Support saving browser metrics to disk and reading them during next run.

chrome/browser/chrome_browser_field_trials.cc

Index: chrome/browser/chrome_browser_field_trials.cc
diff --git a/chrome/browser/chrome_browser_field_trials.cc b/chrome/browser/chrome_browser_field_trials.cc
index 648167695f6b70b9ca8b5c986e7c773d7d08c2ca..6fbf87017cfcbf7d6cefae7a7b2d15fa49e796b4 100644
--- a/chrome/browser/chrome_browser_field_trials.cc
+++ b/chrome/browser/chrome_browser_field_trials.cc
@@ -8,12 +8,15 @@
 
 #include "base/command_line.h"
 #include "base/feature_list.h"
+#include "base/files/file_path.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/persistent_histogram_allocator.h"
+#include "base/path_service.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
+#include "chrome/common/chrome_paths.h"
 #include "chrome/common/chrome_switches.h"
 #include "components/metrics/metrics_pref_names.h"
 
@@ -39,6 +42,20 @@ void InstantiatePersistentHistograms() {
         kAllocatorName);

[Ilya Sherman, 2016/04/20 17:10:33]

Could you pass the destination file name here, so that it is indeed passed to
the constructor, and therefore possible to mark as const?

[bcwhite, 2016/04/22 15:18:05]

This has been moved to it's rightful final resting place in
browser_main_runner.cc.  The pathname can't be set from content/ because it
doesn't have access to the destination directory as that is defined in chrome/.

[Ilya Sherman, 2016/04/25 19:48:40]

Is there not an embedder interface that could be used to provide the destination
directory?  That would be the more typical design structure for accessing data
that's only known to the chrome/ layer.

[bcwhite, 2016/04/25 20:37:17]

I don't understand what you mean by that.

[Ilya Sherman, 2016/04/25 21:12:10]

For example, the ChromeMetricsServiceClient class implements the
metrics::MetricsServiceClient interface to provide data to the //metrics
component, even though that data is only available from the //chrome layer.

[bcwhite, 2016/04/26 13:32:29]

When things can be instantiated at any time, that's possible.  This has to be
instantiated as early in the start-up sequence as possible.

Chrome/ code could later hook into the object to inject an object that provides
this kind of information but that's not really any different than just injecting
the information when there is only one entry.

[Ilya Sherman, 2016/04/26 20:01:24]

Hmm, I guess I didn't look very carefully at where this setup was happening. 
Why are you choosing to implement this here, rather than a tiny bit later, in
ChromeBrowserMainParts::SetupMetricsAndFieldTrials(), where all of the other
metrics code is set up?

[bcwhite, 2016/04/26 21:04:26]

It's important to start it as early as possible so that it catches as many
histograms as possible.  Once it's created, it can never be moved so if it gets
created on the heap before the allocator gets created, it'll never be
persistent.

[Ilya Sherman, 2016/04/27 20:31:33]

Are there actually histograms that are recorded between
BrowserMainRunnerImpl::Initialize() and when the rest of the metrics
initialization happens?  Similarly, are there any metrics recorded after the
metrics system is generally shut down?  jar@ thought pretty carefully about
where metrics initialization and shutdown happen, and I would be kind of
surprised if there were metrics being recorded before/after those.  (Especially
after, since they'd be uniformly discarded currently.)

     base::GlobalHistogramAllocator::Get()->CreateTrackingHistograms(
         kAllocatorName);
+
+    // Set the destination for where these will be persisted. This shouldn't
+    // be necessary because it's done in chrome_metrics_service_client.cc where
+    // the reading of this file is configured but field trials don't get done
+    // until after that point so there is no global allocator to configure
+    // when that code runs. This won't be a problem when this (eventually)
+    // becomes standard and is enabled during early startup.
+    base::FilePath metrics_file;
+    if (base::PathService::Get(chrome::DIR_USER_DATA, &metrics_file)) {
+      metrics_file = metrics_file.AppendASCII(kAllocatorName)
+                         .AddExtension(FILE_PATH_LITERAL(".pma"));
+      base::GlobalHistogramAllocator::Get()
+          ->SetPersistentLocation(metrics_file);
+    }
   }
 }