fix cat-file leak in get with -J

Bugfix: When -J was enabled, getting files leaked a ever-growing number of
git cat-file processes.

(Since commit dd39e9e255)

The leak happened when mergeState called stopNonConcurrentSafeCoProcesses.
While stopNonConcurrentSafeCoProcesses usually manages to stop everything,
there was a race condition where cat-file processes were leaked. Because
catFileStop modifies Annex.catfilehandles in a non-concurrency safe way,
and could clobber modifications made in between. Which should have been ok,
since originally catFileStop was only used at shutdown.

Note the comment on catFileStop saying it should only be used when nothing
else is using the handles. It would be possible to make catFileStop
race-safe, but it should just not be used in a situation where a race is
possible. So I didn't bother.

Instead, the fix is just not to stop any processes in mergeState. Because
in order for mergeState to be called, dupState must have been run, and it
enables concurrency mode, stops any non-concurrent processes, and so all
processes that are running are concurrency safea. So there is no need to
stop them when merging state. Indeed, stopping them would be extra work,
even if there was not this bug.

Sponsored-by: Dartmouth College's Datalad project

Annex/Action.hs

@@ -13,7 +13,6 @@ module Annex.Action (
 	startup,
 	shutdown,
 	stopCoProcesses,
-	stopNonConcurrentSafeCoProcesses,
 ) where
 
 import qualified Data.Map as M
@@ -85,14 +84,8 @@ shutdown nocommit = do
 {- Stops all long-running child processes, including git query processes. -}
 stopCoProcesses :: Annex ()
 stopCoProcesses = do
-	stopNonConcurrentSafeCoProcesses
-	emptyTransferrerPool
-
-{- Stops long-running child processes that use handles that are not safe
- - for multiple threads to access at the same time. -}
-stopNonConcurrentSafeCoProcesses :: Annex ()
-stopNonConcurrentSafeCoProcesses = do
 	catFileStop
 	checkAttrStop
 	hashObjectStop
 	checkIgnoreStop
+	emptyTransferrerPool

Annex/Concurrent.hs

@@ -101,14 +101,10 @@ dupState = do
 		, Annex.errcounter = 0
 		}
 
-{- Merges the passed AnnexState into the current Annex state.
- - Also closes various handles in it. -}
+{- Merges the passed AnnexState into the current Annex state. -}
 mergeState :: AnnexState -> Annex ()
 mergeState st = do
-	rd <- Annex.getRead id
-	st' <- liftIO $ (fst . snd)
-		<$> run (st, rd) stopNonConcurrentSafeCoProcesses
-	forM_ (M.toList $ Annex.cleanupactions st') $
+	forM_ (M.toList $ Annex.cleanupactions st) $
 		uncurry addCleanupAction
-	Annex.Queue.mergeFrom st'
-	changeState $ \s -> s { errcounter = errcounter s + errcounter st' }
+	Annex.Queue.mergeFrom st
+	changeState $ \s -> s { errcounter = errcounter s + errcounter st }

CHANGELOG

@@ -1,5 +1,7 @@
 git-annex (8.20211118) UNRELEASED; urgency=medium
 
+  * Bugfix: When -J was enabled, getting files leaked a ever-growing
+    number of git cat-file processes.
   * importfeed: Display url before starting youtube-dl download.
 
  -- Joey Hess <id@joeyh.name>  Wed, 17 Nov 2021 13:18:44 -0400

doc/forum/git-annex_causing_zombie_git_processes_to_build_up.mdwn

@@ -23,3 +23,5 @@ I'm unsure how to debug what is wrong, so seeking guidance.
 Tomorrow I'll start to see if I can reproduce with older/newer versions of git-annex.
 
 [[!tag forumbug]]
+
+[[bugs/done]]

doc/forum/git-annex_causing_zombie_git_processes_to_build_up/comment_13_41e0fada90d795b9be12929ed66231e1._comment

@@ -0,0 +1,7 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 13"""
+ date="2021-11-19T16:22:47Z"
+ content="""
+Fixed.
+"""]]