[draft][browser][MT] ConfigureAwait(false) ignored when JSSynchronizationContext is installed

src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs

@@ -136,6 +136,10 @@ public class Task : IAsyncResult, IDisposable
         private int StateFlagsForDebugger => m_stateFlags; // Private property used by a debugger to access this Task's state flags
         private TaskStateFlags StateFlags => (TaskStateFlags)(m_stateFlags & ~(int)TaskStateFlags.OptionsMask); // Private property used to help with debugging
 
+#if FEATURE_WASM_THREADS
+        private static Type? s_jsSynchronizationContextType;
+#endif
+
         [Flags]
         internal enum TaskStateFlags
         {
@@ -2515,6 +2519,14 @@ internal void SetContinuationForAwait(
                     goto HaveTaskContinuation;
                 }
             }
+#if FEATURE_WASM_THREADS
+            // when JSSynchronizationContext is installed, we use it regardless of continueOnCapturedContext
+            else if (SynchronizationContext.Current is SynchronizationContext syncCtx && syncCtx.GetType() == s_jsSynchronizationContextType)
+            {
+                tc = new SynchronizationContextAwaitTaskContinuation(syncCtx, continuationAction, flowExecutionContext);
+                goto HaveTaskContinuation;
+            }
+#endif
 
             if (flowExecutionContext)
             {

src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Interop/JavaScriptExports.cs

@@ -225,7 +225,7 @@ public static void InstallSynchronizationContext (JSMarshalerArgument* arguments
             ref JSMarshalerArgument arg_exc = ref arguments_buffer[0]; // initialized by caller in alloc_stack_frame()
             try
             {
-                JSHostImplementation.InstallWebWorkerInterop(true);
+                JSHostImplementation.InstallWebWorkerInterop(true, true);
             }
             catch (Exception ex)
             {

src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSHostImplementation.cs

@@ -199,8 +199,12 @@ public static JSObject CreateCSOwnedProxy(nint jsHandle)
         }
 
 #if FEATURE_WASM_THREADS
-        public static void InstallWebWorkerInterop(bool installJSSynchronizationContext)
+        public static void InstallWebWorkerInterop(bool installJSSynchronizationContext, bool isMainThread)
         {
+            if (isMainThread)
+            {
+                SetJSSynchronizationContextType();
+            }
             Interop.Runtime.InstallWebWorkerInterop(installJSSynchronizationContext);
             if (installJSSynchronizationContext)
             {
@@ -236,6 +240,7 @@ public static void UninstallWebWorkerInterop()
 
         private static FieldInfo? thread_id_Field;
         private static FieldInfo? external_eventloop_Field;
+        private static FieldInfo? task_s_jsSynchronizationContextType_Field;
 
         // FIXME: after https://github.com/dotnet/runtime/issues/86040 replace with
         // [UnsafeAccessor(UnsafeAccessorKind.Field, Name="external_eventloop")]
@@ -261,6 +266,16 @@ public static IntPtr GetNativeThreadId()
             return (int)(long)thread_id_Field.GetValue(Thread.CurrentThread)!;
         }
 
+        [DynamicDependency(DynamicallyAccessedMemberTypes.NonPublicFields, "System.Threading.Tasks.Task", "System.Private.CoreLib")]
+        public static void SetJSSynchronizationContextType()
+        {
+            if (task_s_jsSynchronizationContextType_Field == null)
+            {
+                task_s_jsSynchronizationContextType_Field = typeof(Task).GetField("s_jsSynchronizationContextType", BindingFlags.NonPublic | BindingFlags.Static)!;
+            }
+            task_s_jsSynchronizationContextType_Field.SetValue(null, typeof(JSSynchronizationContext));
+        }
+
 #endif
 
     }

src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/WebWorker.cs

@@ -31,7 +31,7 @@ public static Task<T> RunAsync<T>(Func<Task<T>> body, CancellationToken cancella
                         return;
                     }
 
-                    JSHostImplementation.InstallWebWorkerInterop(true);
+                    JSHostImplementation.InstallWebWorkerInterop(true, false);
                     var childScheduler = TaskScheduler.FromCurrentSynchronizationContext();
                     Task<T> res = body();
                     // This code is exiting thread main() before all promises are resolved.
@@ -68,7 +68,7 @@ public static Task RunAsyncVoid(Func<Task> body, CancellationToken cancellationT
                         return;
                     }
 
-                    JSHostImplementation.InstallWebWorkerInterop(true);
+                    JSHostImplementation.InstallWebWorkerInterop(true, false);
                     var childScheduler = TaskScheduler.FromCurrentSynchronizationContext();
                     Task res = body();
                     // This code is exiting thread main() before all promises are resolved.
@@ -105,7 +105,7 @@ public static Task Run(Action body, CancellationToken cancellationToken)
                         return;
                     }
 
-                    JSHostImplementation.InstallWebWorkerInterop(false);
+                    JSHostImplementation.InstallWebWorkerInterop(false, false);
                     try
                     {
                         body();

src/mono/sample/wasm/browser-threads-minimal/Program.cs

@@ -137,7 +137,7 @@ public static async Task<string> FetchBackground(string url)
                 Console.WriteLine($"smoke: FetchBackground 2 ManagedThreadId:{Thread.CurrentThread.ManagedThreadId}, SynchronizationContext: {SynchronizationContext.Current?.GetType().FullName ?? "null"}");
                 var x=JSHost.ImportAsync(fetchhelper, "./fetchhelper.js");
                 Console.WriteLine($"smoke: FetchBackground 3A ManagedThreadId:{Thread.CurrentThread.ManagedThreadId}, SynchronizationContext: {SynchronizationContext.Current?.GetType().FullName ?? "null"}");
-                using var import = await x;
+                using var import = await x.ConfigureAwait(false);
                 Console.WriteLine($"smoke: FetchBackground 3B ManagedThreadId:{Thread.CurrentThread.ManagedThreadId}, SynchronizationContext: {SynchronizationContext.Current?.GetType().FullName ?? "null"}");
                 var r = await GlobalThisFetch(url);
                 Console.WriteLine($"smoke: FetchBackground 4 ManagedThreadId:{Thread.CurrentThread.ManagedThreadId}, SynchronizationContext: {SynchronizationContext.Current?.GetType().FullName ?? "null"}");