Experiment with redirecting all BrowserThreads (but UI/IO) to TaskScheduler

content/browser/browser_thread_impl.cc

Index: content/browser/browser_thread_impl.cc
diff --git a/content/browser/browser_thread_impl.cc b/content/browser/browser_thread_impl.cc
index 18eff7c8672e37ecf83381b566474c4d75fc9af3..33099cd0e1b4920d02e95ad5fec1474576dc7883 100644
--- a/content/browser/browser_thread_impl.cc
+++ b/content/browser/browser_thread_impl.cc
@@ -12,10 +12,12 @@
 #include "base/bind.h"
 #include "base/compiler_specific.h"
 #include "base/lazy_instance.h"
+#include "base/logging.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/profiler/scoped_tracker.h"
 #include "base/run_loop.h"
-#include "base/single_thread_task_runner.h"
+#include "base/task_scheduler/task_scheduler.h"
 #include "base/threading/platform_thread.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "build/build_config.h"
@@ -52,6 +54,7 @@ static const char* GetThreadName(BrowserThread::ID thread) {
 
 // An implementation of SingleThreadTaskRunner to be used in conjunction
 // with BrowserThread.
+// TODO(gab): This can go away in favor of |g_globals->task_runners|.
 class BrowserThreadTaskRunner : public base::SingleThreadTaskRunner {
  public:
   explicit BrowserThreadTaskRunner(BrowserThread::ID identifier)
@@ -99,31 +102,45 @@ struct BrowserThreadTaskRunners {
 base::LazyInstance<BrowserThreadTaskRunners>::Leaky g_task_runners =
     LAZY_INSTANCE_INITIALIZER;
 
+// State of a given BrowserThread::ID in chronological order throughout the
+// browser process' lifetime.
+enum BrowserThreadState {
+  // BrowserThread::ID isn't associated with anything yet.
+  UNINITIALIZED = 0,
+  // BrowserThread::ID is associated but the underlying thread doesn't run yet.

[Avi (use Gerrit), 2016/11/08 16:16:15]

"... underlying thread hasn't run yet."

[gab, 2016/11/08 20:40:44]

Done.

+  INITIALIZED,
+  // BrowserThread::ID is associated to a live thread.
+  RUNNING,
+  // BrowserThread::ID is associated to a thread having been or being shutdown.
+  SHUTDOWN
+};
+
 struct BrowserThreadGlobals {
   BrowserThreadGlobals()
       : blocking_pool(
             new base::SequencedWorkerPool(3,
                                           "BrowserBlocking",
                                           base::TaskPriority::USER_VISIBLE)) {
-    memset(threads, 0, BrowserThread::ID_COUNT * sizeof(threads[0]));
-    memset(thread_ids, 0, BrowserThread::ID_COUNT * sizeof(thread_ids[0]));
+    memset(states, 0, BrowserThread::ID_COUNT * sizeof(states[0]));
     memset(thread_delegates, 0,
            BrowserThread::ID_COUNT * sizeof(thread_delegates[0]));

[Avi (use Gerrit), 2016/11/08 16:16:15]

Does xxx = { 0 } syntax suffice to zero these? I'm not a fan of memset. :\

[gab, 2016/11/08 20:40:44]

I just tried = {}; as a member initializer and it seems to work (confirmed with
runtime DCHECKs in concstructor locally -- and in practice the DCHECKs in
SetDelegate() and BrowserThreadImpl() would also fire if this didn't work so I
didn't keep the ones in constructor) :-)!

PS: { 0 } just means "initialize the first entry to zero and default init the
rest", {} means "default init everything" (I was also shocked when I learned
this ;-)).

   }
 
-  // This lock protects |threads| and |thread_ids|. Do not read or modify those
+  // This lock protects |task_runners| and |states|. Do not read or modify those
   // arrays without holding this lock. Do not block while holding this lock.
   base::Lock lock;
 
-  // This array is protected by |lock|. IDs in this array are populated as soon
-  // as their respective thread is started and are never reset.
-  base::PlatformThreadId thread_ids[BrowserThread::ID_COUNT];
+  // This array is filled either as the underlying threads start and invoke
+  // Init() or in RedirectThreadIDToTaskRunner() for threads that are being
+  // redirected. It is not emptied during shutdown in order to support
+  // RunsTasksOnCurrentThread() until the very end.
+  scoped_refptr<base::SingleThreadTaskRunner>
+      task_runners[BrowserThread::ID_COUNT];
 
-  // This array is protected by |lock|. The threads are not owned by this
-  // array. Typically, the threads are owned on the UI thread by
-  // BrowserMainLoop. BrowserThreadImpl objects remove themselves from this
-  // array upon destruction.
-  BrowserThreadImpl* threads[BrowserThread::ID_COUNT];
+  // Holds the state of each BrowserThread::ID. This needs to be a separate bit
+  // because |threads| isn't a good signal per redirected IDs remaining null and
+  // |task_runners| isn't either per not being cleaned up on shutdown.
+  BrowserThreadState states[BrowserThread::ID_COUNT];
 
   // Only atomic operations are used on this array. The delegates are not owned
   // by this array, rather by whoever calls BrowserThread::SetDelegate.
@@ -140,13 +157,6 @@ base::LazyInstance<BrowserThreadGlobals>::Leaky
 BrowserThreadImpl::BrowserThreadImpl(ID identifier)
     : Thread(GetThreadName(identifier)), identifier_(identifier) {
   Initialize();
-
-  // Unit tests may create multiple TestBrowserThreadBundles, causing the same
-  // BrowserThread ID to be reinitialized. We explicitly clear the thread ID
-  // here so Start() can sanity check.
-  BrowserThreadGlobals& globals = g_globals.Get();
-  base::AutoLock lock(globals.lock);
-  globals.thread_ids[identifier] = base::kInvalidThreadId;
 }
 
 BrowserThreadImpl::BrowserThreadImpl(ID identifier,
@@ -159,7 +169,12 @@ BrowserThreadImpl::BrowserThreadImpl(ID identifier,
   // which runs on the current thread.
   BrowserThreadGlobals& globals = g_globals.Get();
   base::AutoLock lock(globals.lock);
-  globals.thread_ids[identifier] = base::PlatformThread::CurrentId();
+
+  DCHECK(!globals.task_runners[identifier_]);
+  globals.task_runners[identifier_] = message_loop->task_runner();
+
+  DCHECK_EQ(globals.states[identifier_], BrowserThreadState::INITIALIZED);
+  globals.states[identifier_] = BrowserThreadState::RUNNING;
 }
 
 // static
@@ -185,14 +200,25 @@ void BrowserThreadImpl::FlushThreadPoolHelperForTesting() {
 void BrowserThreadImpl::Init() {
   BrowserThreadGlobals& globals = g_globals.Get();
 
+  {
+    base::AutoLock lock(globals.lock);
+
+    // Grab the underlying thread's TaskRunner. This needs to happen right away
+    // as CurrentlyOn() depends on it.
+    globals.task_runners[identifier_] = message_loop()->task_runner();
+
+    DCHECK_EQ(globals.states[identifier_], BrowserThreadState::INITIALIZED);
+    globals.states[identifier_] = BrowserThreadState::RUNNING;
+  }
+
   if (BrowserThread::CurrentlyOn(BrowserThread::DB) ||
       BrowserThread::CurrentlyOn(BrowserThread::FILE) ||
       BrowserThread::CurrentlyOn(BrowserThread::FILE_USER_BLOCKING) ||
       BrowserThread::CurrentlyOn(BrowserThread::PROCESS_LAUNCHER) ||
       BrowserThread::CurrentlyOn(BrowserThread::CACHE)) {
-    base::MessageLoop* message_loop = base::MessageLoop::current();
-    message_loop->DisallowNesting();
-    message_loop->DisallowTaskObservers();
+    // Nesting and task observers are not allowed on redirected threads.
+    message_loop()->DisallowNesting();
+    message_loop()->DisallowTaskObservers();
   }
 
   using base::subtle::AtomicWord;
@@ -314,7 +340,8 @@ void BrowserThreadImpl::CleanUp() {
   // to prevent a race with accessing the message loop in PostTaskHelper(),
   // remove this thread from the global array now.
   base::AutoLock lock(globals.lock);
-  globals.threads[identifier_] = nullptr;
+  DCHECK_EQ(globals.states[identifier_], BrowserThreadState::RUNNING);
+  globals.states[identifier_] = BrowserThreadState::SHUTDOWN;
 }
 
 void BrowserThreadImpl::Initialize() {
@@ -323,8 +350,8 @@ void BrowserThreadImpl::Initialize() {
   base::AutoLock lock(globals.lock);
   DCHECK_GE(identifier_, 0);
   DCHECK_LT(identifier_, ID_COUNT);
-  DCHECK_EQ(globals.threads[identifier_], nullptr);
-  globals.threads[identifier_] = this;
+  DCHECK_EQ(globals.states[identifier_], BrowserThreadState::UNINITIALIZED);
+  globals.states[identifier_] = BrowserThreadState::INITIALIZED;
 }
 
 BrowserThreadImpl::~BrowserThreadImpl() {
@@ -335,38 +362,45 @@ BrowserThreadImpl::~BrowserThreadImpl() {
 
   BrowserThreadGlobals& globals = g_globals.Get();
   base::AutoLock lock(globals.lock);
-  globals.threads[identifier_] = nullptr;
-#ifndef NDEBUG
+  // This thread should have gone through Cleanup() as part of Stop() and be in
+  // the SHUTDOWN state already (unless it uses an externally provided
+  // MessageLoop instead of a real underlying thread and thus doesn't go through
+  // Cleanup()).
+  if (using_external_message_loop_) {
+    DCHECK_EQ(globals.states[identifier_], BrowserThreadState::RUNNING);
+    globals.states[identifier_] = BrowserThreadState::SHUTDOWN;
+  } else {
+    DCHECK_EQ(globals.states[identifier_], BrowserThreadState::SHUTDOWN);
+  }
+#if DCHECK_IS_ON()
   // Double check that the threads are ordered correctly in the enumeration.
   for (int i = identifier_ + 1; i < ID_COUNT; ++i) {
-    DCHECK(!globals.threads[i]) <<
-        "Threads must be listed in the reverse order that they die";
+    DCHECK_EQ(globals.states[i], BrowserThreadState::SHUTDOWN)
+        << "Threads must be listed in the reverse order that they die";
   }
 #endif
 }
 
-bool BrowserThreadImpl::Start() {
-  return StartWithOptions(base::Thread::Options());
-}
-
-bool BrowserThreadImpl::StartWithOptions(const Options& options) {
-  // The global thread table needs to be locked while a new thread is
-  // starting, as the new thread can asynchronously start touching the
-  // table (and other thread's message_loop).
+// static
+void BrowserThreadImpl::RedirectThreadIDToTaskRunner(
+    BrowserThread::ID identifier,
+    scoped_refptr<base::SingleThreadTaskRunner> task_runner) {
   BrowserThreadGlobals& globals = g_globals.Get();
   base::AutoLock lock(globals.lock);
-  DCHECK_EQ(globals.thread_ids[identifier_], base::kInvalidThreadId);
-  bool result = Thread::StartWithOptions(options);
-  globals.thread_ids[identifier_] = GetThreadId();
-  return result;
+  if (task_runner) {
+    DCHECK(!globals.task_runners[identifier]);
+    DCHECK_EQ(globals.states[identifier], BrowserThreadState::UNINITIALIZED);
+
+    globals.task_runners[identifier] = std::move(task_runner);
+    globals.states[identifier] = BrowserThreadState::RUNNING;
+  } else {
+    // Cancelling redirection should only be done on previously redirected
+    // threads.
+    DCHECK_EQ(globals.states[identifier], BrowserThreadState::RUNNING);
+    globals.states[identifier] = BrowserThreadState::SHUTDOWN;
+  }
 }
 
-bool BrowserThreadImpl::StartAndWaitForTesting() {
-  if (!Start())
-    return false;
-  WaitUntilThreadStarted();
-  return true;
-}
 // static
 bool BrowserThreadImpl::PostTaskHelper(
     BrowserThread::ID identifier,
@@ -391,22 +425,23 @@ bool BrowserThreadImpl::PostTaskHelper(
   if (!target_thread_outlives_current)
     globals.lock.Acquire();
 
-  base::MessageLoop* message_loop =
-      globals.threads[identifier] ? globals.threads[identifier]->message_loop()
-                                  : nullptr;
-  if (message_loop) {
+  const bool accepting_tasks =
+      globals.states[identifier] == BrowserThreadState::RUNNING;
+  if (accepting_tasks) {
+    base::SingleThreadTaskRunner* task_runner =
+        globals.task_runners[identifier].get();
+    DCHECK(task_runner);
     if (nestable) {
-      message_loop->task_runner()->PostDelayedTask(from_here, task, delay);
+      task_runner->PostDelayedTask(from_here, task, delay);
     } else {
-      message_loop->task_runner()->PostNonNestableDelayedTask(from_here, task,
-                                                              delay);
+      task_runner->PostNonNestableDelayedTask(from_here, task, delay);
     }
   }
 
   if (!target_thread_outlives_current)
     globals.lock.Release();
 
-  return !!message_loop;
+  return accepting_tasks;
 }
 
 // static
@@ -457,7 +492,8 @@ bool BrowserThread::IsThreadInitialized(ID identifier) {
   base::AutoLock lock(globals.lock);
   DCHECK_GE(identifier, 0);
   DCHECK_LT(identifier, ID_COUNT);
-  return globals.threads[identifier] != nullptr;
+  return globals.states[identifier] == BrowserThreadState::INITIALIZED ||
+         globals.states[identifier] == BrowserThreadState::RUNNING;
 }
 
 // static
@@ -466,7 +502,8 @@ bool BrowserThread::CurrentlyOn(ID identifier) {
   base::AutoLock lock(globals.lock);
   DCHECK_GE(identifier, 0);
   DCHECK_LT(identifier, ID_COUNT);
-  return base::PlatformThread::CurrentId() == globals.thread_ids[identifier];
+  return globals.task_runners[identifier] &&
+         globals.task_runners[identifier]->RunsTasksOnCurrentThread();
 }
 
 // static
@@ -492,8 +529,7 @@ bool BrowserThread::IsMessageLoopValid(ID identifier) {
   base::AutoLock lock(globals.lock);
   DCHECK_GE(identifier, 0);
   DCHECK_LT(identifier, ID_COUNT);
-  return globals.threads[identifier] &&
-         globals.threads[identifier]->message_loop();
+  return globals.states[identifier] == BrowserThreadState::RUNNING;
 }
 
 // static
@@ -547,7 +583,6 @@ bool BrowserThread::GetCurrentThreadIdentifier(ID* identifier) {
   if (g_globals == nullptr)
     return false;
 
-  base::MessageLoop* cur_message_loop = base::MessageLoop::current();
   BrowserThreadGlobals& globals = g_globals.Get();
   // Profiler to track potential contention on |globals.lock|. This only does
   // real work on canary and local dev builds, so the cost of having this here
@@ -555,9 +590,9 @@ bool BrowserThread::GetCurrentThreadIdentifier(ID* identifier) {
   tracked_objects::ScopedTracker tracking_profile(FROM_HERE);
   base::AutoLock lock(globals.lock);
   for (int i = 0; i < ID_COUNT; ++i) {
-    if (globals.threads[i] &&
-        globals.threads[i]->message_loop() == cur_message_loop) {
-      *identifier = globals.threads[i]->identifier_;
+    if (globals.task_runners[i] &&
+        globals.task_runners[i]->RunsTasksOnCurrentThread()) {
+      *identifier = static_cast<ID>(i);
       return true;
     }
   }