Fix long-standing race in net-tokio reading after a disconnect event

If rust-lightning tells us to disconnect a socket after we read
some bytes from the socket, but before we actually give those bytes
to rust-lightning, we may end up calling rust-lightning with a
Descriptor that isn't registered anymore.

Sadly, there really isn't a good way to solve this, and it should
be a pretty quick event, so we just busy-wait.

Author: TheBlueMatt

lightning-net-tokio/src/lib.rs

@@ -70,7 +70,7 @@ use lightning::ln::peer_handler;
 use lightning::ln::peer_handler::SocketDescriptor as LnSocketTrait;
 use lightning::ln::msgs::ChannelMessageHandler;
 
-use std::task;
+use std::{task, thread};
 use std::net::SocketAddr;
 use std::sync::{Arc, Mutex, MutexGuard};
 use std::sync::atomic::{AtomicU64, Ordering};
@@ -101,6 +101,11 @@ struct Connection {
 	// socket. To wake it up (without otherwise changing its state, we can push a value into this
 	// Sender.
 	read_waker: mpsc::Sender<()>,
+	// When we are told by rust-lightning to disconnect, we can't return to rust-lightning until we
+	// are sure we won't call any more read/write PeerManager functions with the same connection.
+	// This is set to true if we're in such a condition (with disconnect checked before with the
+	// top-level mutex held) and false when we can return.
+	block_disconnect_socket: bool,
 	read_paused: bool,
 	rl_requested_disconnect: bool,
 	id: u64,
@@ -135,35 +140,43 @@ impl Connection {
 				} }
 			}
 
+			macro_rules! prepare_read_write_call {
+				() => { {
+					let mut us_lock = us.lock().unwrap();
+					if us_lock.rl_requested_disconnect {
+						shutdown_socket!("disconnect_socket() call from RL", Disconnect::CloseConnection);
+					}
+					us_lock.block_disconnect_socket = true;
+				} }
+			}
+
 			let read_paused = us.lock().unwrap().read_paused;
 			tokio::select! {
 				v = write_avail_receiver.recv() => {
 					assert!(v.is_some()); // We can't have dropped the sending end, its in the us Arc!
-					if us.lock().unwrap().rl_requested_disconnect {
-						shutdown_socket!("disconnect_socket() call from RL", Disconnect::CloseConnection);
-					}
+					prepare_read_write_call!();
 					if let Err(e) = peer_manager.write_buffer_space_avail(&mut our_descriptor) {
 						shutdown_socket!(e, Disconnect::CloseConnection);
 					}
+					us.lock().unwrap().block_disconnect_socket = false;
 				},
 				_ = read_wake_receiver.recv() => {},
 				read = reader.read(&mut buf), if !read_paused => match read {
 					Ok(0) => shutdown_socket!("Connection closed", Disconnect::PeerDisconnected),
 					Ok(len) => {
-						if us.lock().unwrap().rl_requested_disconnect {
-							shutdown_socket!("disconnect_socket() call from RL", Disconnect::CloseConnection);
-						}
+						prepare_read_write_call!();
 						let read_res = peer_manager.read_event(&mut our_descriptor, &buf[0..len]);
+						let mut us_lock = us.lock().unwrap();
 						match read_res {
 							Ok(pause_read) => {
-								let mut us_lock = us.lock().unwrap();
 								if pause_read {
 									us_lock.read_paused = true;
 								}
 								Self::event_trigger(&mut us_lock);
 							},
 							Err(e) => shutdown_socket!(e, Disconnect::CloseConnection),
 						}
+						us_lock.block_disconnect_socket = false;
 					},
 					Err(e) => shutdown_socket!(e, Disconnect::PeerDisconnected),
 				},
@@ -195,8 +208,8 @@ impl Connection {
 
 		(reader, write_receiver, read_receiver,
 		Arc::new(Mutex::new(Self {
-			writer: Some(writer), event_notify, write_avail, read_waker,
-			read_paused: false, rl_requested_disconnect: false,
+			writer: Some(writer), event_notify, write_avail, read_waker, read_paused: false,
+			block_disconnect_socket: false, rl_requested_disconnect: false,
 			id: ID_COUNTER.fetch_add(1, Ordering::AcqRel)
 		})))
 	}
@@ -403,15 +416,18 @@ impl peer_handler::SocketDescriptor for SocketDescriptor {
 	}
 
 	fn disconnect_socket(&mut self) {
-		let mut us = self.conn.lock().unwrap();
-		us.rl_requested_disconnect = true;
-		us.read_paused = true;
-		// Wake up the sending thread, assuming it is still alive
-		let _ = us.write_avail.try_send(());
-		// TODO: There's a race where we don't meet the requirements of disconnect_socket if the
-		// read task is about to call a PeerManager function (eg read_event or write_event).
-		// Ideally we need to release the us lock and block until we have confirmation from the
-		// read task that it has broken out of its main loop.
+		{
+			let mut us = self.conn.lock().unwrap();
+			us.rl_requested_disconnect = true;
+			us.read_paused = true;
+			// Wake up the sending thread, assuming it is still alive
+			let _ = us.write_avail.try_send(());
+			// Happy-path return:
+			if !us.block_disconnect_socket { return; }
+		}
+		while self.conn.lock().unwrap().block_disconnect_socket {
+			thread::yield_now();
+		}
 	}
 }
 impl Clone for SocketDescriptor {