feat(lib): let another thread cancel a client's connect and login

A caller that gives up on a login had no way to reach the client: the
thread that owns it sits in login() for up to 45 seconds, and disconnect()
needs the client to itself. Until that thread returned, the abandoned
client kept connecting and still wrote its login frame the moment the
server finished the TCP handshake. When that happened after a replacement
had logged in under the same name, the server kicked the replacement.

Client::cancel_handle() hands out a CancelHandle that can be moved to any
thread. After cancel():
- the server actor writes nothing more. The check and the write share a
  lock with cancel(), so once cancel() returns no frame leaves, whether it
  was waiting in the queue, in the dispatcher channel, or mid-flush;
- login() returns NotConnected within 100 ms, and hands no login to the
  actor at all when the client was already cancelled;
- connect() refuses before binding the listener when the client was
  already cancelled. A cancel that lands during that check can still let
  the bind through.
Either way the owner drops the client as before, which disconnects it and
gives its port back.

A frame already written before cancel() returned cannot be recalled. The
API is additive, and login() on a client nobody cancels behaves as before.

Soulseek.NET tears the whole connection down when ConnectAsync is
cancelled, and Nicotine+ drops outgoing messages for a socket it closed;
this gives soulseek-rs the same end state from another thread.

soulseek-rs-lib/src/actor/server_actor.rs

@@ -1,5 +1,5 @@
 use crate::actor::{Actor, ActorHandle, ConnectionState};
-use crate::client::ClientOperation;
+use crate::client::{CancelHandle, ClientOperation};
 use crate::dispatcher::MessageDispatcher;
 use crate::message::server::AdminMessageHandler;
 use crate::message::server::CantConnectToPeerHandler;
@@ -288,6 +288,7 @@ pub struct ServerActor {
     shared_folder_count: u32,
     shared_file_count: u32,
     session: SessionWatch,
+    cancel: CancelHandle,
 }
 
 /// The messages a client sends right after a successful login: its shared-file
@@ -340,6 +341,7 @@ impl ServerActor {
             shared_folder_count,
             shared_file_count,
             session: SessionWatch::default(),
+            cancel: CancelHandle::new(),
         }
     }
 
@@ -348,6 +350,10 @@ impl ServerActor {
         self.session = session;
     }
 
+    pub fn set_cancel(&mut self, cancel: CancelHandle) {
+        self.cancel = cancel;
+    }
+
     fn initiate_connection(&mut self) -> bool {
         let stream = (self.address.get_host(), self.address.get_port())
             .to_socket_addrs()
@@ -563,6 +569,10 @@ impl ServerActor {
     }
 
     fn send_message(&mut self, message: Message) {
+        let cancel = self.cancel.clone();
+        let Some(_live) = cancel.live() else {
+            return;
+        };
         let Some(stream) = self.stream.as_mut() else {
             error!("[server] Cannot send message: stream is None");
             return;

soulseek-rs-lib/src/actor/server_actor/tests.rs

@@ -188,6 +188,44 @@ fn a_login_with_nobody_listening_is_refused_at_once() {
     );
 }
 
+#[test]
+fn a_cancelled_actor_never_writes_the_login_it_queued() {
+    use std::io::Read;
+
+    let listener = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+    let addr = listener.local_addr().unwrap();
+    let client = TcpStream::connect(addr).unwrap();
+    let (mut server_side, _) = listener.accept().unwrap();
+    client.set_nonblocking(true).unwrap();
+
+    let cancel = CancelHandle::new();
+    let mut actor = parked_actor(addr.port());
+    actor.set_cancel(cancel.clone());
+    actor.stream = Some(client);
+    actor.connection_state = ConnectionState::Connecting {
+        since: Instant::now(),
+    };
+    let (response, _verdict) = std::sync::mpsc::channel();
+    actor.handle_login(
+        "u".into(),
+        "p".into(),
+        ClientVersion::default(),
+        response,
+    );
+    assert_eq!(actor.queued_messages.len(), 1);
+
+    cancel.cancel();
+    actor.tick();
+    assert!(matches!(actor.connection_state, ConnectionState::Connected));
+    actor.on_stop();
+
+    server_side
+        .set_read_timeout(Some(Duration::from_secs(5)))
+        .unwrap();
+    let mut written = Vec::new();
+    assert_eq!(server_side.read_to_end(&mut written).unwrap(), 0);
+}
+
 #[test]
 fn a_successful_login_marks_the_session_live_again() {
     let mut actor = parked_actor(1);

soulseek-rs-lib/src/client/cancel.rs

@@ -0,0 +1,24 @@
+use std::sync::{Arc, Mutex, MutexGuard, PoisonError};
+
+#[derive(Clone)]
+pub struct CancelHandle(Arc<Mutex<bool>>);
+
+impl CancelHandle {
+    pub(crate) fn new() -> Self {
+        Self(Arc::new(Mutex::new(false)))
+    }
+
+    pub fn cancel(&self) {
+        *self.0.lock().unwrap_or_else(PoisonError::into_inner) = true;
+    }
+
+    #[must_use]
+    pub fn is_cancelled(&self) -> bool {
+        *self.0.lock().unwrap_or_else(PoisonError::into_inner)
+    }
+
+    pub(crate) fn live(&self) -> Option<MutexGuard<'_, bool>> {
+        let guard = self.0.lock().unwrap_or_else(PoisonError::into_inner);
+        (!*guard).then_some(guard)
+    }
+}

soulseek-rs-lib/src/client/connection.rs

@@ -5,9 +5,11 @@ use super::{
     TcpStream, error, info, mpsc, thread, trace, warn,
 };
 use std::sync::atomic::Ordering;
+use std::sync::mpsc::RecvTimeoutError;
 use std::time::{Duration, Instant};
 
 const LISTENER_RELEASE_TIMEOUT: Duration = Duration::from_secs(2);
+const LOGIN_POLL_INTERVAL: Duration = Duration::from_millis(100);
 
 /// Ceiling on the wait for a login verdict. Generous enough for a slow server
 /// and a retrying connect, short enough that an unattended caller ends.
@@ -90,6 +92,10 @@ impl Client {
         ctx.shares = shares;
         ctx.shared_directories.clone_from(&self.shared_directories);
 
+        if self.cancel.is_cancelled() {
+            return Err(SoulseekRs::NotConnected);
+        }
+
         // Bind before logging in: the port we advertise has to be the port we
         // hold, and a bind that fails outright is the caller's to see rather
         // than a panic on a thread nobody is watching.
@@ -111,6 +117,7 @@ impl Client {
             shared_file_count,
         );
         server_actor.set_session_watch(self.session.clone());
+        server_actor.set_cancel(self.cancel.clone());
 
         self.server_handle = Some(ctx.actor_system.spawn_with_handle(
             server_actor,
@@ -202,7 +209,9 @@ impl Client {
     /// control.
     pub fn login(&self) -> Result<bool> {
         info!("Logging in as {}", self.username);
-        if let Some(handle) = &self.server_handle {
+        if let Some(handle) = &self.server_handle
+            && !self.cancel.is_cancelled()
+        {
             let (tx, rx) = std::sync::mpsc::channel();
             let _ = handle.send(ServerMessage::Login {
                 username: self.username.clone(),
@@ -211,9 +220,19 @@ impl Client {
                 response: tx,
             });
 
-            match rx.recv_timeout(LOGIN_RESPONSE_TIMEOUT) {
-                Ok(result) => result,
-                Err(_) => Err(SoulseekRs::Timeout),
+            let deadline = Instant::now() + LOGIN_RESPONSE_TIMEOUT;
+            loop {
+                match rx.recv_timeout(LOGIN_POLL_INTERVAL) {
+                    Ok(result) => return result,
+                    Err(RecvTimeoutError::Timeout)
+                        if self.cancel.is_cancelled() =>
+                    {
+                        return Err(SoulseekRs::NotConnected);
+                    }
+                    Err(RecvTimeoutError::Timeout)
+                        if Instant::now() < deadline => {}
+                    Err(_) => return Err(SoulseekRs::Timeout),
+                }
             }
         } else {
             Err(SoulseekRs::NotConnected)

soulseek-rs-lib/src/client/mod.rs

@@ -649,6 +649,7 @@ pub struct Client {
     /// port it bound.
     listener_stopped: Arc<AtomicBool>,
     listener_released: Arc<AtomicBool>,
+    cancel: CancelHandle,
 }
 
 impl Drop for Client {
@@ -684,6 +685,7 @@ impl Client {
             session: SessionWatch::default(),
             listener_stopped: Arc::new(AtomicBool::new(false)),
             listener_released: Arc::new(AtomicBool::new(false)),
+            cancel: CancelHandle::new(),
         }
     }
 
@@ -724,6 +726,11 @@ impl Client {
         self.session.loss()
     }
 
+    #[must_use]
+    pub fn cancel_handle(&self) -> CancelHandle {
+        self.cancel.clone()
+    }
+
     /// The directories whose files are currently shared with other peers.
     #[must_use]
     pub fn shared_directories(&self) -> Vec<String> {
@@ -849,6 +856,7 @@ impl Client {
     }
 }
 
+mod cancel;
 mod children;
 mod connection;
 mod context;
@@ -861,5 +869,7 @@ mod social;
 mod upload_queue;
 mod uploads;
 
+pub use cancel::CancelHandle;
+
 #[cfg(test)]
 mod tests;

soulseek-rs-lib/src/lib.rs

@@ -32,7 +32,7 @@ pub mod prelude {
 
 // Re-export commonly used types
 pub use actor::server_actor::{PeerAddress, UserMessage};
-pub use client::{Client, ClientSettings};
+pub use client::{CancelHandle, Client, ClientSettings};
 pub use error::{Result, SoulseekRs};
 pub use message::peer::{SharedDirectory, SharedFileEntry};
 pub use peer::{ConnectionType, ParseConnectionTypeError};

soulseek-rs-lib/tests/e2e.rs

@@ -5621,3 +5621,74 @@ fn a_dropped_client_has_released_its_listener_port_when_drop_returns() {
         );
     }
 }
+
+#[test]
+fn a_cancelled_client_never_binds_its_port() {
+    let _gate = SERVER_GATE
+        .lock()
+        .unwrap_or_else(std::sync::PoisonError::into_inner);
+    let port = free_port().expect("a port to hold");
+    let mut client = Client::with_settings(ClientSettings {
+        username: "e2e_cancelled_bind".to_string(),
+        password: "pw".to_string(),
+        server_address: PeerAddress::new("127.0.0.1".to_string(), 1),
+        enable_listen: true,
+        listen_port: port,
+        shared_directories: Vec::new(),
+        accept_children: false,
+        version: ClientVersion::default(),
+    });
+
+    client.cancel_handle().cancel();
+
+    assert!(client.connect().is_err());
+    assert_eq!(client.listen_port(), None);
+    assert!(std::net::TcpListener::bind(("0.0.0.0", port)).is_ok());
+}
+
+#[test]
+fn a_cancel_ends_a_login_waiting_for_its_verdict_and_nothing_follows_it() {
+    let server =
+        std::net::TcpListener::bind("127.0.0.1:0").expect("a silent server");
+    let mut client = Client::with_settings(ClientSettings {
+        username: "e2e_cancelled_login".to_string(),
+        password: "pw".to_string(),
+        server_address: PeerAddress::new(
+            "127.0.0.1".to_string(),
+            server.local_addr().expect("server address").port(),
+        ),
+        enable_listen: false,
+        listen_port: 0,
+        shared_directories: Vec::new(),
+        accept_children: false,
+        version: ClientVersion::default(),
+    });
+    client.connect().expect("start the session");
+    let (mut conn, _) = server.accept().expect("the client dials the server");
+    conn.set_read_timeout(Some(Duration::from_secs(5)))
+        .expect("read timeout");
+    let cancel = client.cancel_handle();
+
+    std::thread::scope(|scope| {
+        let login = scope.spawn(|| client.login());
+        let frame =
+            read_framed(&mut conn).expect("the login reaches the server");
+        assert_eq!(frame.get_message_code(), 1);
+
+        let cancelled_at = Instant::now();
+        cancel.cancel();
+        assert!(login.join().expect("the login thread").is_err());
+        assert!(cancelled_at.elapsed() < Duration::from_secs(5));
+    });
+    client
+        .connect_peer("e2e_nobody")
+        .expect("the actor takes a request");
+    drop(client);
+
+    let mut rest = Vec::new();
+    assert_eq!(
+        conn.read_to_end(&mut rest)
+            .expect("the server sees the session end"),
+        0
+    );
+}