sync::mpsc: add miri reproducer of double free

Signed-off-by: Petros Angelatos <[email protected]>

Author: petrosagg

Diff for: library/std/src/sync/mpmc/list.rs

@@ -213,6 +213,11 @@ impl<T> Channel<T> {
                     .compare_exchange(block, new, Ordering::Release, Ordering::Relaxed)
                     .is_ok()
                 {
+                    // This yield point leaves the channel in a half-initialized state where the
+                    // tail.block pointer is set but the head.block is not. This is used to
+                    // facilitate the test in src/tools/miri/tests/pass/issues/issue-139553.rs
+                    #[cfg(miri)]
+                    crate::thread::yield_now();
                     self.head.block.store(new, Ordering::Release);
                     block = new;
                 } else {

Diff for: src/tools/miri/tests/pass/issues/issue-139553.rs

@@ -0,0 +1,45 @@
+//@compile-flags: -Zmiri-preemption-rate=0 -Zmiri-compare-exchange-weak-failure-rate=0
+use std::sync::mpsc::channel;
+use std::thread;
+
+/// This test aims to trigger a race condition that causes a double free in the unbounded channel
+/// implementation. The test relies on a particular thread scheduling to happen as annotated by the
+/// comments below.
+fn main() {
+    let (s1, r) = channel::<u64>();
+    let s2 = s1.clone();
+
+    let t1 = thread::spawn(move || {
+        // 1. The first action executed is an attempt to send the first value in the channel. This
+        //    will begin to initialize the channel but will stop at a critical momement as
+        //    indicated by the `yield_now()` call in the `start_send` method of the implementation.
+        let _ = s1.send(42);
+        // 4. The sender is re-scheduled and it finishes the initialization of the channel by
+        //    setting head.block to the same value as tail.block. It then proceeds to publish its
+        //    value but observes that the channel has already disconnected (due to the concurrent
+        //    call of `discard_all_messages`) and aborts the send.
+    });
+    std::thread::yield_now();
+
+    // 2. A second sender attempts to send a value while the channel is in a half-initialized
+    //    state. Here, half-initialized means that the `tail.block` pointer points to a valid block
+    //    but `head.block` is still null. This condition is ensured by the yield of step 1. When
+    //    this call returns the channel state has tail.index != head.index, tail.block != NULL, and
+    //    head.block = NULL.
+    s2.send(42).unwrap();
+    // 3. This thread continues with dropping the one and only receiver. When all receivers are
+    //    gone `discard_all_messages` will attempt to drop all currently sent values and
+    //    de-allocate all the blocks. If `tail.block != NULL` but `head.block = NULL` the
+    //    implementation waits for the initializing sender to finish by spinning/yielding.
+    drop(r);
+    // 5. This thread is rescheduled and `discard_all_messages` observes the head.block pointer set
+    //    by step 4 and proceeds with deallocation. In the problematic version of the code
+    //    `head.block` is simply read via an `Acquire` load and not swapped with NULL. After this
+    //    call returns the channel state has tail.index = head.index, tail.block = NULL, and
+    //    head.block != NULL.
+    t1.join().unwrap();
+    // 6. The last sender (s1) is dropped here which also attempts to cleanup any data in the
+    //    channel. It observes `tail.index = head.index` and so it doesn't attempt to cleanup any
+    //    messages but it also observes that `head.block != NULL` and attempts to deallocate it.
+    //    This is however already deallocated by `discard_all_messages`, leading to a double free.
+}