Log the freshness of the Variations seed in a histogram.

chrome/browser/metrics/variations/variations_service.cc

Index: chrome/browser/metrics/variations/variations_service.cc
diff --git a/chrome/browser/metrics/variations/variations_service.cc b/chrome/browser/metrics/variations/variations_service.cc
index 00c7816705d3638bc190824bbf3f7101bc93150f..4c576d9101ec0bdb8d443f64632d22c6d90e64d9 100644
--- a/chrome/browser/metrics/variations/variations_service.cc
+++ b/chrome/browser/metrics/variations/variations_service.cc
@@ -177,6 +177,20 @@ bool VariationsService::CreateTrialsFromSeed() {
     }
   }
 
+  // Log the "freshness" of the seed that was just used. The freshness is the
+  // time between the last successful seed download and now.
+  base::Time now = base::Time::Now();

[Alexei Svitkine (slow), 2013/02/22 16:26:25]

Move this into the if statement. Make it const.

[SteveT, 2013/02/22 18:12:03]

Done.

+  int64 last_fetch_time_internal =

[Alexei Svitkine (slow), 2013/02/22 16:26:25]

Nit: const.

[SteveT, 2013/02/22 18:12:03]

Done.

+      local_state_->GetInt64(prefs::kVariationsLastFetchTime);
+  if (last_fetch_time_internal) {
+    DCHECK_GE(now.ToInternalValue(), last_fetch_time_internal);

[Alexei Svitkine (slow), 2013/02/22 16:26:25]

This may happen if the user changed his system time between, which is something
that could happen (and can't really be avoided). Don't DCHECK on it.

I think we can just log these anyway - so that we have the info about them in
the logs (assuming the histogram supports negative values).

[SteveT, 2013/02/22 18:12:03]

Removed the DCHECK.

I think the histogram might just log it in the smallest bucket. Ilya, can you
comment on what happens if we pass a negative sample into a histogram?

[Ilya Sherman, 2013/02/22 23:26:16]

Yes, negative values end up in the lowest bucket, which is the underflow bucket.

+    base::TimeDelta delta =

[Alexei Svitkine (slow), 2013/02/22 16:26:25]

Nit: const.

[SteveT, 2013/02/22 18:12:03]

Done.

+        now - base::Time::FromInternalValue(last_fetch_time_internal);
+    // Log the value in number of minutes.
+    UMA_HISTOGRAM_CUSTOM_COUNTS("Variations.SeedFreshness", delta.InMinutes(),
+        1, base::TimeDelta::FromDays(7).InMinutes(), 50);

[Alexei Svitkine (slow), 2013/02/22 16:26:25]

I would say let's use 30 days, just to get a more complete story (for folks who
don't relaunch their browser as much).

How did you come up with the 50 for the last parameter?

[SteveT, 2013/02/22 18:12:03]

Done.
UMA_HISTOGRAM_CUSTOM_COUNTS_10000 has 50 buckets. 7 days = 10080 minutes and our
unit is in minutes. We didn't use UMA_HISTOGRAM_CUSTOM_COUNTS_10000 because
expanding out the InMinutes() here makes the code easier to read.

+  }
+
   return true;
 }
 
@@ -215,6 +229,7 @@ void VariationsService::RegisterPrefs(PrefRegistrySimple* registry) {
   registry->RegisterStringPref(prefs::kVariationsSeed, std::string());
   registry->RegisterInt64Pref(prefs::kVariationsSeedDate,
                               base::Time().ToInternalValue());
+  registry->RegisterInt64Pref(prefs::kVariationsLastFetchTime, 0);
 }
 
 // static
@@ -288,6 +303,10 @@ void VariationsService::OnURLFetchComplete(const net::URLFetcher* source) {
           base::TimeDelta::FromMilliseconds(kServerTimeResolutionMs),
           latency);
     }
+
+    // Record the time of the most recent successful fetch.
+    local_state_->SetInt64(prefs::kVariationsLastFetchTime,
+                           base::Time::Now().ToInternalValue());
   }
 
   if (response_code != net::HTTP_OK) {