started implementing git-annex-shell proxy

So far, it negotiates VERSION with both parties. This is a tricky dance.

Untested.

Command/P2PStdIO.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2018 Joey Hess <id@joeyh.name>
+ - Copyright 2018-2024 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -10,10 +10,12 @@ module Command.P2PStdIO where
 import Command
 import P2P.IO
 import P2P.Annex
+import P2P.Proxy
 import qualified P2P.Protocol as P2P
 import qualified Annex
 import Annex.UUID
 import qualified CmdLine.GitAnnexShell.Checks as Checks
+import Remote.Helper.Ssh (openP2PSshConnection', closeP2PSshConnection)
 
 import System.IO.Error
 
@@ -35,6 +37,12 @@ start theiruuid = startingCustomOutput (ActionItemOther Nothing) $ do
 			(True, _) -> P2P.ServeReadOnly
 			(False, True) -> P2P.ServeAppendOnly
 			(False, False) -> P2P.ServeReadWrite
+	Annex.getState Annex.proxyremote >>= \case
+		Nothing -> performLocal theiruuid servermode
+		Just r -> performProxy theiruuid servermode r
+
+performLocal :: UUID -> P2P.ServerMode -> CommandPerform
+performLocal theiruuid servermode = do
 	myuuid <- getUUID
 	let conn = stdioP2PConnection Nothing
 	let server = do
@@ -48,3 +56,31 @@ start theiruuid = startingCustomOutput (ActionItemOther Nothing) $ do
 		Left e -> giveup (describeProtoFailure e)
   where
 	done = next $ return True
+
+performProxy :: UUID -> P2P.ServerMode -> Remote -> CommandPerform
+performProxy clientuuid servermode remote = do
+	clientside <- ClientSide
+		<$> liftIO (mkRunState $ Serving clientuuid Nothing)
+		<*> pure (stdioP2PConnection Nothing)
+	getClientProtocolVersion clienterrhandler remote clientside $ \case
+		Nothing -> done
+		Just (clientmaxversion, othermsg) ->
+			connectremote clientmaxversion $ \remoteside ->
+				proxy clienterrhandler done servermode
+					clientside remoteside othermsg
+  where
+	-- FIXME: Support special remotes and non-ssh git remotes.
+	connectremote clientmaxversion cont = 
+		openP2PSshConnection' remote clientmaxversion >>= \case
+			Just conn@(P2P.IO.OpenConnection (remoterunst, remoteconn, _)) ->
+				cont (RemoteSide remoterunst remoteconn)
+					`finally` liftIO (closeP2PSshConnection conn)
+			_  -> giveup "Unable to connect to remote."
+
+	clienterrhandler cont a = a >>= \case
+		-- Avoid displaying an error when the client hung up on us.
+		Left (ProtoFailureIOError e) | isEOFError e -> done
+		Left e -> giveup (describeProtoFailure e)
+		Right v -> cont v
+	
+	done = next $ return True