Support serial number requests in variations service.

chrome/browser/metrics/variations_service.cc

Index: chrome/browser/metrics/variations_service.cc
===================================================================
--- chrome/browser/metrics/variations_service.cc	(revision 149502)
+++ chrome/browser/metrics/variations_service.cc	(working copy)
@@ -98,12 +98,15 @@
 }  // namespace
 
 VariationsService::VariationsService()
-    : variations_server_url_(GetVariationsServerURL()) {
+    : variations_server_url_(GetVariationsServerURL()),
+      create_trials_from_seed_called_(false) {
 }
 
 VariationsService::~VariationsService() {}
 
 bool VariationsService::CreateTrialsFromSeed(PrefService* local_prefs) {
+  create_trials_from_seed_called_ = true;

[Ilya Sherman, 2012/08/02 23:40:49]

nit: Is it ok that this method can return |false| after this line?  Is the
DCHECK below that uses this variable just making sure that we don't lose the
optimization we gain in non-exceptional cases from caching the seed?

[Alexei Svitkine (slow), 2012/08/02 23:51:02]

Right, it's just making sure we don't lose the optimization. It's not necessary
that |variations_serial_number_| actually got set (and it's valid for it not to
be - e.g. on first run).

+
   TrialsSeed seed;
   if (!LoadTrialsSeedFromPref(local_prefs, &seed))
     return false;
@@ -132,6 +135,10 @@
 void VariationsService::StartRepeatedVariationsSeedFetch() {
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 
+  // Check that |CreateTrialsFromSeed| was called, which is necessary to
+  // retrieve the serial number that will be sent to the server.
+  DCHECK(create_trials_from_seed_called_);
+
   // Perform the first fetch.
   FetchVariationsSeed();
 
@@ -157,6 +164,10 @@
   pending_seed_request_->SetRequestContext(
       g_browser_process->system_request_context());
   pending_seed_request_->SetMaxRetries(kMaxRetrySeedFetch);
+  if (!variations_serial_number_.empty()) {
+    pending_seed_request_->AddExtraRequestHeader("If-Match:" +
+                                                 variations_serial_number_);
+  }
   pending_seed_request_->Start();
 }
 
@@ -169,9 +180,10 @@
     DVLOG(1) << "Variations server request failed.";
     return;
   }
+
   if (request->GetResponseCode() != 200) {
     DVLOG(1) << "Variations server request returned non-200 response code: "
-            << request->GetResponseCode();
+             << request->GetResponseCode();
     return;
   }
 
@@ -214,6 +226,7 @@
   local_prefs->SetString(prefs::kVariationsSeed, base64_seed_data);
   local_prefs->SetInt64(prefs::kVariationsSeedDate,
                         seed_date.ToInternalValue());
+  variations_serial_number_ = seed.serial_number();
   return true;
 }
 
@@ -415,6 +428,7 @@
     local_prefs->ClearPref(prefs::kVariationsSeed);
     return false;
   }
+  variations_serial_number_ = seed->serial_number();
   return true;
 }