optimisation for borg

Skip needing to list importable contents when unchanged since last time.

Remote/Borg.hs

@@ -26,6 +26,7 @@ import Utility.Metered
 import qualified Remote.Helper.ThirdPartyPopulated as ThirdPartyPopulated
 import Logs.Export
 
+import Data.Either
 import Text.Read
 import Control.Exception (evaluate)
 import Control.DeepSeq
@@ -122,8 +123,6 @@ borgSetup _ mu _ c _gc = do
 	-- persistant state, so it can vary between hosts.
 	gitConfigSpecialRemote u c [("borgrepo", borgrepo)]
 
-	-- TODO: untrusted by default, but allow overriding that
-
 	return (c, u)
 
 borgLocal :: BorgRepo -> Bool
@@ -132,18 +131,20 @@ borgLocal = notElem ':'
 -- XXX the tree generated by using this does not seem to get grafted into
 -- the git-annex branch, so would be subject to being lost to GC.
 -- Is this a general problem affecting importtree too?
-listImportableContentsM :: UUID -> BorgRepo -> Annex (ImportableContents (ContentIdentifier, ByteSize))
+listImportableContentsM :: UUID -> BorgRepo -> Annex (Maybe (ImportableContents (ContentIdentifier, ByteSize)))
 listImportableContentsM u borgrepo = prompt $ do
 	imported <- getImported u
 	ls <- withborglist borgrepo "{barchive}{NUL}" $ \as ->
 		forM as $ \archivename ->
 			case M.lookup archivename imported of
-				Just getfast -> getfast
-				Nothing -> 
+				Just getfast -> return $ Left getfast
+				Nothing -> Right <$>
 					let archive = borgrepo ++ "::" ++ decodeBS' archivename
 					in withborglist archive "{size}{NUL}{path}{NUL}" $
 						liftIO . evaluate . force . parsefilelist archivename
-	return $ mkimportablecontents ls
+	if all isLeft ls
+		then return Nothing -- unchanged since last time, avoid work
+		else Just . mkimportablecontents <$> mapM (either id pure) ls
   where
 	withborglist what format a = do
 		let p = (proc "borg" ["list", what, "--format", format])