Fix race condition in channel completion

src/libraries/System.Threading.Channels/src/System/Threading/Channels/UnboundedChannel.cs

@@ -231,6 +231,16 @@ public override bool TryComplete(Exception? error)
                 ChannelUtilities.FailOperations(blockedReadersHead, ChannelUtilities.CreateInvalidCompletionException(error));
                 ChannelUtilities.SetOrFailOperations(waitingReadersHead, result: false, error: error);
 
+                // If we didn't complete above because IsEmpty was false, recheck. A concurrent TryRead
+                // may have dequeued the last item but not yet called CompleteIfDone, or its CompleteIfDone
+                // may have seen _doneWriting as null. Since no new items can be enqueued at this point,
+                // if the queue is now empty, we should complete. ChannelUtilities.Complete is idempotent
+                // and thread-safe.
+                if (!completeTask && parent._items.IsEmpty)
+                {
+                    ChannelUtilities.Complete(parent._completion, error);
+                }
+
                 // Successfully transitioned to completed.
                 return true;
             }

src/libraries/System.Threading.Channels/tests/System.Threading.Channels.Tests.csproj

@@ -16,6 +16,7 @@
     <Compile Include="TestBase.cs" />
     <Compile Include="UnboundedChannelTests.cs" />
     <Compile Include="$(CommonTestPath)System\Diagnostics\DebuggerAttributes.cs" Link="Common\System\Diagnostics\DebuggerAttributes.cs" />
+    <Compile Include="$(CommonTestPath)System\Threading\Tasks\TaskTimeoutExtensions.cs" Link="Common\System\Threading\Tasks\TaskTimeoutExtensions.cs" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">

src/libraries/System.Threading.Channels/tests/UnboundedChannelTests.cs

@@ -167,6 +167,41 @@ public void AllowSynchronousContinuations_CompletionTask_ContinuationsInvokedAcc
             ((IAsyncResult)r).AsyncWaitHandle.WaitOne(); // avoid inlining the continuation
             r.GetAwaiter().GetResult();
         }
+
+        [OuterLoop]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
+        public async Task Completion_CompletesAfterConcurrentTryReadAndTryComplete()
+        {
+            for (int iter = 0; iter < 100_000; iter++)
+            {
+                Channel<int> channel = CreateChannel();
+
+                channel.Writer.TryWrite(1);
+                channel.Writer.TryWrite(2);
+
+                channel.Reader.TryRead(out _);
+
+                using var barrier = new Barrier(2);
+                Task t1 = Task.Run(() =>
+                {
+                    barrier.SignalAndWait();
+                    channel.Reader.TryRead(out _);
+                });
+
+                Task t2 = Task.Run(() =>
+                {
+                    barrier.SignalAndWait();
+                    channel.Writer.TryComplete();
+                });
+
+                await Task.WhenAll(t1, t2);
+
+                channel.Reader.TryRead(out _);
+
+                Task completionTask = channel.Reader.Completion;
+                await completionTask.WaitAsync(TimeSpan.FromSeconds(10));
+            }
+        }
     }
 
     public abstract class SingleReaderUnboundedChannelTests : UnboundedChannelTests