dup stdio handles for P2P proxy

Special remotes might output to stdout, or read from stdin, which would
mess up the P2P protocol. So dup the handles to avoid any such problem.

P2P/IO.hs

@@ -1,6 +1,6 @@
 {- P2P protocol, IO implementation
  -
- - Copyright 2016-2018 Joey Hess <id@joeyh.name>
+ - Copyright 2016-2024 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -16,6 +16,7 @@ module P2P.IO
 	, ConnIdent(..)
 	, ClosableConnection(..)
 	, stdioP2PConnection
+	, stdioP2PConnectionDupped
 	, connectPeer
 	, closeConnection
 	, serveUnixSocket
@@ -104,6 +105,20 @@ stdioP2PConnection g = P2PConnection
 	, connIdent = ConnIdent Nothing
 	}
 
+-- P2PConnection using stdio, but with the handles first duplicated,
+-- to avoid anything that might output to stdio (eg a program run by a
+-- special remote) from interfering with the connection.
+stdioP2PConnectionDupped :: Maybe Git.Repo -> IO P2PConnection
+stdioP2PConnectionDupped g = do
+	(readh, writeh) <- dupIoHandles
+	return $ P2PConnection
+		{ connRepo = g
+		, connCheckAuth = const False
+		, connIhdl = P2PHandle readh
+		, connOhdl = P2PHandle writeh
+		, connIdent = ConnIdent Nothing
+		}
+
 -- Opens a connection to a peer. Does not authenticate with it.
 connectPeer :: Maybe Git.Repo -> P2PAddress -> IO P2PConnection
 connectPeer g (TorAnnex onionaddress onionport) = do