support modifying and running git add on an unlocked file that used an URL key

Avoids the smudge --clean filter failing because URL keys do not support
genKey. Instead the modified content will be added using the default
backend.

This commit was sponsored by Jochen Bartl on Patreon.

Command/Smudge.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2015-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -28,6 +28,7 @@ import Utility.Metered
 import Annex.InodeSentinal
 import Utility.InodeCache
 import Config.GitConfig
+import qualified Types.Backend
 
 import qualified Data.ByteString as S
 import qualified Data.ByteString.Lazy as L
@@ -120,28 +121,31 @@ clean file = do
 			-- Optimization for the case when the file is already
 			-- annexed and is unmodified.
 			case oldkey of
-				Nothing -> doingest oldkey
+				Nothing -> doingest Nothing
 				Just ko -> ifM (isUnmodifiedCheap ko file)
 					( liftIO $ emitPointer ko
-					, doingest oldkey
+					, updateingest ko
 					)
 		, liftIO $ L.hPut stdout b
 		)
 	
-	doingest oldkey = do
-		-- Look up the backend that was used for this file
-		-- before, so that when git re-cleans a file its
-		-- backend does not change.
-		oldbackend <- maybe
-			(pure Nothing)
-			(maybeLookupBackendVariety . fromKey keyVariety)
-			oldkey
+	-- Use the same backend that was used before, when possible.
+	-- If the old key's backend does not support generating keys,
+	-- use the default backend.
+	updateingest oldkey =
+		maybeLookupBackendVariety (fromKey keyVariety oldkey) >>= \case
+			Nothing -> doingest Nothing
+			Just oldbackend -> case Types.Backend.genKey oldbackend of
+				Just _ -> doingest (Just oldbackend)
+				Nothing -> doingest Nothing
+	
+	doingest preferredbackend = do
 		-- Can't restage associated files because git add
 		-- runs this and has the index locked.
 		let norestage = Restage False
 		liftIO . emitPointer
 			=<< postingest
-			=<< (\ld -> ingest' oldbackend nullMeterUpdate ld Nothing norestage)
+			=<< (\ld -> ingest' preferredbackend nullMeterUpdate ld Nothing norestage)
 			=<< lockDown cfg (fromRawFilePath file)
 
 	postingest (Just k, _) = do