allow `0` to mean "The OS should pick an available TCP port for us to listen to"

prototypes/ptth_quic_client_gui/src/main.rs

@@ -51,6 +51,27 @@ struct Port {
 	forwarding_instance: Option <ForwardingInstance>,
 }
 
+impl Port {
+	pub fn open_port (&mut self, rt: &Runtime, connection_p2_p3: quinn::Connection)
+	-> anyhow::Result <()> 
+	{
+		let params = self.gui.get_params ()?;
+		
+		let _guard = rt.enter ();
+		let forwarding_instance = rt.block_on (ForwardingInstance::new (
+			connection_p2_p3,
+			params,
+		))?;
+		
+		self.gui.input_client_port.set_value (&forwarding_instance.local_port ().to_string ());
+		
+		self.forwarding_instance.replace (forwarding_instance);
+		self.gui.set_forwarding (true);
+		
+		Ok (())
+	}
+}
+
 struct GuiPort {
 	row: fltk::group::Flex,
 	input_client_port: Input,
@@ -67,17 +88,7 @@ impl GuiClient <'_> {
 		port_idx: usize,
 	) -> anyhow::Result <()>
 	{
-		let params = self.ports [port_idx].gui.get_params ()?;
+		self.ports [port_idx].open_port (&self.rt, connection_p2_p3)?;
-		
-		let _guard = self.rt.enter ();
-		let forwarding_instance = self.rt.block_on (ForwardingInstance::new (
-			connection_p2_p3,
-			params,
-		))?;
-		
-		self.ports [port_idx].forwarding_instance.replace (forwarding_instance);
-		
-		self.ports [port_idx].gui.set_forwarding (true);
 		self.sync_status ();
 		
 		Ok (())
@@ -152,12 +163,12 @@ fn main () -> anyhow::Result <()> {
 	{
 		let mut row = Flex::default ().row ();
 		
-		let mut l = Frame::default ().with_label ("Local port");
-		row.set_size (&mut l, 80);
 		let mut l = Frame::default ().with_label ("Server ID");
 		row.set_size (&mut l, 120);
 		let mut l = Frame::default ().with_label ("Server port");
 		row.set_size (&mut l, 80);
+		let mut l = Frame::default ().with_label ("Local port");
+		row.set_size (&mut l, 80);
 		row.end ();
 		
 		col.set_size (&mut row, 30);
@@ -249,19 +260,19 @@ impl GuiPort {
 		
 		let mut row = Flex::default ().row ();
 		
-		let mut input_client_port = Input::default ();
 		let mut input_server_id = Input::default ();
 		let mut input_server_port = Input::default ();
+		let mut input_client_port = Input::default ();
 		let mut but_open = Button::default ().with_label ("Open");
 		let mut but_close = Button::default ().with_label ("Close");
 		
-		row.set_size (&mut input_client_port, 80);
 		row.set_size (&mut input_server_id, 120);
 		row.set_size (&mut input_server_port, 80);
+		row.set_size (&mut input_client_port, 80);
 		row.set_size (&mut but_open, 80);
 		row.set_size (&mut but_close, 80);
 		
-		input_client_port.set_value ("5901");
+		input_client_port.set_value ("0");
 		input_server_id.set_value ("bogus_server");
 		input_server_port.set_value ("5900");
 		

prototypes/quic_demo/src/client_proxy.rs

@@ -9,6 +9,7 @@ use crate::prelude::*;
 pub struct ForwardingInstance {
 	task: JoinHandle <anyhow::Result <()>>,
 	shutdown_flag: watch::Sender <bool>,
+	local_port: u16,
 }
 
 impl ForwardingInstance {
@@ -20,7 +21,8 @@ impl ForwardingInstance {
 		let (shutdown_flag, shutdown_flag_rx) = tokio::sync::watch::channel (true);
 		
 		let listener = TcpListener::bind (("127.0.0.1", params.client_tcp_port)).await?;
-		trace! ("Accepting local TCP connections from P1 on {}", params.client_tcp_port);
+		let local_port = listener.local_addr ()?.port ();
+		trace! ("Accepting local TCP connections from P1 on {}", local_port);
 		
 		let task = tokio::spawn (forward_port (
 			listener,
@@ -32,6 +34,7 @@ impl ForwardingInstance {
 		Ok (Self {
 			task,
 			shutdown_flag,
+			local_port,
 		})
 	}
 	
@@ -44,6 +47,10 @@ impl ForwardingInstance {
 		.context ("inside ForwardingInstance task")?;
 		Ok (())
 	}
+	
+	pub fn local_port (&self) -> u16 {
+		self.local_port
+	}
 }
 
 pub struct ForwardingParams {