proxying special remotes

This is early, but already working for CHECKPRESENT.

However, when the special remote throws an exception on checkPresent,
this happens:

[2024-06-28 13:22:18.520884287] (P2P.IO) [ThreadId 4] P2P > ERROR directory /home/joey/tmp/bench2/dir is not accessible
[2024-06-28 13:22:18.521053135] (P2P.IO) [ThreadId 4] P2P < ERROR expected SUCCESS or FAILURE
git-annex: client error: expected SUCCESS or FAILURE
(fixing location log) p2pstdio: 1 failed

  ** Based on the location log, x
  ** was expected to be present, but its content is missing.
failed

Annex/Proxy.hs

@@ -11,11 +11,25 @@ import Annex.Common
 import P2P.Proxy
 import P2P.Protocol
 import P2P.IO
+import qualified Remote
+import qualified Types.Remote as Remote
+import qualified Remote.Git
 import Remote.Helper.Ssh (openP2PShellConnection', closeP2PShellConnection)
+import Annex.Concurrent
 
--- FIXME: Support special remotes.
-proxySshRemoteSide :: ProtocolVersion -> Bypass -> Remote -> Annex RemoteSide
-proxySshRemoteSide clientmaxversion bypass r = mkRemoteSide r $
+import Control.Concurrent.STM
+import Control.Concurrent.Async
+import qualified Data.ByteString.Lazy as L
+
+proxyRemoteSide :: ProtocolVersion -> Bypass -> Remote -> Annex RemoteSide
+proxyRemoteSide clientmaxversion bypass r
+	| Remote.remotetype r == Remote.Git.remote = 
+		proxyGitRemoteSide clientmaxversion bypass r
+	| otherwise =
+		proxySpecialRemoteSide clientmaxversion r
+
+proxyGitRemoteSide :: ProtocolVersion -> Bypass -> Remote -> Annex RemoteSide
+proxyGitRemoteSide clientmaxversion bypass r = mkRemoteSide r $
 	openP2PShellConnection' r clientmaxversion bypass >>= \case
 		Just conn@(OpenConnection (remoterunst, remoteconn, _)) ->
 			return $ Just 
@@ -24,3 +38,84 @@ proxySshRemoteSide clientmaxversion bypass r = mkRemoteSide r $
 				, void $ liftIO $ closeP2PShellConnection conn
 				)
 		_  -> return Nothing
+
+proxySpecialRemoteSide :: ProtocolVersion -> Remote -> Annex RemoteSide
+proxySpecialRemoteSide clientmaxversion r = mkRemoteSide r $ do
+	let protoversion = min clientmaxversion maxProtocolVersion
+	remoterunst <- Serving (Remote.uuid r) Nothing <$>
+		liftIO (newTVarIO protoversion)
+	ihdl <- liftIO newEmptyTMVarIO
+	ohdl <- liftIO newEmptyTMVarIO
+	endv <- liftIO newEmptyTMVarIO
+	worker <- liftIO . async =<< forkState
+		(proxySpecialRemote protoversion r ihdl ohdl endv)
+	let remoteconn = P2PConnection
+		{ connRepo = Nothing
+		, connCheckAuth = const False
+		, connIhdl = P2PHandleTMVar ihdl
+		, connOhdl = P2PHandleTMVar ohdl
+		, connIdent = ConnIdent (Just (Remote.name r))
+		}
+	let closeremoteconn = do
+		liftIO $ atomically $ putTMVar endv ()
+		join $ liftIO (wait worker)
+	return $ Just
+		( remoterunst
+		, remoteconn
+		, closeremoteconn
+		)
+
+-- Proxy for the special remote, speaking the P2P protocol.
+proxySpecialRemote
+	:: ProtocolVersion
+	-> Remote
+	-> TMVar (Either L.ByteString Message)
+	-> TMVar (Either L.ByteString Message)
+	-> TMVar ()
+	-> Annex ()
+proxySpecialRemote protoversion r ihdl ohdl endv = go
+  where
+	go = receivemessage >>= \case
+		Just (BYPASS _) -> go
+		Just (CHECKPRESENT k) -> do
+			tryNonAsync (Remote.checkPresent r k) >>= \case
+				Right True -> sendmessage SUCCESS
+				Right False -> sendmessage FAILURE
+				Left err -> sendmessage (ERROR (show err))
+			go
+		Just (LOCKCONTENT _) -> do
+			-- Special remotes do not support locking content.
+			sendmessage FAILURE
+			go
+		Just (REMOVE k) -> do
+			tryNonAsync (Remote.removeKey r k) >>= \case
+				Right () -> sendmessage SUCCESS
+				Left _ -> sendmessage FAILURE
+			go
+		Just (PUT af k) -> giveup "TODO PUT" -- XXX
+		Just (GET offset af k) -> giveup "TODO GET" -- XXX
+		Just (CONNECT _) -> 
+			-- Not supported and the protocol ends here.
+			sendmessage $ CONNECTDONE (ExitFailure 1)	
+		Just NOTIFYCHANGE -> do
+			sendmessage (ERROR "NOTIFYCHANGE unsupported for a special remote")
+			go
+		Just _ -> giveup "protocol error"
+		Nothing -> return ()
+
+	getnextmessageorend = 
+		liftIO $ atomically $ 
+			(Right <$> takeTMVar ohdl)
+				`orElse`
+			(Left <$> takeTMVar endv)
+
+	receivemessage = getnextmessageorend >>= \case
+		Right (Right m) -> return (Just m)
+		Right (Left _b) -> giveup "unexpected ByteString received from P2P MVar"
+		Left () -> return Nothing
+	--receivebytestring = liftIO (atomically $ takeTMVar ohdl) >>= \case
+	--	Left b -> return b
+	--	Right _m -> giveup "did not receive ByteString from P2P MVar"
+
+	sendmessage m = liftIO $ atomically $ putTMVar ihdl (Right m)
+	sendbytestring b = liftIO $ atomically $ putTMVar ihdl (Left b)