support P2PAnnex in connectPeer

This is probably enough to support accessing remotes using p2p-annex:: urls.
Not tested yet of course since there is not yet support for serving the
other side of such a connection, or for setting up such a connection.

P2P.Generic has an implementation of the whole interface to the
git-annex-p2p-<netname> commands.

P2P/Generic.hs

@@ -0,0 +1,57 @@
+{- P2P protocol, generic transports.
+ -
+ - See doc/design/generic_p2p_transport.mdwn
+ -
+ - Copyright 2025 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module P2P.Generic where
+
+import Common
+import P2P.Address
+
+genericP2PCommand :: P2PNetName -> String
+genericP2PCommand (P2PNetName netname) = "git-annex-p2p-" ++ netname
+
+connectGenericP2P :: P2PNetName -> UnderlyingP2PAddress -> CreateProcess
+connectGenericP2P netname (UnderlyingP2PAddress address) =
+	(proc (genericP2PCommand netname) [address])
+		{ std_in = CreatePipe
+		, std_out = CreatePipe
+		}
+
+socketGenericP2P :: P2PNetName -> UnderlyingP2PAddress -> CreateProcess
+socketGenericP2P netname (UnderlyingP2PAddress address) =
+	(proc (genericP2PCommand netname) ["socket", address])
+		{ std_in = CreatePipe
+		}
+
+addressGenericP2P :: P2PNetName -> CreateProcess
+addressGenericP2P netname =
+	(proc (genericP2PCommand netname) ["address"])
+		{ std_in = CreatePipe
+		}
+
+getSocketGenericP2P :: P2PNetName -> UnderlyingP2PAddress -> IO (Maybe (OsPath, ProcessHandle))
+getSocketGenericP2P netname address = do
+	(Just hin, Nothing, Nothing, pid) <- createProcess $
+		socketGenericP2P netname address
+	hGetLineUntilExitOrEOF pid hin >>= \case
+		Just l | not (null l) -> return $ Just (toOsPath l, pid)
+		_ -> return Nothing
+
+getAddressGenericP2P :: P2PNetName -> IO [P2PAddress]
+getAddressGenericP2P netname = do
+	(Just hin, Nothing, Nothing, pid) <- createProcess $
+		addressGenericP2P netname
+	go [] hin pid
+  where
+	go addrs hin pid = hGetLineUntilExitOrEOF pid hin >>= \case
+		Just l
+			| not (null l) -> 
+				let addr = P2PAnnex netname (UnderlyingP2PAddress l)
+				in go (addr:addrs) hin pid
+			| otherwise -> go addrs hin pid
+		Nothing -> return addrs