sync: When listing contents on an import remote fails, proceed with other syncing instead of aborting

Switch listContents to being a proper CommandStart, so if it throws an
exception, it will be treated like any other command action that fails.

downloadImport apparently does not ever throw an exception,
and itself uses commandAction, so it can't be a CommandStart.

Annex/Import.hs

@@ -214,8 +214,8 @@ buildImportTrees basetree msubdir importable = History
 {- Downloads all new ContentIdentifiers as needed to generate Keys. 
  - Supports concurrency when enabled.
  -
- - If any download fails, the whole thing fails, but it will resume where
- - it left off.
+ - If any download fails, the whole thing fails with Nothing, 
+ - but it will resume where it left off.
  -}
 downloadImport :: Remote -> ImportTreeConfig -> ImportableContents (ContentIdentifier, ByteSize) -> Annex (Maybe (ImportableContents Key))
 downloadImport remote importtreeconfig importablecontents = do

CHANGELOG

@@ -9,6 +9,8 @@ git-annex (7.20190323) UNRELEASED; urgency=medium
     content.
   * Made git-annex sync --content much faster when all the remotes it's
     syncing with are export/import remotes.
+  * sync: When listing contents on an import remote fails, proceed with
+    other syncing instead of aborting.
 
  -- Joey Hess <id@joeyh.name>  Tue, 09 Apr 2019 14:07:53 -0400
 

Command/Import.hs

@@ -33,6 +33,8 @@ import Git.Types
 import Git.Branch
 import Types.Import
 
+import Control.Concurrent.STM
+
 cmd :: Command
 cmd = notBareRepo $
 	withGlobalOptions [jobsOption, jsonOptions, fileMatchingOptions] $
@@ -265,10 +267,20 @@ seekRemote remote branch msubdir = do
 	
 	parentcommit <- fromtrackingbranch Git.Ref.sha
 	let importcommitconfig = ImportCommitConfig parentcommit ManualCommit importmessage
+	let commitimport = commitRemote remote branch tb parentcommit importtreeconfig importcommitconfig
 
-	importable <- download importtreeconfig =<< listcontents
-	void $ includeCommandAction $
-		commitRemote remote branch tb parentcommit importtreeconfig importcommitconfig importable
+	importabletvar <- liftIO $ newTVarIO Nothing
+	void $ includeCommandAction (listContents remote importabletvar)
+	liftIO (atomically (readTVar importabletvar)) >>= \case
+		Nothing -> return ()
+		Just importable -> downloadImport remote importtreeconfig importable >>= \case
+			Nothing -> warning $ concat
+				[ "Failed to import some files from "
+				, Remote.name remote
+				, ". Re-run command to resume import."
+				]
+			Just imported -> void $
+				includeCommandAction $ commitimport imported
   where
 	importmessage = "import from " ++ Remote.name remote
 
@@ -276,20 +288,14 @@ seekRemote remote branch msubdir = do
 
 	fromtrackingbranch a = inRepo $ a (fromRemoteTrackingBranch tb)
 
-	listcontents = do
-		showStart' "list" (Just (Remote.name remote))
-		Remote.listImportableContents (Remote.importActions remote) >>= \case
-			Nothing -> do
-				showEndFail
-				giveup $ "Unable to list contents of " ++ Remote.name remote
-			Just importable -> do
-				showEndOk
-				return importable
-
-	download importtreeconfig importablecontents =
-		downloadImport remote importtreeconfig importablecontents >>= \case
-			Nothing -> giveup $ "Failed to import some files from " ++ Remote.name remote ++ ". Re-run command to resume import."
-			Just importable -> return importable
+listContents :: Remote -> TVar (Maybe (ImportableContents (ContentIdentifier, Remote.ByteSize))) -> CommandStart
+listContents remote tvar = do
+	showStart' "list" (Just (Remote.name remote))
+	next $ Remote.listImportableContents (Remote.importActions remote) >>= \case
+		Nothing -> giveup $ "Unable to list contents of " ++ Remote.name remote
+		Just importable -> next $ do
+			liftIO $ atomically $ writeTVar tvar (Just importable)
+			return True
 
 commitRemote :: Remote -> Branch -> RemoteTrackingBranch -> Maybe Sha -> ImportTreeConfig -> ImportCommitConfig -> ImportableContents Key -> CommandStart
 commitRemote remote branch tb parentcommit importtreeconfig importcommitconfig importable = do