Implement querying CPU time from /proc/stat on Linux for systemInfo.cpu API

chrome/browser/extensions/api/system_info_cpu/cpu_info_provider_linux.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/system_info_cpu/cpu_info_provider.h"
 
+#include <cstdio>
+#include <iostream>
+
+#include "base/file_util.h"
+#include "base/format_macros.h"
+
 namespace extensions {
 
+namespace {
+
+const char kProcStat[] = "/proc/stat";
+
+}  // namespace
+
 bool CpuInfoProvider::QueryCpuTimePerProcessor(std::vector<CpuTime>* times) {

[benwells, 2012/09/21 07:10:10]

DCHECK times here. This provides
(a) a statement of intent in the code that times can't be NULL
(b) a nice error in debug mode if it is

[Hongbo Min, 2012/09/21 08:30:56]

Done.

-  // TODO(hongbo): Query the cpu time from /proc/stat.
-  return false;
+  std::vector<CpuTime> results;

[benwells, 2012/09/21 07:10:10]

Move results down to just before while loop / after the early return. Why even
have a results variable? Can't you just use times?

[Hongbo Min, 2012/09/21 08:30:56]

It is just a coding preference. See http://codereview.chromium.org/10916197/.
The reviewer alexeypa seems to prefer that style.

+
+  std::string contents;
+  if (!file_util::ReadFileToString(FilePath(kProcStat), &contents))
+     return false;
+
+  std::istringstream iss(contents);
+  int count = 0;

[benwells, 2012/09/21 07:10:10]

Replace int count with bool first, since you just want to know if an entry is
the first one or not.

[Hongbo Min, 2012/09/21 08:30:56]

Remove it and call std::getline before the loop to skip the first line instead.

+  uint64 user = 0, nice = 0, sys = 0, idle = 0;
+  std::string line;
+  while (std::getline(iss, line)) {
+    ++count;
+    // Ignore the first line because it is just an aggregated number of

[benwells, 2012/09/21 07:10:10]

Mihai had asked for a comment in cpu_info_provider.h to be updated but I can't
see that file in the patch. Was it updated?

[benwells, 2012/09/21 08:09:14]

Oh, I see it in the next review.

+    // all cpuN lines, or the line not starting 'cpu' string.
+    if (count == 1 || line.compare(0, 3, "cpu") != 0)
+      continue;
+
+    sscanf(line.c_str(), "%*s %"PRIu64" %"PRIu64" %"PRIu64" %"PRIu64,
+           &user, &nice, &sys, &idle);
+
+    CpuTime time;
+    time.kernel = sys;
+    time.user = user + nice;
+    time.idle = idle;
+    results.push_back(time);
+  }
+  times->swap(results);
+  return true;
 }
 
 }  // namespace extensions