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 72345852c0f5e7e64f5c3d501839dda16748c5e5..459fc3cbc4dc939a6d887cedfc73dee28d3e45d0 100644
--- a/content/browser/browser_thread_impl.cc
+++ b/content/browser/browser_thread_impl.cc
@@ -4,18 +4,18 @@
 
 #include "content/browser/browser_thread_impl.h"
 
-#include <string.h>
-
 #include <string>
 
 #include "base/atomicops.h"
 #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/synchronization/waitable_event.h"
 #include "base/threading/platform_thread.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "build/build_config.h"
@@ -52,6 +52,8 @@ static const char* GetThreadName(BrowserThread::ID thread) {
 
 // An implementation of SingleThreadTaskRunner to be used in conjunction
 // with BrowserThread.
+// TODO(gab): Consider replacing this with |g_globals->task_runners| -- only
+// works if none are requested before starting the threads.
 class BrowserThreadTaskRunner : public base::SingleThreadTaskRunner {
  public:
   explicit BrowserThreadTaskRunner(BrowserThread::ID identifier)
@@ -99,6 +101,20 @@ 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 with a BrowserThreadImpl instance but the
+  // underlying thread hasn't started yet.
+  INITIALIZED,
+  // BrowserThread::ID is associated to a TaskRunner and is accepting tasks.
+  RUNNING,
+  // BrowserThread::ID no longer accepts tasks.
+  SHUTDOWN
+};
+
 using BrowserThreadDelegateAtomicPtr = base::subtle::AtomicWord;
 
 struct BrowserThreadGlobals {
@@ -106,24 +122,21 @@ struct 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]));
-  }
+                                          base::TaskPriority::USER_VISIBLE)) {}
 
-  // 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.
+  BrowserThreadState states[BrowserThread::ID_COUNT] = {};
 
   // Only atomic operations are used on this pointer. The delegate isn't owned
   // by BrowserThreadGlobals, rather by whoever calls
@@ -141,13 +154,6 @@ 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,
@@ -156,11 +162,16 @@ BrowserThreadImpl::BrowserThreadImpl(ID identifier,
   SetMessageLoop(message_loop);
   Initialize();
 
-  // If constructed with an explicit message loop, this is a fake BrowserThread
-  // which runs on the current thread.
+  // If constructed with an explicit message loop, this is a fake
+  // BrowserThread 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_] = task_runner();
+
+  DCHECK_EQ(globals.states[identifier_], BrowserThreadState::INITIALIZED);
+  globals.states[identifier_] = BrowserThreadState::RUNNING;
 }
 
 // static
@@ -186,14 +197,27 @@ void BrowserThreadImpl::FlushThreadPoolHelperForTesting() {
 void BrowserThreadImpl::Init() {
   BrowserThreadGlobals& globals = g_globals.Get();
 
+#if DCHECK_IS_ON()
+  {
+    base::AutoLock lock(globals.lock);
+    // |globals| should already have been initialized for |identifier_| in
+    // BrowserThreadImpl::StartWithOptions(). If this isn't the case it's likely
+    // because this BrowserThreadImpl's owner incorrectly used Thread::Start.*()
+    // instead of BrowserThreadImpl::Start.*().
+    DCHECK_EQ(globals.states[identifier_], BrowserThreadState::RUNNING);
+    DCHECK(globals.task_runners[identifier_]);
+    DCHECK(globals.task_runners[identifier_]->RunsTasksOnCurrentThread());
+  }
+#endif  // DCHECK_IS_ON()
+
   if (identifier_ == BrowserThread::DB ||
       identifier_ == BrowserThread::FILE ||
       identifier_ == BrowserThread::FILE_USER_BLOCKING ||
       identifier_ == BrowserThread::PROCESS_LAUNCHER ||
       identifier_ == 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();
   }
 
   if (identifier_ == BrowserThread::IO) {
@@ -305,12 +329,13 @@ void BrowserThreadImpl::CleanUp() {
       reinterpret_cast<BrowserThreadDelegate*>(delegate)->CleanUp();
   }
 
-  // PostTaskHelper() accesses the message loop while holding this lock.
-  // However, the message loop will soon be destructed without any locking. So
-  // to prevent a race with accessing the message loop in PostTaskHelper(),
-  // remove this thread from the global array now.
+  // Change the state to SHUTDOWN so that PostTaskHelper stops accepting tasks
+  // for this thread. Do not clear globals.task_runners[identifier_] so that
+  // BrowserThread::CurrentlyOn() works from the MessageLoop's
+  // DestructionObservers.
   base::AutoLock lock(globals.lock);
-  globals.threads[identifier_] = nullptr;
+  DCHECK_EQ(globals.states[identifier_], BrowserThreadState::RUNNING);
+  globals.states[identifier_] = BrowserThreadState::SHUTDOWN;
 }
 
 void BrowserThreadImpl::Initialize() {
@@ -319,8 +344,19 @@ 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;
+}
+
+// static
+void BrowserThreadImpl::ResetGlobalsForTesting(BrowserThread::ID identifier) {
+  BrowserThreadGlobals& globals = g_globals.Get();
+
+  base::AutoLock lock(globals.lock);
+  DCHECK_EQ(globals.states[identifier], BrowserThreadState::SHUTDOWN);
+  globals.states[identifier] = BrowserThreadState::UNINITIALIZED;
+  globals.task_runners[identifier] = nullptr;
+  SetIOThreadDelegate(nullptr);
 }
 
 BrowserThreadImpl::~BrowserThreadImpl() {
@@ -331,12 +367,22 @@ 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(globals.states[i] == BrowserThreadState::SHUTDOWN ||
+           globals.states[i] == BrowserThreadState::UNINITIALIZED)
+        << "Threads must be listed in the reverse order that they die";
   }
 #endif
 }
@@ -346,14 +392,24 @@ bool BrowserThreadImpl::Start() {
 }
 
 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).
   BrowserThreadGlobals& globals = g_globals.Get();
+
+  // Holding the lock is necessary when kicking off the thread to ensure
+  // |states| and |task_runners| are updated before it gets to query them.
   base::AutoLock lock(globals.lock);
-  DCHECK_EQ(globals.thread_ids[identifier_], base::kInvalidThreadId);
+
   bool result = Thread::StartWithOptions(options);
-  globals.thread_ids[identifier_] = GetThreadId();
+
+  // Although the thread is starting asynchronously, the MessageLoop is already
+  // ready to accept tasks and as such this BrowserThreadImpl is considered as
+  // "running".
+  DCHECK(!globals.task_runners[identifier_]);
+  globals.task_runners[identifier_] = task_runner();
+  DCHECK(globals.task_runners[identifier_]);
+
+  DCHECK_EQ(globals.states[identifier_], BrowserThreadState::INITIALIZED);
+  globals.states[identifier_] = BrowserThreadState::RUNNING;
+
   return result;
 }
 
@@ -363,6 +419,70 @@ bool BrowserThreadImpl::StartAndWaitForTesting() {
   WaitUntilThreadStarted();
   return true;
 }
+
+// static
+void BrowserThreadImpl::RedirectThreadIDToTaskRunner(
+    BrowserThread::ID identifier,
+    scoped_refptr<base::SingleThreadTaskRunner> task_runner) {
+  DCHECK(task_runner);
+
+  BrowserThreadGlobals& globals = g_globals.Get();
+  base::AutoLock lock(globals.lock);
+
+  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;
+}
+
+// static
+void BrowserThreadImpl::StopRedirectionOfThreadID(
+    BrowserThread::ID identifier) {
+  BrowserThreadGlobals& globals = g_globals.Get();
+  base::AutoLock auto_lock(globals.lock);
+
+  DCHECK(globals.task_runners[identifier]);
+
+  // Change the state to SHUTDOWN to stop accepting new tasks. Note: this is
+  // different from non-redirected threads which continue accepting tasks while
+  // being joined and only quit when idle. However, any tasks for which this
+  // difference matters was already racy as any thread posting a task after the
+  // Signal task below can't be synchronized with the joining thread. Therefore,
+  // that task could already come in before or after the join had completed in
+  // the non-redirection world. Entering SHUTDOWN early merely skews this race
+  // towards making it less likely such a task is accepted by the joined thread
+  // which is fine.
+  DCHECK_EQ(globals.states[identifier], BrowserThreadState::RUNNING);
+  globals.states[identifier] = BrowserThreadState::SHUTDOWN;
+
+  // Wait for all pending tasks to complete.
+  base::WaitableEvent flushed(base::WaitableEvent::ResetPolicy::MANUAL,
+                              base::WaitableEvent::InitialState::NOT_SIGNALED);
+  globals.task_runners[identifier]->PostTask(
+      FROM_HERE,
+      base::Bind(&base::WaitableEvent::Signal, base::Unretained(&flushed)));
+  {
+    base::AutoUnlock auto_lock(globals.lock);
+    flushed.Wait();
+  }
+
+  // Only reset the task runner after running pending tasks so that
+  // BrowserThread::CurrentlyOn() works in their scope.
+  globals.task_runners[identifier] = nullptr;
+
+  // Note: it's still possible for tasks to be posted to that task runner after
+  // this point (e.g. through a previously obtained ThreadTaskRunnerHandle or by
+  // one of the last tasks re-posting to its ThreadTaskRunnerHandle) but the
+  // BrowserThread API itself won't accept tasks. Such tasks are ultimately
+  // guaranteed to run before TaskScheduler::Shutdown() returns but may break
+  // the assumption in PostTaskHelper that BrowserThread::ID A > B will always
+  // succeed to post to B. This is pretty much the only observable difference
+  // between a redirected thread and a real one and is one we're willing to live
+  // with for this experiment. TODO(gab): fix this before enabling the
+  // experiment by default on trunk, http://crbug.com/653916.
+}
+
 // static
 bool BrowserThreadImpl::PostTaskHelper(
     BrowserThread::ID identifier,
@@ -374,7 +494,8 @@ bool BrowserThreadImpl::PostTaskHelper(
   DCHECK_LT(identifier, ID_COUNT);
   // Optimization: to avoid unnecessary locks, we listed the ID enumeration in
   // order of lifetime.  So no need to lock if we know that the target thread
-  // outlives current thread.
+  // outlives current thread as that implies the current thread only ever sees
+  // the target thread in its RUNNING state.
   // Note: since the array is so small, ok to loop instead of creating a map,
   // which would require a lock because std::map isn't thread safe, defeating
   // the whole purpose of this optimization.
@@ -387,22 +508,23 @@ 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
@@ -453,7 +575,8 @@ 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
@@ -462,7 +585,8 @@ 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
@@ -488,8 +612,7 @@ 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
@@ -543,7 +666,6 @@ 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
@@ -551,9 +673,9 @@ 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;
     }
   }