[wasm] De-flake deserializer test

The NativeModule that was serialized in another isolate might be
temporarily kept alive by background threads. By keeping a weak pointer
to the module, we can wait for it to really die.
This happens very rarely, and the module will die pretty quickly, so
busy-waiting is fine in this case.

R=thibaudm@chromium.org

Bug: v8:10148
Change-Id: I8c4645acfccd04a820ef3f694cad3eb15e75acb6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2016585Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65942}

test/cctest/wasm/test-wasm-serialization.cc

@@ -127,6 +127,9 @@ class WasmSerializationTest {
     Isolate* serialization_isolate =
         reinterpret_cast<Isolate*>(serialization_v8_isolate);
     ErrorThrower thrower(serialization_isolate, "");
+    // Keep a weak pointer so we can check that the native module dies after
+    // serialization (when the isolate is disposed).
+    std::weak_ptr<NativeModule> weak_native_module;
     {
       HandleScope scope(serialization_isolate);
       v8::Local<v8::Context> serialization_context =
@@ -140,6 +143,9 @@ class WasmSerializationTest {
               ModuleWireBytes(buffer.begin(), buffer.end()));
       Handle<WasmModuleObject> module_object =
           maybe_module_object.ToHandleChecked();
+      weak_native_module = module_object->shared_native_module();
+      // Check that the native module exists at this point.
+      CHECK(weak_native_module.lock());
 
       v8::Local<v8::Object> v8_module_obj =
           v8::Utils::ToLocal(Handle<JSObject>::cast(module_object));
@@ -162,6 +168,12 @@ class WasmSerializationTest {
     // and forces de-serialization in the new isolate.
     serialization_v8_isolate->Dispose();
 
+    // Busy-wait for the NativeModule to really die. Background threads might
+    // temporarily keep it alive (happens very rarely, see
+    // https://crbug.com/v8/10148).
+    while (weak_native_module.lock()) {
+    }
+
     serialized_bytes_ = {data_.buffer.get(), data_.size};
 
     v8::HandleScope new_scope(CcTest::isolate());