![\"\"](\"http:\/\/blogs.igalia.com\/gyuyoung\/files\/2020\/05\/chromium-arch.png\")
<\/p>\nChromium has a multi-process architecture to become more secure and robust like modern operating systems, and it means that Chromium has a lot of processes communicating with each other. For example, renderer process, browser process, GPU process, utility process, and so on. Those processes have been communicating using IPC [1].<\/p>\n
<\/p>\n
As a long-term intent, the Chromium team wanted to refactor Chromium into a large set of smaller services. To achieve that, they had considered below questions [3]<\/p>\n
They learned much from using the legacy Chromium IPC and maintaining Chromium dependencies over the past years. They felt a more robust messaging layer could allow them to integrate a large number of components without link-time interdependencies as well as help to build more and better features, faster, and with much less cost to users. So, that’s why Chromium team begins to make the Mojo<\/a> communication framework.<\/p>\n From the performance perspective, Mojo is 3 times faster than IPC, and \u2153 less context switching compared to the old IPC in Chrome [3]. Also, we can remove unnecessary layers like content\/renderer layer to communicate between different processes. Because combined with generated code from the Mojom IDL, we can easily connect interface clients and implementations across arbitrary inter-process boundaries. Lastly, Mojo is a collection of runtime libraries providing a platform-agnostic abstraction of common IPC primitives. So, we can build higher-level bindings APIs to simplify messaging for developers writing C++, Java, Javascript.<\/p>\n Igalia has been working on the migration since this year in earnest. But, hundreds of IPCs still remain in Chromium. The below chart shows the progress of migrating legacy IPC to Mojo [4].<\/p>\n Let’s take a look at the key terminology before starting the migration briefly.<\/p>\n In the following example, we migrate WebTestHostMsg_SimulateWebNotificationClose<\/strong> to illustrate the conversion from legacy IPC to Mojo.<\/p>\n We begin to migrate WebTestHostMsg_SimulateWebNotificationClose <\/b>to WebTestClient <\/b>interface from here. First, let’s see an overall call flow through simple diagrams. [5]<\/span><\/p>\n File: content\/shell\/renderer\/web_test\/blink_test_runner.cc<\/a><\/em> There were a lot of flaky web test failures after finishing the migration of WebTestHostMsg to Mojo. The failures were caused by using ‘Remote’ instead of ‘AssociatedRemote’ for WebTestClient<\/strong> interface in the BlinkTestRunner class. Because BlinkTestRunner was using the WebTestControlHost<\/strong> interface for ‘PrintMessage’ as an ‘AssociatedRemote’. But, ‘Remote’ used by WebTestClient<\/strong> didn’t guarantee the message order between ‘PrintMessage’ and ‘InitiateCaptureDump’ message implemented by different interfaces(WebTestControlHost vs. WebTestClient<\/strong>). Thus, tests had often finished before receiving all logs. The actual results could be different from the expected results.<\/span><\/p>\n Changing Remote<\/strong> with AssociatedRemote<\/strong> for the WebTestClient interface solved the flaky test issues.<\/span><\/p>\n [1] Inter-process Communication (IPC)<\/span><\/a> Chromium IPC Chromium has a multi-process architecture to become more secure and robust like modern operating systems, and it means that Chromium has a lot of processes communicating with each other. For example, renderer process, browser process, GPU process, utility process, and so on. Those processes have been communicating using IPC [1]. Why is Mojo […]<\/p>\n","protected":false},"author":52,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1,3],"tags":[4,15,14],"class_list":["post-625","post","type-post","status-publish","format-standard","hentry","category-igalia-chromium","category-igalia-planet","tag-chromium","tag-ipc","tag-mojo"],"_links":{"self":[{"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/posts\/625","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/users\/52"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/comments?post=625"}],"version-history":[{"count":113,"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/posts\/625\/revisions"}],"predecessor-version":[{"id":820,"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/posts\/625\/revisions\/820"}],"wp:attachment":[{"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/media?parent=625"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/categories?post=625"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.igalia.com\/gyuyoung\/wp-json\/wp\/v2\/tags?post=625"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Status of migrating legacy IPC to Mojo<\/h2>\n
![\"\"](\"http:\/\/blogs.igalia.com\/gyuyoung\/files\/2020\/05\/mojo-conversion-status.jpg\")
<\/p>\nMojo Terminology<\/h1>\n
\n
Example of migrating a legacy IPC to Mojo<\/h1>\n
The existing WebTestHostMsg_SimulateWebNotificationClose IPC<\/h2>\n
\n
\nFile: content\/shell\/common\/web_test\/web_test_messages.h<\/em>
\n<\/strong><\/strong><\/p>\nIPC_MESSAGE_ROUTED2(<\/span>WebTestHostMsg_SimulateWebNotificationClose<\/b>,<\/span>\n std::string \/*title*\/,<\/span> \u00a0bool \/*by_user*\/)<\/span><\/pre>\n<\/li>\n
\nFile: content\/shell\/renderer\/web_test\/blink_test_runner.cc<\/em><\/strong><\/strong><\/strong><\/p>\nvoid BlinkTestRunner::<\/span>SimulateWebNotificationClose<\/b>(\n const std::string& title,<\/span>\u00a0bool by_user) {<\/span>\n \u00a0Send(new WebTestHostMsg_SimulateWebNotificationClose(\n routing_id(), title,<\/span>\u00a0by_user));<\/span>\n}<\/span><\/pre>\n<\/li>\n
\nFile: content\/shell\/browser\/web_test\/web_test_message_filter.cc<\/em>
\n<\/strong><\/strong><\/p>\nbool WebTestMessageFilter::OnMessageReceived(\n const IPC::Message& message) {<\/span>\n \u00a0bool handled = true;<\/span>\n \u00a0IPC_BEGIN_MESSAGE_MAP(WebTestMessageFilter, message)<\/span>\n \u00a0\u00a0\u00a0IPC_MESSAGE_HANDLER(\n<\/span> WebTestHostMsg_SimulateWebNotificationClose<\/b>,<\/span>\n OnSimulateWebNotificationClose)<\/span><\/pre>\n<\/li>\n
\nFile: content\/shell\/browser\/web_test\/web_test_message_filter.cc<\/strong><\/em>
\n<\/strong><\/li>\n<\/ol>\nvoid WebTestMessageFilter::<\/span>OnSimulateWebNotificationClose<\/b>(\n const std::string& title, bool by_user) {<\/span>\n \u00a0DCHECK_CURRENTLY_ON(BrowserThread::UI);<\/span>\n \u00a0GetMockPlatformNotificationService()->\n SimulateClose(title, by_user);<\/span>\n}<\/span><\/pre>\nCall flow after migrating the legacy IPC to Mojo<\/h2>\n
\n
\n![\"\"](\"http:\/\/blogs.igalia.com\/gyuyoung\/files\/2020\/05\/call-flow-1.jpg\")
<\/li>\n
\n![\"\"](\"http:\/\/blogs.igalia.com\/gyuyoung\/files\/2020\/05\/call-flow-2.jpg\")
<\/li>\n
\n![\"\"](\"http:\/\/blogs.igalia.com\/gyuyoung\/files\/2020\/05\/call-flow-3.jpg\")
<\/li>\n<\/ol>\nMigrate the legacy IPC to Mojo<\/h2>\n
\n
\nFile: content\/shell\/common\/web_test\/web_test.mojom<\/a><\/em>
\n<\/strong><\/strong><\/p>\nmodule content.mojom;<\/span>\n\n\/\/ Web test messages sent from the renderer process to the\n<\/span>\/\/<\/span> browser.<\/span> \ninterface <\/span>WebTestClient<\/b> {<\/span>\n \u00a0\/\/ Simulates closing a titled web notification depending on the user<\/span>\n \u00a0\/\/ click.<\/span>\n \u00a0\/\/ \u00a0 - |title|: the title of the notification.<\/span>\n \u00a0\/\/ \u00a0 - |by_user|: whether the user clicks the notification.<\/span>\n \u00a0SimulateWebNotificationClose(string title, bool by_user);<\/b>\n};<\/span><\/pre>\n<\/li>\n
\nFile: content\/shell\/BUILD.gn<\/a><\/strong><\/em><\/p>\nmojom(\"web_test_common_mojom\") {<\/span>\n \u00a0sources = [\n<\/span> \u00a0\u00a0\"common\/web_test\/fake_bluetooth_chooser.mojom\",\n \u00a0\u00a0\u00a0\"common\/web_test\/web_test.mojom\",<\/b><\/span>\n \u00a0\u00a0\u00a0\"common\/web_test\/web_test_bluetooth_fake_adapter_setter.mojom\",<\/span>\n ]<\/span><\/pre>\n<\/li>\n
\nFile: content\/shell\/browser\/web_test\/web_test_client_impl.h<\/a><\/em>
\n<\/strong><\/strong><\/p>\n#include \"content\/shell\/common\/web_test.mojom.h\"<\/span>\n\nclass WebTestClientImpl : public mojom::WebTestClient {<\/b>\n public:<\/span>\n \u00a0WebTestClientImpl() = default;<\/span>\n \u00a0~WebTestClientImpl() override = default;<\/span>\n\n\u00a0\u00a0WebTestClientImpl(const WebTestClientImpl&) = delete;<\/span>\n \u00a0WebTestClientImpl& operator=(const WebTestClientImpl&) = delete;<\/span>\n\n\u00a0\u00a0static void Create(\n mojo::PendingReceiver<mojom::WebTestClient> receiver);<\/span>\n private:<\/span>\n \u00a0\/\/ WebTestClient implementation.<\/span>\n void SimulateWebNotificationClose(const std::string& title,<\/b>\n \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0bool by_user) override;<\/b>\n};<\/pre>\n<\/li>\n
\nFile: content\/shell\/browser\/web_test\/web_test_client_impl.cc<\/a><\/strong><\/em><\/p>\nvoid WebTestClientImpl::<\/span>SimulateWebNotificationClose<\/b>(\n const std::string& title,<\/span>\u00a0bool by_user) {\n<\/span> \u00a0DCHECK_CURRENTLY_ON(BrowserThread::UI);\n<\/span> \u00a0GetMockPlatformNotificationService()->\n SimulateClose(title, by_user);\n<\/span>}<\/span><\/pre>\n<\/li>\n
\nFile: content\/shell\/renderer\/web_test\/blink_test_runner.h<\/a><\/strong><\/em><\/p>\nmojo::AssociatedRemote<mojom::WebTestClient>&\n GetWebTestClientRemote()<\/strong>;\n<\/span>mojo::AssociatedRemote<mojom::WebTestClient>\n web_test_client_remote_<\/strong>;<\/span><\/pre>\n
\n<\/strong><\/p>\nmojo::AssociatedRemote<mojom::WebTestClient>&\n<\/span>BlinkTestRunner::<\/span>GetWebTestClientRemote<\/b>() {\n<\/span> if (!web_test_client_remote_) {\n<\/span> \u00a0\u00a0\u00a0RenderThread::Get()->GetChannel()->\n<\/span> GetRemoteAssociatedInterface<\/b>(<\/span>&web_test_client_remote_);\n<\/span> \u00a0\u00a0\u00a0web_test_client_remote_.set_disconnect_handler(\n<\/span> \u00a0\u00a0\u00a0\u00a0\u00a0 base::BindOnce( \n &BlinkTestRunner::HandleWebTestClientDisconnected,\n<\/span> \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0base::Unretained(this)));\n<\/span> \u00a0}\n<\/span> \u00a0return web_test_client_remote_;\n<\/span>}<\/span><\/pre>\n<\/li>\n
\n File: content\/shell\/browser\/web_test\/web_test_content_browser_client.cc<\/a><\/strong><\/em><\/p>\nvoid WebTestContentBrowserClient::<\/span>ExposeInterfacesToRenderer<\/b> {\n<\/span> \u00a0...\n<\/span> \u00a0associated_registry->AddInterface(base::BindRepeating(\n<\/span> \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0&WebTestContentBrowserClient::BindWebTestController,\n<\/span> \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0render_process_host->GetID(),\n<\/span> \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0BrowserContext::GetDefaultStoragePartition(\n browser_context())));\n<\/span> }\n<\/span> void WebTestContentBrowserClient::<\/span>BindWebTestController<\/b>(\n<\/span> \u00a0\u00a0\u00a0int render_process_id,\n<\/span> \u00a0\u00a0\u00a0StoragePartition* partition,\n<\/span> \u00a0\u00a0\u00a0mojo::PendingAssociatedReceiver<mojom::WebTestClient>\n receiver) {\n<\/span> \u00a0WebTestClientImpl::Create<\/b>(render_process_id,\n \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/span>\u00a0\u00a0\u00a0partition->GetQuotaManager(),\n<\/span> \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0\u00a0partition->GetDatabaseTracker(),\n<\/span> \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0partition->GetNetworkContext(),\n<\/span> \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0std::move(receiver));\n<\/span> } <\/span><\/pre>\n<\/li>\n
\nFile: <\/strong>content\/shell\/renderer\/web_test\/blink_test_runner.cc<\/a><\/b><\/em><\/p>\nvoid BlinkTestRunner::SimulateWebNotificationClose<\/strong>(\n const std::string& title, bool by_user) {\n GetWebTestClientRemote()->\n SimulateWebNotificationClose(title, by_user);\n}<\/pre>\n<\/li>\n
\nFile: content\/shell\/browser\/web_test\/web_test_client_impl.cc<\/a><\/strong><\/em><\/p>\nvoid WebTestClientImpl::SimulateWebNotificationClose<\/strong>(\n const std::string& title, bool by_user) {\n DCHECK_CURRENTLY_ON(BrowserThread::UI);\n GetMockPlatformNotificationService()->\n SimulateClose(title, by_user);\n}<\/pre>\n<\/li>\n<\/ol>\n<\/h2>\n
Appendix: A case study of Regression<\/h2>\n
\n
![\"\"](\"http:\/\/blogs.igalia.com\/gyuyoung\/files\/2020\/05\/regression.jpg\")
<\/p>\n
\n
\n[2] Mojo in Chromium<\/a>
\n[3] Mojo & Servicification Performance Notes<\/a>
\n[4] Chrome IPC legacy Conversion Status<\/a>
\n[5] Convert Legacy IPC to Mojo<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"