avoid no longer necessary piping of ssh stderr for p2pstdio

This was needed when supporting old git-annex-shell that do not support
p2pstdio yet, in order to cleanly fall back to the old interface without
error messages being displayed. That is no longer supported, so simplify
to not intercept error messages.

Sponsored-by: Dartmouth College's DANDI project

Remote/Helper/Ssh.hs

@@ -1,6 +1,6 @@
 {- git-annex remote access with ssh and git-annex-shell
  -
- - Copyright 2011-2018 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2022 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -27,7 +27,6 @@ import qualified P2P.IO as P2P
 import qualified P2P.Annex as P2P
 
 import Control.Concurrent.STM
-import Control.Concurrent.Async
 
 toRepo :: ConsumeStdin -> Git.Repo -> RemoteGitConfig -> SshCommand -> Annex (FilePath, [CommandParam])
 toRepo cs r gc remotecmd = do
@@ -178,18 +177,15 @@ rsyncParams r direction = do
 	gc = gitconfig r
 
 -- A connection over ssh to git-annex shell speaking the P2P protocol.
-type P2PSshConnection = P2P.ClosableConnection
+type P2PSshConnection = P2P.ClosableConnection 
-	(P2P.RunState, P2P.P2PConnection, ProcessHandle, TVar StderrHandlerState)
+	(P2P.RunState, P2P.P2PConnection, ProcessHandle)
-
-data StderrHandlerState = DiscardStderr | DisplayStderr | EndStderrHandler
 
 closeP2PSshConnection :: P2PSshConnection -> IO (P2PSshConnection, Maybe ExitCode)
 closeP2PSshConnection P2P.ClosedConnection = return (P2P.ClosedConnection, Nothing)
-closeP2PSshConnection (P2P.OpenConnection (_st, conn, pid, stderrhandlerst)) =
+closeP2PSshConnection (P2P.OpenConnection (_st, conn, pid)) =
 	-- mask async exceptions, avoid cleanup being interrupted
 	uninterruptibleMask_ $ do
 		P2P.closeConnection conn
-		atomically $ writeTVar stderrhandlerst EndStderrHandler
 		exitcode <- waitForProcess pid
 		return (P2P.ClosedConnection, Just exitcode)
 
@@ -244,11 +240,10 @@ openP2PSshConnection r connpool = do
 		Just (cmd, params) -> start cmd params =<< getRepo r
   where
 	start cmd params repo = liftIO $ do
-		(Just from, Just to, Just err, pid) <- createProcess $
+		(Just from, Just to, Nothing, pid) <- createProcess $
 			(proc cmd (toCommand params))
 				{ std_in = CreatePipe
 				, std_out = CreatePipe
-				, std_err = CreatePipe
 				}
 		pidnum <- getPid pid
 		let conn = P2P.P2PConnection
@@ -259,17 +254,14 @@ openP2PSshConnection r connpool = do
 			, P2P.connIdent = P2P.ConnIdent $
 				Just $ "ssh connection " ++ show pidnum
 			}
-		stderrhandlerst <- newStderrHandler err pid
 		runst <- P2P.mkRunState P2P.Client
-		let c = P2P.OpenConnection (runst, conn, pid, stderrhandlerst)
+		let c = P2P.OpenConnection (runst, conn, pid)
 		-- When the connection is successful, the remote
 		-- will send an AUTH_SUCCESS with its uuid.
 		let proto = P2P.postAuth $
 			P2P.negotiateProtocolVersion P2P.maxProtocolVersion
 		tryNonAsync (P2P.runNetProto runst conn proto) >>= \case
-			Right (Right (Just theiruuid)) | theiruuid == uuid r -> do
+			Right (Right (Just theiruuid)) | theiruuid == uuid r ->
-				atomically $ 
-					writeTVar stderrhandlerst DisplayStderr
 				return $ Just c
 			_ -> do
 				(cclosed, exitcode) <- closeP2PSshConnection c
@@ -284,25 +276,6 @@ openP2PSshConnection r connpool = do
 		modifyTVar' connpool $
 			maybe (Just P2PSshUnsupported) Just
 
-newStderrHandler :: Handle -> ProcessHandle -> IO (TVar StderrHandlerState)
-newStderrHandler errh ph = do
-	-- stderr from git-annex-shell p2pstdio is initially discarded
-	-- because old versions don't support the command. Once it's known
-	-- to be running, this is changed to DisplayStderr.
-	v <- newTVarIO DiscardStderr
-	void $ async $ go v
-	return v
-  where
-	go v = do
-		hGetLineUntilExitOrEOF ph errh >>= \case
-			Nothing -> hClose errh
-			Just l -> atomically (readTVar v) >>= \case
-				DiscardStderr -> go v
-				DisplayStderr -> do
-					hPutStrLn stderr l
-					go v
-				EndStderrHandler -> hClose errh
-
 -- Runs a P2P Proto action on a remote when it supports that,
 -- otherwise the fallback action.
 runProto :: Remote -> P2PSshConnectionPool -> Annex a -> P2P.Proto a -> Annex (Maybe a)
@@ -319,7 +292,7 @@ runProto r connpool onerr proto = Just <$>
 
 runProtoConn :: P2P.Proto a -> P2PSshConnection -> Annex (P2PSshConnection, Maybe a)
 runProtoConn _ P2P.ClosedConnection = return (P2P.ClosedConnection, Nothing)
-runProtoConn a conn@(P2P.OpenConnection (runst, c, _, _)) = do
+runProtoConn a conn@(P2P.OpenConnection (runst, c, _)) = do
 	P2P.runFullProto runst c a >>= \case
 		Right r -> return (conn, Just r)
 		-- When runFullProto fails, the connection is no longer