fix: Potential event loop hang if shutdown is sent when paused

bittorrent/src/event_loop.rs

@@ -718,6 +718,11 @@ impl<'scope, 'state: 'scope> EventLoop {
                         self.state = EventLoopState::ShuttingDown { listener_fd };
                         connection_manager.disconnect_all(io, state_ref);
                     }
+                    // Break out of the command loop for the potential of us being paused.
+                    // It's the same reason `Resume` does. We can do this unconditionally
+                    // since command left in the channel is for a torrent that is
+                    // going away is pretty meaningless anyways
+                    break;
                 }
             }
         }

bittorrent/tests/pause_shutdown.rs

@@ -0,0 +1,85 @@
+mod common;
+
+use std::{collections::HashMap, net::TcpListener, sync::mpsc::RecvTimeoutError, time::Duration};
+
+use heapless::spsc;
+use vortex_bittorrent::{Command, Config, PeerId, State, Torrent, TorrentEvent};
+
+use common::{create_test_torrent, init_test_environment};
+
+/// How long the event loop is given to shut down after being asked to. Only ever
+/// waited out when the shutdown is broken, a working one takes well under a second
+const SHUTDOWN_TIMEOUT: Duration = Duration::from_secs(15);
+
+/// A paused torrent iterates its command channel with the blocking iterator, so
+/// `Stop` has to be enough on its own to get the event loop out of there. It used
+/// to only return once the channel disconnected on top of that, which hangs any
+/// client that keeps its command sender alive while joining the torrent thread.
+#[test]
+fn stop_while_paused_shuts_down() {
+    init_test_environment();
+
+    let test_files: HashMap<String, Vec<u8>> =
+        [("file.txt".to_string(), b"BitTorrent Test Data!".repeat(200))]
+            .into_iter()
+            .collect();
+
+    let torrent_name = format!("test_pause_shutdown_{}", rand::random::<u32>());
+    let (torrent_dir, metadata) = create_test_torrent(&test_files, &torrent_name, 16384);
+
+    let state = State::from_metadata_and_root(
+        PeerId::generate(),
+        metadata,
+        torrent_dir.path().clone(),
+        Config::default(),
+    )
+    .expect("Failed to create state");
+    let mut torrent = Torrent::new(state);
+
+    let (command_tx, command_rc) = std::sync::mpsc::sync_channel(64);
+    // Leaked so that the event loop can run on a detached thread: a shutdown that
+    // never happens then fails the assertion below instead of blocking the test
+    // forever on a join
+    let event_q: &'static mut spsc::Queue<TorrentEvent, 512> =
+        Box::leak(Box::new(spsc::Queue::new()));
+    let (event_tx, mut event_rc) = event_q.split();
+
+    let (stopped_tx, stopped_rc) = std::sync::mpsc::channel();
+    std::thread::spawn(move || {
+        let listener = TcpListener::bind("127.0.0.1:0").unwrap();
+        torrent.start(event_tx, command_rc, listener).unwrap();
+        stopped_tx.send(()).unwrap();
+    });
+
+    let mut pause_sent = false;
+    loop {
+        let Some(event) = event_rc.dequeue() else {
+            continue;
+        };
+        match event {
+            TorrentEvent::Running { port } => {
+                log::info!("Running on port {port}, requesting pause");
+                assert!(!pause_sent, "The torrent should never be resumed");
+                command_tx.send(Command::Pause).unwrap();
+                pause_sent = true;
+            }
+            TorrentEvent::Paused => {
+                log::info!("Paused, requesting shutdown");
+                command_tx.send(Command::Stop).unwrap();
+                break;
+            }
+            _ => {}
+        }
+    }
+
+    // `command_tx` is deliberately kept alive here, so the command channel never
+    // disconnects and only the `Stop` itself can end the event loop
+    match stopped_rc.recv_timeout(SHUTDOWN_TIMEOUT) {
+        Ok(()) => log::info!("The event loop shut down while paused"),
+        Err(RecvTimeoutError::Timeout) => {
+            panic!("The event loop did not shut down after being stopped while paused")
+        }
+        Err(RecvTimeoutError::Disconnected) => panic!("The event loop thread died"),
+    }
+    drop(command_tx);
+}