make WorkerStage an open type

Rather than limiting it to PerformStage and CleanupStage, this opens it
up so any number of stages can be added as needed by commands.

Each concurrent command has a set of stages that it uses, and only
transitions between those can block waiting for a free slot in the
worker pool. Calling enteringStage for some other stage does not block,
and has very little overhead.

Note that while before the Annex state was duplicated on the first call
to commandAction, this now happens earlier, in startConcurrency.
That means that seek stage actions should that use startConcurrency
and then modify Annex state won't modify the state of worker threads
they then start. I audited all of them, and only Command.Seek
did so; prepMerge changes the working directory and so has to come
before startConcurrency.

Also, the remote list is built before duplicating the state, which means
that it gets built earlier now than it used to. This would only have an
effect of making commands that end up not needing to perform any actions
unncessary build the remote list (only when they're run with concurrency
enable), but that's a minor overhead compared to commands seeking
through the work tree and determining they don't need to do anything.

Annex/Concurrent.hs

@@ -1,6 +1,6 @@
 {- git-annex concurrent state
  -
- - Copyright 2015 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2019 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -9,9 +9,15 @@ module Annex.Concurrent where
 
 import Annex
 import Annex.Common
-import Annex.Action
 import qualified Annex.Queue
+import Annex.CatFile
+import Annex.CheckAttr
+import Annex.HashObject
+import Annex.CheckIgnore
+import Types.WorkerPool
 
+import Control.Concurrent
+import Control.Concurrent.STM
 import qualified Data.Map as M
 
 {- Allows forking off a thread that uses a copy of the current AnnexState
@@ -59,3 +65,83 @@ mergeState st = do
 		uncurry addCleanup
 	Annex.Queue.mergeFrom st'
 	changeState $ \s -> s { errcounter = errcounter s + errcounter st' }
+
+{- Stops all long-running git query processes. -}
+stopCoProcesses :: Annex ()
+stopCoProcesses = do
+	catFileStop
+	checkAttrStop
+	hashObjectStop
+	checkIgnoreStop
+
+{- Runs an action and makes the current thread have the specified stage
+ - while doing so. If too many other threads are running in the specified
+ - stage, waits for one of them to become idle.
+ -
+ - 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.
+ -
+ - Also a noop if the stage is not one of the stages that the worker pool
+ - uses.
+ -
+ - The pool needs to continue to contain the same number of worker threads
+ - 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.
+ -}
+enteringStage :: WorkerStage -> Annex a -> Annex a
+enteringStage newstage a = do
+	mytid <- liftIO myThreadId
+	tv <- Annex.getState Annex.workers
+	let setup = changeStageTo mytid tv newstage
+	let cleanup Nothing = noop
+	let cleanup (Just oldstage) = changeStageTo mytid tv oldstage
+	bracket setup cleanup (const a)
+
+changeStageTo :: ThreadId -> TMVar (WorkerPool AnnexState) -> WorkerStage -> Annex (Maybe WorkerStage)
+changeStageTo mytid tv newstage = liftIO $ atomically $ do
+	pool <- takeTMVar tv
+	case pool of
+		WorkerPool usedstages _
+			| memberStage newstage usedstages ->
+				case removeThreadIdWorkerPool mytid pool of
+					Just ((myaid, oldstage), WorkerPool usedstages' l)
+						| oldstage /= newstage -> do
+							(idlest, restpool) <- waitWorkerSlot usedstages' newstage l
+							let pool' = addWorkerPool (IdleWorker idlest oldstage) $
+								addWorkerPool (ActiveWorker myaid newstage) restpool
+							putTMVar tv pool'
+							return (Just oldstage)
+					_ -> do
+						putTMVar tv pool
+						return Nothing
+		_ -> do
+			putTMVar tv pool
+			return Nothing
+
+-- | Waits until there's an idle worker in the worker pool
+-- for its initial stage, removes it from the pool, and returns its state.
+--
+-- If the worker pool is not already allocated, returns Nothing.
+waitInitialWorkerSlot :: TMVar (WorkerPool Annex.AnnexState) -> STM (Maybe (Annex.AnnexState, WorkerStage))
+waitInitialWorkerSlot tv =
+	takeTMVar tv >>= \case
+		UnallocatedWorkerPool -> do
+			putTMVar tv UnallocatedWorkerPool 
+			return Nothing
+		WorkerPool usedstages l -> do
+			let stage = initialStage usedstages
+			(st, pool') <- waitWorkerSlot usedstages stage l
+			putTMVar tv pool'
+			return $ Just (st, stage)
+
+-- | Waits until there's an idle worker for the specified stage, and returns
+-- its state and a WorkerPool containing all the other workers.
+waitWorkerSlot :: UsedStages -> WorkerStage -> [Worker Annex.AnnexState] -> STM (Annex.AnnexState, WorkerPool Annex.AnnexState)
+waitWorkerSlot usedstages wantstage = findidle []
+  where
+	findidle _ [] = retry
+	findidle c ((IdleWorker st stage):rest) 
+		| stage == wantstage = return (st, WorkerPool usedstages (c ++ rest))
+	findidle c (w:rest) = findidle (w:c) rest