avoid failure to lock content of removed file causing drop etc to fail

This was already prevented in other ways, but as seen in commit
c30fd24d91, those were a bit fragile.
And I'm not sure races were avoided in every case before. At least a
race between two separate git-annex processes, dropping the same
content, seemed possible.

This way, if locking fails, and the content is not present, it will
always do the right thing. Also, it avoids the overhead of an unncessary
inAnnex check for every file.

This commit was sponsored by Denis Dzyubenko on Patreon.

Annex/Content.hs

@@ -194,13 +194,15 @@ contentLockFile key = Just <$> calcRepo (gitAnnexContentLock key)
  - rather than running the action.
  -}
 lockContentShared :: Key -> (VerifiedCopy -> Annex a) -> Annex a
-lockContentShared key a = lockContentUsing lock key $ ifM (inAnnex key)
+lockContentShared key a = lockContentUsing lock key notpresent $
+	ifM (inAnnex key)
 		( do
 			u <- getUUID
 			withVerifiedCopy LockedCopy u (return True) a
-	, giveup $ "failed to lock content: not present"
+		, notpresent
 		)
   where
+	notpresent = giveup $ "failed to lock content: not present"
 #ifndef mingw32_HOST_OS
 	lock contentfile Nothing = tryLockShared Nothing contentfile
 	lock _ (Just lockfile) = posixLocker tryLockShared lockfile
@@ -212,9 +214,12 @@ lockContentShared key a = lockContentUsing lock key $ ifM (inAnnex key)
  - might remove it.
  -
  - If locking fails, throws an exception rather than running the action.
+ -
+ - But, if locking fails because the the content is not present, runs the
+ - fallback action instead.
  -}
-lockContentForRemoval :: Key -> (ContentRemovalLock -> Annex a) -> Annex a
-lockContentForRemoval key a = lockContentUsing lock key $ 
+lockContentForRemoval :: Key -> Annex a -> (ContentRemovalLock -> Annex a) -> Annex a
+lockContentForRemoval key fallback a = lockContentUsing lock key fallback $ 
 	a (ContentRemovalLock key)
   where
 #ifndef mingw32_HOST_OS
@@ -251,22 +256,31 @@ winLocker takelock _ (Just lockfile) = do
 winLocker _ _ Nothing = return Nothing
 #endif
 
-lockContentUsing :: ContentLocker -> Key -> Annex a -> Annex a
-lockContentUsing locker key a = do
+{- The fallback action is run if the ContentLocker throws an IO exception
+ - and the content is not present. It's not guaranteed to always run when
+ - the content is not present, because the content file is not always
+ - the file that is locked eg on Windows a different file is locked. -}
+lockContentUsing :: ContentLocker -> Key -> Annex a -> Annex a -> Annex a
+lockContentUsing locker key fallback a = do
 	contentfile <- fromRawFilePath <$> calcRepo (gitAnnexLocation key)
 	lockfile <- contentLockFile key
 	bracket
 		(lock contentfile lockfile)
-		(unlock lockfile)
-		(const a)
+		(either (const noop) (unlock lockfile))
+		go
   where
 	alreadylocked = giveup "content is locked"
 	failedtolock e = giveup $ "failed to lock content: " ++ show e
 
-	lock contentfile lockfile =
-		(maybe alreadylocked return 
-			=<< locker contentfile lockfile)
-		`catchIO` failedtolock
+	lock contentfile lockfile = tryIO $
+		maybe alreadylocked return 
+			=<< locker contentfile lockfile
+	
+	go (Right _) = a
+	go (Left e) = ifM (inAnnex key)
+		( failedtolock e
+		, fallback 
+		)
 
 #ifndef mingw32_HOST_OS
 	unlock mlockfile lck = do

Annex/Drop.hs

@@ -17,7 +17,6 @@ import qualified Command.Drop
 import Command
 import Annex.Wanted
 import Annex.SpecialRemote.Config
-import Annex.Content
 import qualified Database.Keys
 import Git.FilePath
 

Assistant/Unused.hs

@@ -76,7 +76,7 @@ expireUnused duration = do
 	forM_ oldkeys $ \k -> do
 		debug ["removing old unused key", serializeKey k]
 		liftAnnex $ tryNonAsync $ do
-			lockContentForRemoval k removeAnnex
+			lockContentForRemoval k noop removeAnnex
 			logStatus k InfoMissing
   where
 	boundry = durationToPOSIXTime <$> duration

Assistant/Upgrade.hs

@@ -99,7 +99,7 @@ startDistributionDownload d = go =<< liftIO . newVersionLocation d =<< liftIO ol
 		, transferKeyData = fromKey id k
 		}
 	cleanup = liftAnnex $ do
-		lockContentForRemoval k removeAnnex
+		lockContentForRemoval k noop removeAnnex
 		setUrlMissing k u
 		logStatus k InfoMissing
 

Command/Drop.hs

@@ -99,11 +99,6 @@ startKeys o from (key, ai) = start' o from key (AssociatedFile Nothing) ai
 
 startLocal :: AssociatedFile -> ActionItem -> NumCopies -> Key -> [VerifiedCopy] -> CommandStart
 startLocal afile ai numcopies key preverified =
-	-- This is a redundant check, because checkContentPresent was
-	-- enabled when seeking. However, when two files have the same key,
-	-- the content may have already been removed, which would cause
-	-- this to fail, so it has to be checked again.
-	stopUnless (inAnnex key) $
 	starting "drop" (OnlyActionOn key ai) $
 		performLocal key afile numcopies preverified
 
@@ -113,7 +108,7 @@ startRemote afile ai numcopies key remote =
 		performRemote key afile numcopies remote
 
 performLocal :: Key -> AssociatedFile -> NumCopies -> [VerifiedCopy] -> CommandPerform
-performLocal key afile numcopies preverified = lockContentForRemoval key $ \contentlock -> do
+performLocal key afile numcopies preverified = lockContentForRemoval key fallback $ \contentlock -> do
 	u <- getUUID
 	(tocheck, verified) <- verifiableCopies key [u]
 	doDrop u (Just contentlock) key afile numcopies [] (preverified ++ verified) tocheck
@@ -130,6 +125,13 @@ performLocal key afile numcopies preverified = lockContentForRemoval key $ \cont
 			notifyDrop afile False
 			stop
 		)
+  where
+	-- This occurs when, for example, two files are being dropped
+	-- and have the same content. The seek stage checks if the content
+	-- is present, but due to buffering, may find it present for the
+	-- second file before the first is dropped. If so, nothing remains
+	-- to be done except for cleaning up.
+	fallback = next $ cleanupLocal key
 
 performRemote :: Key -> AssociatedFile -> NumCopies -> Remote -> CommandPerform
 performRemote key afile numcopies remote = do

Command/DropKey.hs

@@ -47,7 +47,7 @@ start key = starting "dropkey" (mkActionItem key) $
 
 perform :: Key -> CommandPerform
 perform key = ifM (inAnnex key)
-	( lockContentForRemoval key $ \contentlock -> do
+	( lockContentForRemoval key (next $ cleanup key) $ \contentlock -> do
 		removeAnnex contentlock
 		next $ cleanup key
 	, next $ return True

Command/Move.hs

@@ -155,7 +155,7 @@ toPerform dest removewhen key afile fastcheck isthere =
 		RemoveNever -> do
 			setpresentremote
 			next $ return True
-		RemoveSafe -> lockContentForRemoval key $ \contentlock -> do
+		RemoveSafe -> lockContentForRemoval key lockfailed $ \contentlock -> do
 			srcuuid <- getUUID
 			let destuuid = Remote.uuid dest
 			willDropMakeItWorse srcuuid destuuid deststartedwithcopy key afile >>= \case
@@ -189,6 +189,13 @@ toPerform dest removewhen key afile fastcheck isthere =
 			() <- setpresentremote
 			return False
 	
+	-- This occurs when, for example, two files are being dropped
+	-- and have the same content. The seek stage checks if the content
+	-- is present, but due to buffering, may find it present for the
+	-- second file before the first is dropped. If so, nothing remains
+	-- to be done except for cleaning up.
+	lockfailed = next $ Command.Drop.cleanupLocal key
+
 fromStart :: RemoveWhen -> AssociatedFile -> Key -> ActionItem -> Remote -> CommandStart
 fromStart removewhen afile key ai src = 
 	stopUnless (fromOk src key) $

Command/TestRemote.hs

@@ -245,7 +245,7 @@ test runannex mkr mkk =
 		whenwritable r $ isRight <$> tryNonAsync (store r k)
 	, check ("present " ++ show True) $ \r k -> present r k True
 	, check "retrieveKeyFile" $ \r k -> do
-		lockContentForRemoval k removeAnnex
+		lockContentForRemoval k noop removeAnnex
 		get r k
 	, check "fsck downloaded object" fsck
 	, check "retrieveKeyFile resume from 33%" $ \r k -> do
@@ -255,20 +255,20 @@ test runannex mkr mkk =
 			sz <- hFileSize h
 			L.hGet h $ fromInteger $ sz `div` 3
 		liftIO $ L.writeFile tmp partial
-		lockContentForRemoval k removeAnnex
+		lockContentForRemoval k noop removeAnnex
 		get r k
 	, check "fsck downloaded object" fsck
 	, check "retrieveKeyFile resume from 0" $ \r k -> do
 		tmp <- prepTmp k
 		liftIO $ writeFile tmp ""
-		lockContentForRemoval k removeAnnex
+		lockContentForRemoval k noop removeAnnex
 		get r k
 	, check "fsck downloaded object" fsck
 	, check "retrieveKeyFile resume from end" $ \r k -> do
 		loc <- fromRawFilePath <$> Annex.calcRepo (gitAnnexLocation k)
 		tmp <- prepTmp k
 		void $ liftIO $ copyFileExternal CopyAllMetaData loc tmp
-		lockContentForRemoval k removeAnnex
+		lockContentForRemoval k noop removeAnnex
 		get r k
 	, check "fsck downloaded object" fsck
 	, check "removeKey when present" $ \r k -> 
@@ -393,7 +393,7 @@ cleanup rs ks ok
 	| all Remote.readonly rs = return ok
 	| otherwise = do
 		forM_ rs $ \r -> forM_ ks (Remote.removeKey r)
-		forM_ ks $ \k -> lockContentForRemoval k removeAnnex
+		forM_ ks $ \k -> lockContentForRemoval k noop removeAnnex
 		return ok
 
 chunkSizes :: Int -> Bool -> [Int]

Command/Uninit.hs

@@ -115,7 +115,7 @@ removeUnannexed = go []
 	go c [] = return c
 	go c (k:ks) = ifM (inAnnexCheck k $ liftIO . enoughlinks)
 		( do
-			lockContentForRemoval k removeAnnex
+			lockContentForRemoval k noop removeAnnex
 			go c ks
 		, go (k:c) ks
 		)

P2P/Annex.hs

@@ -107,12 +107,14 @@ runLocal runst runner a = case a of
 			Left e -> return $ Left $ ProtoFailureException e
 			Right result -> runner (next result)
 	RemoveContent k next -> do
-		v <- tryNonAsync $
-			ifM (Annex.Content.inAnnex k)
-				( lockContentForRemoval k $ \contentlock -> do
-					removeAnnex contentlock
+		let cleanup = do
 			logStatus k InfoMissing
 			return True
+		v <- tryNonAsync $
+			ifM (Annex.Content.inAnnex k)
+				( lockContentForRemoval k cleanup $ \contentlock -> do
+					removeAnnex contentlock
+					cleanup
 				, return True
 				)
 		case v of

Remote/Git.hs

@@ -436,9 +436,10 @@ dropKey' repo r st@(State connpool duc _ _ _) key
 		( guardUsable repo (giveup "cannot access remote") $
 			commitOnCleanup repo r st $ onLocalFast st $ do
 				whenM (Annex.Content.inAnnex key) $ do
-					Annex.Content.lockContentForRemoval key $ \lock -> do
+					let cleanup = logStatus key InfoMissing
+					Annex.Content.lockContentForRemoval key cleanup $ \lock -> do
 						Annex.Content.removeAnnex lock
-						logStatus key InfoMissing
+						cleanup
 					Annex.Content.saveState True
 		, giveup "remote does not have expected annex.uuid value"
 		)