[flutter] Port: Run handle wait completers on the microtask queue (#11649)

Change-Id: I66c39d9ef7d534094148212940eef45754559d04

shell/platform/fuchsia/dart-pkg/zircon/lib/src/handle.dart

@@ -39,3 +39,17 @@ class Handle extends NativeFieldWrapperClass2 {
 
   Handle duplicate(int rights) native 'Handle_Duplicate';
 }
+
+@pragma('vm:entry-point')
+class _OnWaitCompleteClosure {  // ignore: unused_element
+  // No public constructor - this can only be created from native code.
+  @pragma('vm:entry-point')
+  _OnWaitCompleteClosure(this._callback, this._arg1, this._arg2);
+
+  Function _callback;
+  Object _arg1;
+  Object _arg2;
+
+  @pragma('vm:entry-point')
+  Function get _closure => () => _callback(_arg1, _arg2); // ignore: unused_element
+}

shell/platform/fuchsia/dart-pkg/zircon/sdk_ext/handle_waiter.cc

@@ -12,6 +12,7 @@
 #include "third_party/tonic/dart_binding_macros.h"
 #include "third_party/tonic/dart_library_natives.h"
 #include "third_party/tonic/dart_message_handler.h"
+#include "third_party/tonic/dart_microtask_queue.h"
 #include "third_party/tonic/logging/dart_invoke.h"
 
 using tonic::DartInvokeField;
@@ -81,11 +82,39 @@ void HandleWaiter::OnWaitComplete(async_dispatcher_t* dispatcher,
   // Remove this waiter from the handle.
   handle_->ReleaseWaiter(this);
 
-  // Schedule the callback on the microtask queue.
-  handle_->ScheduleCallback(std::move(callback_), status, signal);
-
   // Clear handle_.
   handle_ = nullptr;
+
+  auto state = callback_.dart_state().lock();
+  FML_DCHECK(state);
+  DartState::Scope scope(state);
+
+  // Put the closure invocation on the microtask queue.
+  Dart_Handle zircon_lib = Dart_LookupLibrary(ToDart("dart:zircon"));
+  FML_DCHECK(!tonic::LogIfError(zircon_lib));
+
+  Dart_Handle owc_type =
+      Dart_GetClass(zircon_lib, ToDart("_OnWaitCompleteClosure"));
+  FML_DCHECK(!tonic::LogIfError(owc_type));
+
+  FML_DCHECK(!callback_.is_empty());
+  std::vector<Dart_Handle> owc_args{callback_.Release(), ToDart(status),
+                                    ToDart(signal->observed)};
+  Dart_Handle owc =
+      Dart_New(owc_type, Dart_Null(), owc_args.size(), owc_args.data());
+  FML_DCHECK(!tonic::LogIfError(owc));
+
+  Dart_Handle closure = Dart_GetField(owc, ToDart("_closure"));
+  FML_DCHECK(!tonic::LogIfError(closure));
+
+  // TODO(issue#tbd): Use tonic::DartMicrotaskQueue::ScheduleMicrotask()
+  // instead when tonic::DartState gets a microtask queue field.
+  Dart_Handle async_lib = Dart_LookupLibrary(ToDart("dart:async"));
+  FML_DCHECK(!tonic::LogIfError(async_lib));
+  std::vector<Dart_Handle> sm_args{closure};
+  Dart_Handle sm_result = Dart_Invoke(async_lib, ToDart("scheduleMicrotask"),
+                                      sm_args.size(), sm_args.data());
+  FML_DCHECK(!tonic::LogIfError(sm_result));
 }
 
 }  // namespace dart