smudge: Warn when encountering a pointer file that has other content appended to it

It will then proceed to add the file the same as if it were any other
file containing possibly annexable content. Usually the file is one that
was annexed before, so the new, probably corrupt content will also be added
to the annex. If the file was not annexed before, the content will be added
to git.

It's not possible for the smudge filter to throw an error here, because
git then just adds the file to git anyway.

Sponsored-by: Dartmouth College's Datalad project

Command/Smudge.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2015-2021 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2022 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -105,7 +105,7 @@ clean file = do
 		then L.length b `seq` return ()
 		else liftIO $ hClose stdin
 	let emitpointer = liftIO . S.hPut stdout . formatPointer
-	clean' file (parseLinkTargetOrPointerLazy b)
+	clean' file (parseLinkTargetOrPointerLazy' b)
 		passthrough
 		discardreststdin
 		emitpointer
@@ -115,7 +115,7 @@ clean file = do
 -- Handles everything except the IO of the file content.
 clean'
 	:: RawFilePath
-	-> Maybe Key
+	-> Either InvalidAppendedPointerFile (Maybe Key)
 	-- ^ If the content provided by git is an annex pointer,
 	-- this is the key it points to.
 	-> Annex ()
@@ -135,19 +135,26 @@ clean' file mk passthrough discardreststdin emitpointer =
   where
 
 	go = case mk of
-		Just k -> do
+		Right (Just k) -> do
 			addingExistingLink file k $ do
 				getMoveRaceRecovery k file
 				passthrough
-		Nothing -> inRepo (Git.Ref.fileRef file) >>= \case
-			Just fileref -> do
-				indexmeta <- catObjectMetaData fileref
-				oldkey <- case indexmeta of
-					Just (_, sz, _) -> catKey' fileref sz
-					Nothing -> return Nothing
-				go' indexmeta oldkey
-			Nothing -> passthrough
-	go' indexmeta oldkey = ifM (shouldAnnex file indexmeta oldkey)
+		Right Nothing -> notpointer
+		Left InvalidAppendedPointerFile -> do
+			toplevelWarning False $
+				"The file \"" ++ fromRawFilePath file ++ "\" looks like git-annex pointer file that has had other content appended to it"
+			notpointer
+
+	notpointer = inRepo (Git.Ref.fileRef file) >>= \case
+		Just fileref -> do
+			indexmeta <- catObjectMetaData fileref
+			oldkey <- case indexmeta of
+				Just (_, sz, _) -> catKey' fileref sz
+				Nothing -> return Nothing
+			notpointer' indexmeta oldkey
+		Nothing -> passthrough
+	
+	notpointer' indexmeta oldkey = ifM (shouldAnnex file indexmeta oldkey)
 		( do
 			discardreststdin