Run tunnels as singleton process (for a --cli-data-dir) (#177002)

* wip on singleton

* wip

* windows support

* wip

* wip

* fix clippy

cli/src/tunnels/singleton_client.rs

@@ -0,0 +1,45 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+use crate::{
+	async_pipe::{socket_stream_split, AsyncPipe},
+	json_rpc::{new_json_rpc, start_json_rpc},
+	log,
+	util::sync::Barrier,
+};
+
+use super::{protocol, shutdown_signal::ShutdownSignal};
+
+pub struct SingletonClientArgs {
+	pub log: log::Logger,
+	pub stream: AsyncPipe,
+	pub shutdown: Barrier<ShutdownSignal>,
+}
+
+struct SingletonServerContext {
+	log: log::Logger,
+}
+
+pub async fn start_singleton_client(args: SingletonClientArgs) {
+	let rpc = new_json_rpc();
+
+	debug!(
+		args.log,
+		"An existing tunnel is running on this machine, connecting to it..."
+	);
+
+	let mut rpc = rpc.methods(SingletonServerContext {
+		log: args.log.clone(),
+	});
+
+	rpc.register_sync("log", |log: protocol::singleton::LogMessageOwned, c| {
+		c.log
+			.emit(log.level, &format!("{}: {}", log.prefix, log.message));
+		Ok(())
+	});
+
+	let (read, write) = socket_stream_split(args.stream);
+	let _ = start_json_rpc(rpc.build(args.log), read, write, (), args.shutdown.clone()).await;
+}