Add the implementation skeleton for systeminfo.cpu API

chrome/browser/extensions/api/systeminfo_cpu/systeminfo_cpu_api.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 "chrome/browser/extensions/api/systeminfo_cpu/systeminfo_cpu_api.h"
+
+#include "base/logging.h"
+#include "content/public/browser/browser_thread.h"
+#include "chrome/browser/extensions/api/systeminfo_cpu/cpu_info_provider.h"
+
+namespace extensions {
+using api::experimental_systeminfo_cpu::CpuInfo;
+using api::experimental_systeminfo_cpu::CpuCoreInfo;
+using content::BrowserThread;
+
+SysteminfoCpuGetFunction::SysteminfoCpuGetFunction() {
+}
+
+SysteminfoCpuGetFunction::~SysteminfoCpuGetFunction() {
+  // Delete the provider that was created on FILE thread.
+  BrowserThread::DeleteSoon(BrowserThread::FILE, FROM_HERE, provider_);
+}
+
+bool SysteminfoCpuGetFunction::RunImpl() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  BrowserThread::PostTask(BrowserThread::FILE, FROM_HERE,
+    base::Bind(&SysteminfoCpuGetFunction::WorkOnFileThread, this));
+  return true;
+}
+
+void SysteminfoCpuGetFunction::GetCpuInfoOnFileThread() {

[Mihai Parparita -not on Chrome, 2012/08/10 20:24:56]

Can you move this to be below WorkOnFileThread, so that that the functions
appear in the file in the same order that they call each-other in?

[Hongbo Min, 2012/08/12 14:22:44]

Done.

+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
+  bool success = false;
+
+  CpuInfo info;
+  if (provider_ && provider_->GetCpuInfo(&info)) {
+    SetResult(info.ToValue().release());
+    success = true;
+  } else {
+    SetError("Error in querying cpu information!");
+  }
+  // Respond on UI thread.
+  BrowserThread::PostTask(
+      BrowserThread::UI, FROM_HERE,
+      base::Bind(&SysteminfoCpuGetFunction::RespondOnUIThread, this, success));
+}
+
+void SysteminfoCpuGetFunction::WorkOnFileThread() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
+  provider_ = CpuInfoProvider::Create();

[Mihai Parparita -not on Chrome, 2012/08/10 20:24:56]

It seems expensive to create a provider per function call, especially if you
have to wait for at least one sample to be collected each time. Is the eventual
plan to have a single shared instance?

[Hongbo Min, 2012/08/12 14:22:44]

Yes, it makes lots of senses to make it as a single shared instance via
Singleton or  base::LazyInstance support.

Here the code for provider creation is actually premature and just for the API
skeleton. Next step I will focus on providing the its refinement and the
provider implementation.

+  // Delay 500ms to give a chance for CPU information sampling.

[Mihai Parparita -not on Chrome, 2012/08/10 20:24:56]

The 500ms delay seems arbitrary and prone to flakiness. Can the provider support
a OnNextSample(callback) method instead?

[Hongbo Min, 2012/08/12 14:22:44]

The 500ms is a tricky for getting cpu usage per core on linux. On linux, the cpu
load is calculated from /proc/stat file. It requires to read /proc/stat file at
a interval, for example, 0.5 seconds. The cpu usage is actually a statistics for
that interval.

On Windows, no such a tricky is needed. 

Here I just remove it and let it going in next reviewing cycle. It is relevant
to provider implementation.

+  BrowserThread::PostDelayedTask(
+      BrowserThread::FILE, FROM_HERE,
+      base::Bind(&SysteminfoCpuGetFunction::GetCpuInfoOnFileThread, this),
+      base::TimeDelta::FromMilliseconds(500));
+}
+
+void SysteminfoCpuGetFunction::RespondOnUIThread(bool success) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  SendResponse(success);
+}
+
+// A mock implementation of CpuInfoProver for temporary usage.
+// Will be moved out when the platform specific implementation are done.
+class MockCpuInfoProvider : public CpuInfoProvider {
+ public:
+  MockCpuInfoProvider() {}
+  virtual ~MockCpuInfoProvider() {}
+  virtual bool GetCpuInfo(CpuInfo* info) OVERRIDE;
+};
+
+bool MockCpuInfoProvider::GetCpuInfo(CpuInfo* info) {
+  if (!info) return false;
+  linked_ptr<CpuCoreInfo> core(new CpuCoreInfo());
+  core->load = 53;
+  info->cores.push_back(core);
+  return true;
+}
+
+// static
+CpuInfoProvider* CpuInfoProvider::Create() {
+  return new MockCpuInfoProvider();
+}
+
+}  // namespace extensions