Remove usage of SequencedWorkerPool::GetNamedSequenceToken from kiosk_app_data.cc.

chrome/browser/chromeos/app_mode/kiosk_app_data.cc

Index: chrome/browser/chromeos/app_mode/kiosk_app_data.cc
diff --git a/chrome/browser/chromeos/app_mode/kiosk_app_data.cc b/chrome/browser/chromeos/app_mode/kiosk_app_data.cc
index e9062b046729ec48521bb9fad1ff73ea4460a0be..9fb7fcabded69ea7911bfbcc3bed0c85c06a1144 100644
--- a/chrome/browser/chromeos/app_mode/kiosk_app_data.cc
+++ b/chrome/browser/chromeos/app_mode/kiosk_app_data.cc
@@ -12,7 +12,6 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/task_scheduler/post_task.h"
-#include "base/threading/sequenced_worker_pool.h"
 #include "base/values.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/chromeos/app_mode/kiosk_app_data_delegate.h"
@@ -78,18 +77,14 @@ class KioskAppData::CrxLoader : public extensions::SandboxedUnpackerClient {
             const base::FilePath& crx_file)
       : client_(client),
         crx_file_(crx_file),
-        success_(false) {
-  }
+        success_(false),
+        task_runner_(base::CreateSequencedTaskRunnerWithTraits(
+            {base::MayBlock(), base::TaskPriority::BACKGROUND,
+             base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN})) {}
 
   void Start() {
-    base::SequencedWorkerPool* pool = BrowserThread::GetBlockingPool();
-    base::SequencedWorkerPool::SequenceToken token =
-        pool->GetNamedSequenceToken("KioskAppData.CrxLoaderWorker");

[fdoray, 2017/05/16 17:58:56]

Before my CL, the named sequence token provided mutual exclusion between tasks
posted from different KioskAppData::CrxLoader instances.

With my CL, mutual exclusion is only between tasks posted from the same
KioskAppData::CrxLoader instance.

Is that ok?

[xiyuan, 2017/05/16 19:41:45]

I would like to keep the existing behavior. Crx unpacking is heavy. If a kiosk
device gets a few kiosk apps pushed down via policy, we could be running crx
unpacking in parallel, which slows down the device.

We have a KioskAppManager::GetBackgroundTaskRunner, which always creates a new
task runner ATM. It is currently used in two places (for ExternalCache and USB
update monitoring). We probably can change it to only create one task runner and
use it everywhere (including for CrxLoader).

[fdoray, 2017/05/19 19:06:52]

The TaskScheduler is already limiting the rate at which
base::TaskPriority::BACKGROUND tasks run. We would prefer if tasks ran on the
same sequence only when there is a data dependency between them. Sharing a
sequence to limit load prevents the TaskScheduler from being smart (e.g. run
more BACKGROUND tasks when the browser is idle).

What do you think?

-    task_runner_ = pool->GetSequencedTaskRunnerWithShutdownBehavior(
-        token,
-        base::SequencedWorkerPool::SKIP_ON_SHUTDOWN);
     task_runner_->PostTask(FROM_HERE,
-                           base::Bind(&CrxLoader::StartOnBlockingPool, this));
+                           base::BindOnce(&CrxLoader::StartInThreadPool, this));
   }
 
   bool success() const { return success_; }
@@ -122,21 +117,21 @@ class KioskAppData::CrxLoader : public extensions::SandboxedUnpackerClient {
       icon_ = install_icon;
       required_platform_version_ = info->required_platform_version;
     }
-    NotifyFinishedOnBlockingPool();
+    NotifyFinishedInThreadPool();
   }
   void OnUnpackFailure(const extensions::CrxInstallError& error) override {
     DCHECK(task_runner_->RunsTasksOnCurrentThread());
 
     success_ = false;
-    NotifyFinishedOnBlockingPool();
+    NotifyFinishedInThreadPool();
   }
 
-  void StartOnBlockingPool() {
+  void StartInThreadPool() {
     DCHECK(task_runner_->RunsTasksOnCurrentThread());
 
     if (!temp_dir_.CreateUniqueTempDir()) {
       success_ = false;
-      NotifyFinishedOnBlockingPool();
+      NotifyFinishedInThreadPool();
       return;
     }
 
@@ -147,7 +142,7 @@ class KioskAppData::CrxLoader : public extensions::SandboxedUnpackerClient {
     unpacker->StartWithCrx(extensions::CRXFileInfo(crx_file_));
   }
 
-  void NotifyFinishedOnBlockingPool() {
+  void NotifyFinishedInThreadPool() {
     DCHECK(task_runner_->RunsTasksOnCurrentThread());
 
     if (!temp_dir_.Delete()) {
@@ -171,7 +166,7 @@ class KioskAppData::CrxLoader : public extensions::SandboxedUnpackerClient {
   base::FilePath crx_file_;
   bool success_;
 
-  scoped_refptr<base::SequencedTaskRunner> task_runner_;
+  const scoped_refptr<base::SequencedTaskRunner> task_runner_;
   base::ScopedTempDir temp_dir_;
 
   // Extracted meta data.