try retrieval from more than one export location when the first fails

Combined with commit 0ffc59d341, this
fixes the case where there are duplicate files on the special remote,
and the first gets modified/deleted, while the second is still present.

directory, adb: Fixed a bug when importtree=yes, and multiple files in the
special remote have the same content, that caused it to refuse to get a
file from the special remote, incorrectly complaining that it had changed,
due to only accepting the inode+mtime of one file (that was since modified
or deleted) and not accepting the inode+mtime of other duplicate files.

Sponsored-by: Max Thoursie on Patreon

Remote/Helper/ExportImport.hs

@@ -327,11 +327,6 @@ adjustExportImport' isexport isimport r rs = do
 		db <- getexportdb dbv
 		liftIO $ Export.getExportTree db k
 	
-	getfirstexportloc dbv k = do
-		getexportlocs dbv k >>= \case
-			[] -> giveup "unknown export location"
-			(l:_) -> return l
-	
 	getexportlocs dbv k = do
 		db <- getexportdb dbv
 		liftIO $ Export.getExportTree db k >>= \case
@@ -340,6 +335,15 @@ adjustExportImport' isexport isimport r rs = do
 				, return []
 				)
 			ls -> return ls
+	
+	tryexportlocs dbv k a = 
+		go Nothing =<< getexportlocs dbv k
+	  where
+		go Nothing [] = giveup "unknown export location"
+		go (Just ex) [] = throwM ex
+		go _ (l:ls) = tryNonAsync (a l) >>= \case
+			Right v -> return v
+			Left e -> go (Just e) ls
 		
 	getkeycids ciddbv k = do
 		db <- getciddb ciddbv
@@ -350,9 +354,8 @@ adjustExportImport' isexport isimport r rs = do
 	-- have replaced with content not of the requested key, the content
 	-- has to be strongly verified.
 	retrieveKeyFileFromExport dbv k _af dest p = ifM (isVerifiable k)
-		( do
-			l <- getfirstexportloc dbv k
-			retrieveExport (exportActions r) k l dest p >>= return . \case
+		( tryexportlocs dbv k $ \loc -> 
+			retrieveExport (exportActions r) k loc dest p >>= return . \case
 				UnVerified -> MustVerify
 				IncompleteVerify iv -> MustFinishIncompleteVerify iv
 				v -> v
@@ -362,9 +365,8 @@ adjustExportImport' isexport isimport r rs = do
 	retrieveKeyFileFromImport dbv ciddbv k af dest p = do
 		cids <- getkeycids ciddbv k
 		if not (null cids)
-			then do
-				l <- getfirstexportloc dbv k
-				snd <$> retrieveExportWithContentIdentifier (importActions r) l cids dest (Left k) p
+			then tryexportlocs dbv k $ \loc ->
+				snd <$> retrieveExportWithContentIdentifier (importActions r) loc cids dest (Left k) p
 			-- In case a content identifier is somehow missing,
 			-- try this instead.
 			else if isexport