Fix AsyncVoidMethodBuilder race condition around SynchronizationContext (#99461)

This fixes a long-standing issue we've seen sporadically over the years but for which we just got a solid repro; the symptom is a sporadic unhandled null reference exception that crashes an app when using an async void method builder and a non-default SynchronizationContext. The issue is that, because of how state management is handled in the builder, the builder itself can be cleared while its SetResult method is running, and that means two reads of the _synchronizationContext field can end up returning a non-null value followed by a null value. The fix is to just cache the field into a local before completing the builder, and then only use the local state after.

Author: stephentoub

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncVoidMethodBuilder.cs

@@ -80,13 +80,27 @@ public void SetResult()
                 TplEventSource.Log.TraceOperationEnd(this.Task.Id, AsyncCausalityStatus.Completed);
             }
 
+            // Grab the context. Calling SetResult will complete the builder which can cause the state
+            // to be cleared out of the builder, so we can't touch anything on this builder after calling Set*.
+            // This clearing is done as part of the AsyncStateMachineBox.MoveNext method after it calls
+            // MoveNext on the state machine: it's possible to have a chain of events like this:
+            // Thread 1: Calls AsyncStateMachineBox.MoveNext, which calls StateMachine.MoveNext.
+            // Thread 1: StateMachine.MoveNext hooks up a continuation and returns
+            //     Thread 2: That continuation runs and calls AsyncStateMachineBox.MoveNext, which calls SetResult on the builder (below)
+            //               which will result in the state machine task being marked completed.
+            // Thread 1: The original AsyncStateMachineBox.MoveNext call continues and sees that the task is now completed
+            // Thread 1: Clears the builder
+            //     Thread 2: Continues in this call to AsyncVoidMethodBuilder. If it touches anything on this instance, it will be cleared.
+            SynchronizationContext? context = _synchronizationContext;
+
             // Mark the builder as completed.  As this is a void-returning method, this mostly
             // doesn't matter, but it can affect things like debug events related to finalization.
+            // Marking the task completed will also then enable the MoveNext code to clear state.
             _builder.SetResult();
 
-            if (_synchronizationContext != null)
+            if (context != null)
             {
-                NotifySynchronizationContextOfCompletion();
+                NotifySynchronizationContextOfCompletion(context);
             }
         }
 
@@ -106,17 +120,18 @@ public void SetException(Exception exception)
                 TplEventSource.Log.TraceOperationEnd(this.Task.Id, AsyncCausalityStatus.Error);
             }
 
-            if (_synchronizationContext != null)
+            SynchronizationContext? context = _synchronizationContext;
+            if (context != null)
             {
                 // If we captured a synchronization context, Post the throwing of the exception to it
                 // and decrement its outstanding operation count.
                 try
                 {
-                    Task.ThrowAsync(exception, targetContext: _synchronizationContext);
+                    Task.ThrowAsync(exception, targetContext: context);
                 }
                 finally
                 {
-                    NotifySynchronizationContextOfCompletion();
+                    NotifySynchronizationContextOfCompletion(context);
                 }
             }
             else
@@ -132,12 +147,12 @@ public void SetException(Exception exception)
         }
 
         /// <summary>Notifies the current synchronization context that the operation completed.</summary>
-        private void NotifySynchronizationContextOfCompletion()
+        private static void NotifySynchronizationContextOfCompletion(SynchronizationContext context)
         {
-            Debug.Assert(_synchronizationContext != null, "Must only be used with a non-null context.");
+            Debug.Assert(context != null, "Must only be used with a non-null context.");
             try
             {
-                _synchronizationContext.OperationCompleted();
+                context.OperationCompleted();
             }
             catch (Exception exc)
             {