enableremote: Better handling of the unusual case where multiple special remotes have been initialized with the same name

Before it would pick one at random, though preferring ones that were not
dead over dead ones.

Now, if one is dead and the other not, it will use the non-dead one. But if
both are not dead, or both dead, it will error out, suggesting the user
clarify what they want to enable.

Sponsored-by: Luke Shumaker on Patreon

Annex/SpecialRemote.hs

@@ -1,6 +1,6 @@
 {- git-annex special remote configuration
  -
- - Copyright 2011-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -25,16 +25,22 @@ import qualified Types.Remote as Remote
 import Git.Types (RemoteName)
 
 import qualified Data.Map as M
-import Data.Ord
 
 {- See if there's an existing special remote with this name.
  -
- - Prefer remotes that are not dead when a name appears multiple times. -}
+ - Remotes that are not dead come first in the list
+ - when a name appears multiple times. -}
 findExisting :: RemoteName -> Annex (Maybe (UUID, RemoteConfig, Maybe (ConfigFrom UUID)))
 findExisting name = do
+	(a, b) <- findExisting' name
+	return (headMaybe (a++b))
+
+{- Dead remotes with the name are in the second list, all others in the
+ - first list. -}
+findExisting' :: RemoteName -> Annex ([(UUID, RemoteConfig, Maybe (ConfigFrom UUID))], [(UUID, RemoteConfig, Maybe (ConfigFrom UUID))])
+findExisting' name = do
 	t <- trustMap
-	headMaybe
-		. sortBy (comparing $ \(u, _, _) -> Down $ M.lookup u t)
+	partition (\(u, _, _) -> M.lookup u t /= Just DeadTrusted)
 		. findByRemoteConfig (\c -> lookupName c == Just name)
 		<$> Logs.Remote.remoteConfigMap