android: Java-based launcher thread

content/browser/browser_main_loop.cc

Index: content/browser/browser_main_loop.cc
diff --git a/content/browser/browser_main_loop.cc b/content/browser/browser_main_loop.cc
index ba276a6acc07aa22fcc09a95cd89497fa8814223..ae864f144ebf85d709bb6c99235c6c329a08f0a4 100644
--- a/content/browser/browser_main_loop.cc
+++ b/content/browser/browser_main_loop.cc
@@ -123,6 +123,7 @@
 #include "base/android/jni_android.h"
 #include "components/tracing/common/graphics_memory_dump_provider_android.h"
 #include "content/browser/android/browser_startup_controller.h"
+#include "content/browser/android/launcher_thread.h"
 #include "content/browser/android/scoped_surface_request_manager.h"
 #include "content/browser/android/tracing_controller_android.h"
 #include "content/browser/media/android/browser_media_player_manager.h"
@@ -986,7 +987,7 @@ int BrowserMainLoop::CreateThreads() {
   base::Thread::Options ui_message_loop_options;
   ui_message_loop_options.message_loop_type = base::MessageLoop::TYPE_UI;
 
-  const bool redirect_nonUInonIO_browser_threads =
+  const bool client_redirect_nonUInonIO_browser_threads =
       GetContentClient()
           ->browser()
           ->RedirectNonUINonIOBrowserThreadsToTaskScheduler();
@@ -1003,6 +1004,14 @@ int BrowserMainLoop::CreateThreads() {
     // updated away from its default.
     std::unique_ptr<BrowserProcessSubThread>* thread_to_start = nullptr;
     base::Thread::Options options;
+    // If |message_loop| is not nullptr, then this BrowserThread will use this
+    // message loop instead of creating a new thread. Note that means this
+    // thread will not be joined on shutdown, and may cause use-after-free if
+    // anything tries to access objects deleted by AtExitManager, such as
+    // non-leaky LazyInstance.
+    base::MessageLoop* message_loop = nullptr;
+    bool redirect_nonUInonIO_browser_threads =

[gab, 2017/03/29 18:14:04]

bool redirect_thread = redirect_nonUInonIO_browser_threads;

(i.e. this individual bool only applies to this thread and should be named as
such -- also removes need to rename top-level bool)

[boliu, 2017/03/29 18:30:07]

Done, but now needs to rename all the usage of the old var in the switch block
:p

+        client_redirect_nonUInonIO_browser_threads;
 
     // Otherwise this thread ID will be backed by a SingleThreadTaskRunner using
     // |non_ui_non_io_task_runner_traits| (which can be augmented below).
@@ -1067,6 +1076,13 @@ int BrowserMainLoop::CreateThreads() {
         TRACE_EVENT_BEGIN1("startup",
             "BrowserMainLoop::CreateThreads:start",
             "Thread", "BrowserThread::PROCESS_LAUNCHER");
+#if defined(OS_ANDROID)
+        // Android specializes Launcher thread so it is accessible in java.
+        // Note Android never does clean shutdown, so shutdown use-after-free
+        // concerns is not a problem in practice.

[gab, 2017/03/29 18:14:04]

s/is/are/

[boliu, 2017/03/29 18:30:07]

Done.

+        redirect_nonUInonIO_browser_threads = false;
+        message_loop = android::LauncherThread::GetMessageLoop();
+#endif
         if (redirect_nonUInonIO_browser_threads) {

[gab, 2017/03/29 18:38:06]

Actually can't we just change this to

if (redirect_nonUInonIO_browser_threads && !message_loop) {
  ...

instead of having a duplicate of the bool merely to skip this block?

[boliu, 2017/03/29 18:42:20]

Hmm, then in theory, that should be changed in all the case blocks, which is no
better than the current thing imo

[gab, 2017/03/29 18:46:53]

Hmmm ok sure.

           non_ui_non_io_task_runner_traits
               .WithPriority(base::TaskPriority::USER_BLOCKING)
@@ -1121,7 +1137,9 @@ int BrowserMainLoop::CreateThreads() {
     BrowserThread::ID id = static_cast<BrowserThread::ID>(thread_id);
 
     if (thread_to_start) {
-      (*thread_to_start).reset(new BrowserProcessSubThread(id));
+      (*thread_to_start)
+          .reset(message_loop ? new BrowserProcessSubThread(id, message_loop)
+                              : new BrowserProcessSubThread(id));
       if (!(*thread_to_start)->StartWithOptions(options))
         LOG(FATAL) << "Failed to start the browser thread: id == " << id;
     } else {