Windows: Fix plugin destruction

[jnschulze]

Fixes flutter/flutter#93234

This small fix prevents plugin registrar destruction callbacks from overwriting each other.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[jnschulze]

@stuartmorgan

[stuartmorgan-g]

LGTM with one question about the test. Thanks for cleaning this up!

@cbracken for second review.

[stuartmorgan-g]

Why does this test need a mock keyboard handler?

[jnschulze]

MockEmbedderApiForKeyboard mocks a couple of embedder API functions required by engine.RunWithEntrypoint. see

engine/shell/platform/windows/testing/test_keyboard.cc

Lines 100 to 192 in 8509510

	void MockEmbedderApiForKeyboard(
	EngineModifier& modifier,
	std::shared_ptr<MockKeyResponseController> response_controller) {
	stored_response_controller = response_controller;
	// This mock handles channel messages.
	modifier.embedder_api()
	.SendPlatformMessage = [](FLUTTER_API_SYMBOL(FlutterEngine) engine,
	const FlutterPlatformMessage* message) {
	if (std::string(message->channel) == std::string("flutter/settings")) {
	return kSuccess;
	}
	if (std::string(message->channel) == std::string("flutter/keyevent")) {
	stored_response_controller->HandleChannelMessage([message](bool handled) {
	auto response = _keyHandlingResponse(handled);
	auto response_handle = message->response_handle;
	if (response_handle->callback != nullptr) {
	response_handle->callback(response->data(), response->size(),
	response_handle->user_data);
	}
	});
	return kSuccess;
	}
	if (std::string(message->channel) == std::string("flutter/textinput")) {
	std::unique_ptr<rapidjson::Document> document =
	flutter::JsonMessageCodec::GetInstance().DecodeMessage(
	message->message, message->message_size);
	if (document == nullptr) {
	return kInvalidArguments;
	}
	stored_response_controller->HandleTextInputMessage(std::move(document));
	return kSuccess;
	}
	return kSuccess;
	};
	// This mock handles key events sent through the embedder API.
	modifier.embedder_api().SendKeyEvent =
	[](FLUTTER_API_SYMBOL(FlutterEngine) engine, const FlutterKeyEvent* event,
	FlutterKeyEventCallback callback, void* user_data) {
	stored_response_controller->HandleEmbedderMessage(
	event, [callback, user_data](bool handled) {
	if (callback != nullptr) {
	callback(handled, user_data);
	}
	});
	return kSuccess;
	};
	// The following mocks enable channel mocking.
	modifier.embedder_api().PlatformMessageCreateResponseHandle =
	[](auto engine, auto data_callback, auto user_data, auto response_out) {
	auto response_handle = new FlutterPlatformMessageResponseHandle();
	response_handle->user_data = user_data;
	response_handle->callback = data_callback;
	*response_out = response_handle;
	return kSuccess;
	};
	modifier.embedder_api().PlatformMessageReleaseResponseHandle =
	[](FLUTTER_API_SYMBOL(FlutterEngine) engine,
	FlutterPlatformMessageResponseHandle* response) {
	delete response;
	return kSuccess;
	};
	// The following mock disables responses for method channels sent from the
	// embedding to the framework. (No code uses the response yet.)
	modifier.embedder_api().SendPlatformMessageResponse =
	[](FLUTTER_API_SYMBOL(FlutterEngine) engine,
	const FlutterPlatformMessageResponseHandle* handle,
	const uint8_t* data, size_t data_length) { return kSuccess; };
	// The following mocks allows RunWithEntrypoint to be run, which creates a
	// non-empty FlutterEngine and enables SendKeyEvent.
	modifier.embedder_api().Run =
	[](size_t version, const FlutterRendererConfig* config,
	const FlutterProjectArgs* args, void* user_data,
	FLUTTER_API_SYMBOL(FlutterEngine) * engine_out) {
	*engine_out = reinterpret_cast<FLUTTER_API_SYMBOL(FlutterEngine)>(1);
	return kSuccess;
	};
	modifier.embedder_api().UpdateLocales =
	[](auto engine, const FlutterLocale** locales, size_t locales_count) {
	return kSuccess;
	};
	modifier.embedder_api().SendWindowMetricsEvent =
	[](auto engine, const FlutterWindowMetricsEvent* event) {
	return kSuccess;
	};
	modifier.embedder_api().Shutdown = [](auto engine) { return kSuccess; };
	}

Should I just mock the required functions explicitly?

[cbracken]

Thanks for cleaning this up!