Create a synthetic field trial for precache.

chrome/browser/precache/precache_util.cc

 // Copyright 2016 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/precache/precache_util.h"
 
+#include <vector>
+
+#include "base/metrics/field_trial.h"
+#include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "chrome/browser/browser_process.h"
+#include "chrome/browser/metrics/chrome_metrics_service_accessor.h"
 #include "chrome/browser/precache/precache_manager_factory.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "components/data_use_measurement/content/content_url_request_classifier.h"
 #include "components/precache/content/precache_manager.h"
+#include "components/variations/metrics_util.h"
 #include "content/public/browser/browser_thread.h"
 #include "net/url_request/url_request.h"
 #include "url/gurl.h"
 
 namespace net {
 class HttpResponseInfo;
 }
 
 namespace {
 
+const char kPrecacheFieldTrialName[] = "Precache";
+const char kPrecacheSyntheticFieldTrialName[] = "PrecacheSynthetic";
+
+// Users who precached in the last |kDaysForPrecacheCandidates| and
+// |kDaysForRecentPrecacheCandidates| days will be placed in separate trial
+// groups.
+const size_t kDaysForPrecacheCandidates = 15;
+const size_t kDaysForRecentPrecacheCandidates = 1;

[jamartin, 2016/12/21 23:54:33]

(nit)

s/kDaysForPrecacheCandidates/kDaysForPrecache/
s/kDaysForRecentPrecacheCandidates/kDaysForPrecacheRecent/

I accept if you want to keep the Candidates suffix but I think there is some
merit in having PrecacheRecent matching the group name.

Or maybe even better, instead of naming the synthetic groups "Precache" and
"PrecacheRecent", which may be confusing since our experiment is called
"Precache" already, why not simply call them "1D" and "15D"?

Reading the documentation I believe you did this to guarantee that the trial
group names are unique. Then, I'd suggest "Precache1D" and "Precache15D".

[twifkak, 2017/01/05 23:53:14]

You read my code incorrectly. Keep in mind that there are two names: the trial
name and the group name. For succinctness, I'll denote with a slash, as
TrialName/GroupName.

The function below maps Precache/Control to zero or more of
PrecacheSynthetic/Control and PrecacheSynthetic/ControlRecent. Ditto for
Precache/Enabled.

I renamed Control{,Recent} to Control{15,1}D, per your suggestion. I got rid of
the constants, since having a `const int kFifteen = 15` is pretty ridiculous. I
also added some comments.

+
 void UpdatePrecacheMetricsAndStateOnUIThread(const GURL& url,
                                              const GURL& referrer,
                                              base::TimeDelta latency,
                                              const base::Time& fetch_time,
                                              const net::HttpResponseInfo& info,
                                              int64_t size,
                                              bool is_user_traffic,
                                              void* profile_id) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 
@@ skipping 29 matching lines @@
   // Record precache metrics when a fetch is completed successfully, if
   // precaching is allowed.
   content::BrowserThread::PostTask(
       content::BrowserThread::UI, FROM_HERE,
       base::Bind(&UpdatePrecacheMetricsAndStateOnUIThread, request->url(),
                  GURL(request->referrer()), latency, base::Time::Now(),
                  request->response_info(), received_content_length,
                  data_use_measurement::IsUserRequest(*request), profile_id));
 }
 
+void RegisterPrecacheSyntheticFieldTrial(const base::Time& last_precache_time) {
+  std::vector<uint32_t> groups;
+  base::TimeDelta time_ago = base::Time::Now() - last_precache_time;
+  std::string group_name =
+      base::FieldTrialList::FindFullName(kPrecacheFieldTrialName);
+  if (time_ago <= base::TimeDelta::FromDays(kDaysForPrecacheCandidates))
+    groups.push_back(metrics::HashName(group_name));
+  if (time_ago <= base::TimeDelta::FromDays(kDaysForRecentPrecacheCandidates))
+    groups.push_back(metrics::HashName(group_name + "Recent"));
+  ChromeMetricsServiceAccessor::RegisterSyntheticMultiGroupFieldTrial(

[jamartin, 2016/12/21 23:54:33]

I couldn't find any other caller for this function. Either I didn't look for it
hard enough or I this is a bit concerning.

[twifkak, 2017/01/05 23:53:14]

It looks like it was implemented in
https://chromium.googlesource.com/chromium/src/+/6a1c881f89fe55559248668026b0...
and its only caller is in
https://chrome-internal.googlesource.com/clank/internal/apps/+/556a4789aeab52....

+      kPrecacheSyntheticFieldTrialName, groups);
+}
+
 }  // namespace precache