Repository: rust-lang-nursery/futures-rs (rust)

Issue:
Race condition in futures::sync::mpsc
It looks like there's a race condition in the MPSC implementation in `futures 0.1.19`, unless my understanding of the intended behavior is incorrect.  If a `Sender::try_send()` happens concurrently with a `Receiver` close/drop, it's possible for `try_send()` to return `Ok(())` even though the item can never be received and will not be dropped until the `Sender` is dropped.  My expectation was to receive an error matching `Err(ref e) if e.is_disconnected()`.  The `Receiver` `Drop` implementation closes the channel and drains any items present, but this can apparently happen just before the `Sender` thinks it has successfully enqueued the item.

I wrote a small program to stress-test this scenario and demonstrate the bug:
https://github.com/simmons/mpsc-stress

I discovered this behavior while stress testing a program which implements a synchronous API by sending a command + `oneshot::Sender` to a future's MPSC, which processes the command and sends the oneshot when complete.  When the described race occurs, the sending thread would deadlock while hopelessly waiting forever for the `oneshot::Receiver` to either receive or indicate disconnection.

If I get a chance in the next few days, I'll see if I can root-cause the problem.  (Unless I'm told that my expectation is incorrect.)

Pull request:
fix race with dropping mpsc::Receiver
Fix a bug where messages sent into a channel at the same time as the
`Receiver` is dropped are never removed.

The impl in 0.3 suffers the same bug.

Fixes #909

Patch:
diff --git a/src/sync/mpsc/mod.rs b/src/sync/mpsc/mod.rs
index 71875796..d9f36924 100644
--- a/src/sync/mpsc/mod.rs
+++ b/src/sync/mpsc/mod.rs
@@ -813,6 +813,12 @@ impl<T> Receiver<T> {
         loop {
             match unsafe { self.inner.message_queue.pop() } {
                 PopResult::Data(msg) => {
+                    // If there are any parked task handles in the parked queue,
+                    // pop one and unpark it.
+                    self.unpark_one();
+                    // Decrement number of messages
+                    self.dec_num_messages();
+
                     return Async::Ready(msg);
                 }
                 PopResult::Empty => {
@@ -863,7 +869,7 @@ impl<T> Receiver<T> {
         let state = decode_state(curr);
 
         // If the channel is closed, then there is no need to park.
-        if !state.is_open && state.num_messages == 0 {
+        if state.is_closed() {
             return TryPark::Closed;
         }
 
@@ -904,8 +910,8 @@ impl<T> Stream for Receiver<T> {
     fn poll(&mut self) -> Poll<Option<T>, ()> {
         loop {
             // Try to read a message off of the message queue.
-            let msg = match self.next_message() {
-                Async::Ready(msg) => msg,
+            match self.next_message() {
+                Async::Ready(msg) => return Ok(Async::Ready(msg)),
                 Async::NotReady => {
                     // There are no messages to read, in this case, attempt to
                     // park. The act of parking will verify that the channel is
@@ -929,17 +935,7 @@ impl<T> Stream for Receiver<T> {
                         }
                     }
                 }
-            };
-
-            // If there are any parked task handles in the parked queue, pop
-            // one and unpark it.
-            self.unpark_one();
-
-            // Decrement number of messages
-            self.dec_num_messages();
-
-            // Return the message
-            return Ok(Async::Ready(msg));
+            }
         }
     }
 }
@@ -948,8 +944,27 @@ impl<T> Drop for Receiver<T> {
     fn drop(&mut self) {
         // Drain the channel of all pending messages
         self.close();
-        while self.next_message().is_ready() {
-            // ...
+
+        loop {
+            match self.next_message() {
+                Async::Ready(_) => {}
+                Async::NotReady => {
+                    let curr = self.inner.state.load(SeqCst);
+                    let state = decode_state(curr);
+
+                    // If the channel is closed, then there is no need to park.
+                    if state.is_closed() {
+                        return;
+                    }
+
+                    // TODO: Spinning isn't ideal, it might be worth
+                    // investigating using a condvar or some other strategy
+                    // here. That said, if this case is hit, then another thread
+                    // is about to push the value into the queue and this isn't
+                    // the only spinlock in the impl right now.
+                    thread::yield_now();
+                }
+            }
         }
     }
 }
@@ -1125,6 +1140,12 @@ impl<T> Inner<T> {
 unsafe impl<T: Send> Send for Inner<T> {}
 unsafe impl<T: Send> Sync for Inner<T> {}
 
+impl State {
+    fn is_closed(&self) -> bool {
+        !self.is_open && self.num_messages == 0
+    }
+}
+
 /*
  *
  * ===== Helpers =====


Test patch:
diff --git a/tests/mpsc-close.rs b/tests/mpsc-close.rs
index 253e0157..061616ae 100644
--- a/tests/mpsc-close.rs
+++ b/tests/mpsc-close.rs
@@ -1,9 +1,12 @@
 extern crate futures;
 
+use std::sync::{Arc, Weak};
 use std::thread;
+use std::time::{Duration, Instant};
 
 use futures::prelude::*;
 use futures::sync::mpsc::*;
+use futures::task;
 
 #[test]
 fn smoke() {
@@ -19,3 +22,131 @@ fn smoke() {
 
     t.join().unwrap()
 }
+
+// Stress test that `try_send()`s occurring concurrently with receiver
+// close/drops don't appear as successful sends.
+#[test]
+fn stress_try_send_as_receiver_closes() {
+    const AMT: usize = 10000;
+    // To provide variable timing characteristics (in the hopes of
+    // reproducing the collision that leads to a race), we busy-re-poll
+    // the test MPSC receiver a variable number of times before actually
+    // stopping.  We vary this countdown between 1 and the following
+    // value.
+    const MAX_COUNTDOWN: usize = 20;
+    // When we detect that a successfully sent item is still in the
+    // queue after a disconnect, we spin for up to 100ms to confirm that
+    // it is a persistent condition and not a concurrency illusion.
+    const SPIN_TIMEOUT_S: u64 = 10;
+    const SPIN_SLEEP_MS: u64 = 10;
+    struct TestRx {
+        rx: Receiver<Arc<()>>,
+        // The number of times to query `rx` before dropping it.
+        poll_count: usize
+    }
+    struct TestTask {
+        command_rx: Receiver<TestRx>,
+        test_rx: Option<Receiver<Arc<()>>>,
+        countdown: usize,
+    }
+    impl TestTask {
+        /// Create a new TestTask
+        fn new() -> (TestTask, Sender<TestRx>) {
+            let (command_tx, command_rx) = channel::<TestRx>(0);
+            (
+                TestTask {
+                    command_rx: command_rx,
+                    test_rx: None,
+                    countdown: 0, // 0 means no countdown is in progress.
+                },
+                command_tx,
+            )
+        }
+    }
+    impl Future for TestTask {
+        type Item = ();
+        type Error = ();
+        fn poll(&mut self) -> Poll<(), ()> {
+            // Poll the test channel, if one is present.
+            if let Some(ref mut rx) = self.test_rx {
+                if let Ok(Async::Ready(v)) = rx.poll() {
+                   let _ = v.expect("test finished unexpectedly!");
+                }
+                self.countdown -= 1;
+                // Busy-poll until the countdown is finished.
+                task::current().notify();
+            }
+            // Accept any newly submitted MPSC channels for testing.
+            match self.command_rx.poll()? {
+                Async::Ready(Some(TestRx { rx, poll_count })) => {
+                    self.test_rx = Some(rx);
+                    self.countdown = poll_count;
+                    task::current().notify();
+                },
+                Async::Ready(None) => return Ok(Async::Ready(())),
+                _ => {},
+            }
+            if self.countdown == 0 {
+                // Countdown complete -- drop the Receiver.
+                self.test_rx = None;
+            }
+            Ok(Async::NotReady)
+        }
+    }
+    let (f, mut cmd_tx) = TestTask::new();
+    let bg = thread::spawn(move || f.wait());
+    for i in 0..AMT {
+        let (mut test_tx, rx) = channel(0);
+        let poll_count = i % MAX_COUNTDOWN;
+        cmd_tx.try_send(TestRx { rx: rx, poll_count: poll_count }).unwrap();
+        let mut prev_weak: Option<Weak<()>> = None;
+        let mut attempted_sends = 0;
+        let mut successful_sends = 0;
+        loop {
+            // Create a test item.
+            let item = Arc::new(());
+            let weak = Arc::downgrade(&item);
+            match test_tx.try_send(item) {
+                Ok(_) => {
+                    prev_weak = Some(weak);
+                    successful_sends += 1;
+                }
+                Err(ref e) if e.is_full() => {}
+                Err(ref e) if e.is_disconnected() => {
+                    // Test for evidence of the race condition.
+                    if let Some(prev_weak) = prev_weak {
+                        if prev_weak.upgrade().is_some() {
+                            // The previously sent item is still allocated.
+                            // However, there appears to be some aspect of the
+                            // concurrency that can legitimately cause the Arc
+                            // to be momentarily valid.  Spin for up to 100ms
+                            // waiting for the previously sent item to be
+                            // dropped.
+                            let t0 = Instant::now();
+                            let mut spins = 0;
+                            loop {
+                                if prev_weak.upgrade().is_none() {
+                                    break;
+                                }
+                                assert!(t0.elapsed() < Duration::from_secs(SPIN_TIMEOUT_S),
+                                    "item not dropped on iteration {} after \
+                                     {} sends ({} successful). spin=({})",
+                                    i, attempted_sends, successful_sends, spins
+                                );
+                                spins += 1;
+                                thread::sleep(Duration::from_millis(SPIN_SLEEP_MS));
+                            }
+                        }
+                    }
+                    break;
+                }
+                Err(ref e) => panic!("unexpected error: {}", e),
+            }
+            attempted_sends += 1;
+        }
+    }
+    drop(cmd_tx);
+    bg.join()
+        .expect("background thread join")
+        .expect("background thread result");
+}