
 		if memberStage newstage (usedStages pool)
 			then case removeThreadIdWorkerPool mytid pool of
 				Just ((myaid, oldstage), pool')
-					| oldstage /= newstage -> do
+					| oldstage /= newstage -> case getIdleWorkerSlot newstage pool' of
+						Nothing -> do
 							putTMVar tv $
 								addWorkerPool (IdleWorker oldstage) pool'
-						return $ Just (myaid, oldstage)
+							return $ Just $ Left (myaid, oldstage)
+						Just pool'' -> do
+							-- optimisation
+							putTMVar tv $
+								addWorkerPool (IdleWorker oldstage) $
+									addWorkerPool (ActiveWorker myaid newstage) pool''
+							return $ Just $ Right oldstage
 					| otherwise -> do
 						putTMVar tv pool
 						return Nothing
@@ -157,11 +166,14 @@ waitInitialWorkerSlot tv = do
 			return v
 
 waitIdleWorkerSlot :: WorkerStage -> WorkerPool Annex.AnnexState -> STM (WorkerPool Annex.AnnexState)
-waitIdleWorkerSlot wantstage pool = do
+waitIdleWorkerSlot wantstage = maybe retry return . getIdleWorkerSlot wantstage
+
+getIdleWorkerSlot :: WorkerStage -> WorkerPool Annex.AnnexState -> Maybe (WorkerPool Annex.AnnexState)
+getIdleWorkerSlot wantstage pool = do
 	l <- findidle [] (workerList pool)
 	return $ pool { workerList = l }
   where
-	findidle _ [] = retry
-	findidle c ((IdleWorker stage):rest) | stage == wantstage =
-		return (c ++ rest)
+	findidle _ [] = Nothing
+	findidle c ((IdleWorker stage):rest)
+		| stage == wantstage = Just (c ++ rest)
 	findidle c (w:rest) = findidle (w:c) rest