rt: do not trace tasks while locking OwnedTasks

tokio/src/runtime/handle.rs

@@ -543,6 +543,14 @@ cfg_taskdump! {
                 scheduler::Handle::MultiThreadAlt(_) => panic!("task dump not implemented for this runtime flavor"),
             }
         }
+
+        /// Produces `true` if the current task is being traced for a dump;
+        /// otherwise false. This function is only public for integration
+        /// testing purposes. Do not rely on it.
+        #[doc(hidden)]
+        pub fn is_tracing() -> bool {
+            super::task::trace::Context::is_tracing()
+        }
     }
 
     cfg_rt_multi_thread! {

tokio/src/runtime/task/mod.rs

@@ -444,6 +444,16 @@ impl<S: Schedule> UnownedTask<S> {
     }
 }
 
+impl<S: 'static> Clone for Task<S> {
+    fn clone(&self) -> Task<S> {
+        // SAFETY: We increment the ref count.
+        unsafe {
+            self.raw.ref_inc();
+            Task::new(self.raw)
+        }
+    }
+}
+
 impl<S: 'static> Drop for Task<S> {
     fn drop(&mut self) {
         // Decrement the ref count

tokio/src/runtime/task/trace/mod.rs

@@ -100,6 +100,16 @@ impl Context {
             Self::try_with_current(|context| f(&context.collector)).expect(FAIL_NO_THREAD_LOCAL)
         }
     }
+
+    /// Produces `true` if the current task is being traced; otherwise false.
+    pub(crate) fn is_tracing() -> bool {
+        Self::with_current_collector(|maybe_collector| {
+            let collector = maybe_collector.take();
+            let result = collector.is_some();
+            maybe_collector.set(collector);
+            result
+        })
+    }
 }
 
 impl Trace {
@@ -268,22 +278,8 @@ pub(in crate::runtime) fn trace_current_thread(
         drop(task);
     }
 
-    // notify each task
-    let mut tasks = vec![];
-    owned.for_each(|task| {
-        // set the notified bit
-        task.as_raw().state().transition_to_notified_for_tracing();
-        // store the raw tasks into a vec
-        tasks.push(task.as_raw());
-    });
-
-    tasks
-        .into_iter()
-        .map(|task| {
-            let ((), trace) = Trace::capture(|| task.poll());
-            trace
-        })
-        .collect()
+    // precondition: We have drained the tasks from the injection queue.
+    trace_owned(owned)
 }
 
 cfg_rt_multi_thread! {
@@ -316,21 +312,35 @@ cfg_rt_multi_thread! {
 
         drop(synced);
 
-        // notify each task
-        let mut traces = vec![];
-        owned.for_each(|task| {
-            // set the notified bit
-            task.as_raw().state().transition_to_notified_for_tracing();
-
-            // trace the task
-            let ((), trace) = Trace::capture(|| task.as_raw().poll());
-            traces.push(trace);
+        // precondition: we have drained the tasks from the local and injection
+        // queues.
+        trace_owned(owned)
+    }
+}
 
-            // reschedule the task
-            let _ = task.as_raw().state().transition_to_notified_by_ref();
-            task.as_raw().schedule();
-        });
+/// Trace the `OwnedTasks`.
+///
+/// # Preconditions
+///
+/// This helper presumes exclusive access to each task. The tasks must not exist
+/// in any other queue.
+fn trace_owned<S>(owned: &OwnedTasks<Arc<S>>) -> Vec<Trace> {
+    // notify each task
+    let mut tasks = vec![];
+    owned.for_each(|task| {
+        // set the notified bit
+        task.as_raw().state().transition_to_notified_for_tracing();
+        // store the tasks into a vec
+        tasks.push(task.clone());
+        // do NOT poll `task` here, since we hold a lock on `owned` and the task
+        // may complete and need to remove itself from `owned`.
+    });
 
-        traces
-    }
+    tasks
+        .into_iter()
+        .map(|task| {
+            let ((), trace) = Trace::capture(|| task.as_raw().poll());
+            trace
+        })
+        .collect()
 }

tokio/tests/dump.rs

@@ -97,3 +97,60 @@ fn multi_thread() {
         );
     });
 }
+
+/// Regression tests for #6035.
+///
+/// These tests ensure that dumping will not deadlock if a future completes
+/// during a trace.
+mod future_completes_during_trace {
+    use super::*;
+
+    use core::future::{poll_fn, Future};
+
+    /// A future that completes only during a trace.
+    fn complete_during_trace() -> impl Future<Output = ()> + Send {
+        use std::task::Poll;
+        poll_fn(|cx| {
+            if Handle::is_tracing() {
+                Poll::Ready(())
+            } else {
+                cx.waker().wake_by_ref();
+                Poll::Pending
+            }
+        })
+    }
+
+    #[test]
+    fn current_thread() {
+        let rt = runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap();
+
+        async fn dump() {
+            let handle = Handle::current();
+            let _dump = handle.dump().await;
+        }
+
+        rt.block_on(async {
+            let _ = tokio::join!(tokio::spawn(complete_during_trace()), dump());
+        });
+    }
+
+    #[test]
+    fn multi_thread() {
+        let rt = runtime::Builder::new_multi_thread()
+            .enable_all()
+            .build()
+            .unwrap();
+
+        async fn dump() {
+            let handle = Handle::current();
+            let _dump = handle.dump().await;
+        }
+
+        rt.block_on(async {
+            let _ = tokio::join!(tokio::spawn(complete_during_trace()), dump());
+        });
+    }
+}