Add a switch to emit browser histograms.

chrome/browser/ui/startup/startup_browser_creator.cc

Index: chrome/browser/ui/startup/startup_browser_creator.cc
diff --git a/chrome/browser/ui/startup/startup_browser_creator.cc b/chrome/browser/ui/startup/startup_browser_creator.cc
index 202f9f8f2c473aeddd0aa22a1a0214921d1e298b..727ae6305f3ef88ddd76b05de46a85fbfe53abc1 100644
--- a/chrome/browser/ui/startup/startup_browser_creator.cc
+++ b/chrome/browser/ui/startup/startup_browser_creator.cc
@@ -14,11 +14,13 @@
 #include "base/command_line.h"
 #include "base/compiler_specific.h"
 #include "base/environment.h"
+#include "base/file_util.h"
 #include "base/files/file_path.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram.h"
+#include "base/metrics/statistics_recorder.h"
 #include "base/path_service.h"
 #include "base/prefs/pref_service.h"
 #include "base/strings/string_number_conversions.h"
@@ -227,6 +229,20 @@ class ProfileLaunchObserver : public content::NotificationObserver {
 base::LazyInstance<ProfileLaunchObserver> profile_launch_observer =
     LAZY_INSTANCE_INITIALIZER;
 
+// Dumps the current set of the browser process's histograms to |output_file|.
+// The file is not overwritten or modified if it exists. This function should
+// only be called in the blocking pool.
+void DumpBrowserHistograms(const base::FilePath& output_file) {
+  base::ThreadRestrictions::AssertIOAllowed();
+
+  if (base::PathExists(output_file))

[marja, 2013/11/14 08:33:10]

I think this is confusing and error prone for the user point of view. I can
imagine the use case for this feature is that you run a command line for dumping
the data several times (at least I heavily recycle my command lines), and it's
easy to make a mistake by specifying a file that is already there from the
previous run, in which case the file would be left untouched, but the user might
think the data was retrieved.

I tried to come up with examples of programs (e.g., unix command line tools)
that work this way, but couldn't think of any. All examples I can think of
overwrite the output file if it already exists.

But as I'm not familiar with the feature itself, it's up to you to decide if
this is what you want.

[grt (UTC plus 2), 2013/11/14 14:44:12]

After more consideration, I think you're right. My main concern with the
overwrite case was an accidental fat-fingering leading to overwriting the wrong
file, but given the purpose of this switch and the burden it puts on callers, I
think overwrite is good. Thanks.

+    return;
+
+  std::string output_string(base::StatisticsRecorder::ToJSON(std::string()));
+  file_util::WriteFile(output_file, output_string.data(),
+                       static_cast<int>(output_string.size()));
+}
+
 }  // namespace
 
 StartupBrowserCreator::StartupBrowserCreator()
@@ -602,6 +618,20 @@ bool StartupBrowserCreator::ProcessCmdLineImpl(
   ui::TouchFactory::SetTouchDeviceListFromCommandLine();
 #endif
 
+  if (!process_startup &&
+      command_line.HasSwitch(switches::kDumpBrowserHistograms)) {
+    // Only handle --dump-browser-histograms from a rendezvous. In this case, do
+    // not open a new browser window even if no output file was given.
+    base::FilePath output_file(
+        command_line.GetSwitchValuePath(switches::kDumpBrowserHistograms));
+    if (!output_file.empty()) {
+      BrowserThread::PostBlockingPoolTask(
+          FROM_HERE,
+          base::Bind(&DumpBrowserHistograms, output_file));
+    }
+    silent_launch = true;
+  }
+
   // If we don't want to launch a new browser window or tab (in the case
   // of an automation request), we are done here.
   if (silent_launch)