Disallow posting tasks to SequencedWorkerPools by default.

content/browser/browser_main_loop.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/browser_main_loop.h"
 
 #include <stddef.h>
 
 #include <string>
 #include <utility>
@@ skipping 13 matching lines @@
 #include "base/metrics/user_metrics.h"
 #include "base/pending_task.h"
 #include "base/power_monitor/power_monitor.h"
 #include "base/power_monitor/power_monitor_device_source.h"
 #include "base/process/process_metrics.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
 #include "base/system_monitor/system_monitor.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/timer/hi_res_timer_manager.h"
 #include "base/trace_event/memory_dump_manager.h"
 #include "base/trace_event/trace_event.h"
 #include "build/build_config.h"
 #include "components/tracing/browser/trace_config_file.h"
 #include "components/tracing/common/process_metrics_memory_dump_provider.h"
 #include "components/tracing/common/trace_to_console.h"
 #include "components/tracing/common/tracing_switches.h"
@@ skipping 683 matching lines @@
       base::kNullProcessId);
   base::trace_event::MemoryDumpManager::GetInstance()->RegisterDumpProvider(
       HostSharedBitmapManager::current(), "HostSharedBitmapManager", nullptr);
   base::trace_event::MemoryDumpManager::GetInstance()->RegisterDumpProvider(
       skia::SkiaMemoryDumpProvider::GetInstance(), "Skia", nullptr);
   base::trace_event::MemoryDumpManager::GetInstance()->RegisterDumpProvider(
       sql::SqlMemoryDumpProvider::GetInstance(), "Sql", nullptr);
 }
 
 int BrowserMainLoop::PreCreateThreads() {
+  DCHECK(!base::SequencedWorkerPool::IsEnabled());

[gab, 2016/11/01 21:48:02]

DCHECK are supposed to document invariants, so a DCHECK followed by an if
doesn't make sense.

[fdoray, 2016/11/02 12:56:09]

Added comment to clarify.

SequencedWorkerPool is not enabled when BrowserMainLoop::PreCreateThreads
enters. In Chrome, it is enabled by parts_->PreCreateThreads() below. In other
products (android webview, cast...) it is enabled by
base::SequencedWorkerPool::EnableForProcess() below (surrounded by an if).

+
   if (parts_) {
     TRACE_EVENT0("startup",
         "BrowserMainLoop::CreateThreads:PreCreateThreads");
 
     result_code_ = parts_->PreCreateThreads();
   }
 
+  // Enable SequencedWorkerPool for this process if it has not already been
+  // enabled by the BrowserMainParts::PreCreateThreads() call above.
+  // TODO(fdoray): Remove this once the SequencedWorkerPool to TaskScheduler
+  // redirection experiment concludes https://crbug.com/622400.
+  if (!base::SequencedWorkerPool::IsEnabled())
+    base::SequencedWorkerPool::EnableForProcess();
+
   const base::CommandLine* command_line =
       base::CommandLine::ForCurrentProcess();
   // Note that we do not initialize a new FeatureList when calling this for
   // the second time.
   base::FeatureList::InitializeInstance(
       command_line->GetSwitchValueASCII(switches::kEnableFeatures),
       command_line->GetSwitchValueASCII(switches::kDisableFeatures));
 
   InitializeMemoryManagementComponent();
 
@@ skipping 838 matching lines @@
     scoped_refptr<base::SingleThreadTaskRunner> worker_task_runner =
         audio_thread_->task_runner();
     audio_manager_ = media::AudioManager::Create(std::move(audio_task_runner),
                                                  std::move(worker_task_runner),
                                                  MediaInternals::GetInstance());
   }
   CHECK(audio_manager_);
 }
 
 }  // namespace content