annex.backend is the new name for what was annex.backends

It takes a single key-value backend, rather than the unncessary and confusing list.
The old option still works if set.

Simplified some old old code too.

This commit was sponsored by Thomas Hochstein on Patreon.

Backend.hs

@@ -7,7 +7,7 @@
 
 module Backend (
 	list,
-	orderedList,
+	defaultBackend,
 	genKey,
 	getBackend,
 	chooseBackend,
@@ -33,40 +33,29 @@ import qualified Data.Map as M
 list :: [Backend]
 list = Backend.Hash.backends ++ Backend.WORM.backends ++ Backend.URL.backends
 
-{- List of backends in the order to try them when storing a new key. -}
-orderedList :: Annex [Backend]
-orderedList = do
-	l <- Annex.getState Annex.backends -- list is cached here
-	if not $ null l
-		then return l
-		else do
-			f <- Annex.getState Annex.forcebackend
-			case f of
-				Just name | not (null name) ->
-					return [lookupname name]
-				_ -> do
-					l' <- gen . annexBackends <$> Annex.getGitConfig
-					Annex.changeState $ \s -> s { Annex.backends = l' }
-					return l'
+{- Backend to use by default when generating a new key. -}
+defaultBackend :: Annex Backend
+defaultBackend = maybe cache return =<< Annex.getState Annex.backend
   where
-	gen [] = list
-	gen ns = map lookupname ns
+	cache = do
+		n <- maybe (annexBackend <$> Annex.getGitConfig) (return . Just)
+			=<< Annex.getState Annex.forcebackend
+		let b = case n of
+			Just name | valid name -> lookupname name
+			_ -> Prelude.head list
+		Annex.changeState $ \s -> s { Annex.backend = Just b }
+		return b
+	valid name = not (null name)
 	lookupname = lookupBackendVariety . parseKeyVariety
 
-{- Generates a key for a file, trying each backend in turn until one
- - accepts it. -}
+{- Generates a key for a file. -}
 genKey :: KeySource -> Maybe Backend -> Annex (Maybe (Key, Backend))
-genKey source trybackend = do
-	bs <- orderedList
-	let bs' = maybe bs (: bs) trybackend
-	genKey' bs' source
-genKey' :: [Backend] -> KeySource -> Annex (Maybe (Key, Backend))
-genKey' [] _ = return Nothing
-genKey' (b:bs) source = do
+genKey source preferredbackend = do
+	b <- maybe defaultBackend return preferredbackend
 	r <- B.getKey b source
-	case r of
-		Nothing -> genKey' bs source
-		Just k -> return $ Just (makesane k, b)
+	return $ case r of
+		Nothing -> Nothing
+		Just k -> Just (makesane k, b)
   where
 	-- keyNames should not contain newline characters.
 	makesane k = k { keyName = map fixbadchar (keyName k) }
@@ -82,13 +71,14 @@ getBackend file k = case maybeLookupBackendVariety (keyVariety k) of
 		return Nothing
 
 {- Looks up the backend that should be used for a file.
- - That can be configured on a per-file basis in the gitattributes file. -}
+ - That can be configured on a per-file basis in the gitattributes file,
+ - or forced with --backend. -}
 chooseBackend :: FilePath -> Annex (Maybe Backend)
 chooseBackend f = Annex.getState Annex.forcebackend >>= go
   where
-	go Nothing =  maybeLookupBackendVariety . parseKeyVariety
+	go Nothing = maybeLookupBackendVariety . parseKeyVariety
 		<$> checkAttr "annex.backend" f
-	go (Just _) = Just . Prelude.head <$> orderedList
+	go (Just _) = Just <$> defaultBackend
 
 {- Looks up a backend by variety. May fail if unsupported or disabled. -}
 lookupBackendVariety :: KeyVariety -> Backend