Merge multiple histogram snapshots into single one for reporting.

base/metrics/histogram_persistence.cc

Index: base/metrics/histogram_persistence.cc
diff --git a/base/metrics/histogram_persistence.cc b/base/metrics/histogram_persistence.cc
index 7c60079ef9ced823ea06d16470e5ff14780ed7a4..9166be027be17bb8ad1babd75fe2a604914b88a9 100644
--- a/base/metrics/histogram_persistence.cc
+++ b/base/metrics/histogram_persistence.cc
@@ -222,11 +222,14 @@ HistogramBase* CreatePersistentHistogram(
           histogram_data.counts_ref, kTypeIdCountsArray);
   if (!counts_data ||
       allocator->GetAllocSize(histogram_data.counts_ref) <
-          histogram_data.bucket_count * sizeof(HistogramBase::AtomicCount)) {
+          2 * histogram_data.bucket_count *
+          sizeof(HistogramBase::AtomicCount)) {

[Alexei Svitkine (slow), 2016/02/09 19:46:51]

Nit: Can you make a variable to store the size and a comment above for the
calculation? Then, the if can be a but more concise.

Or make a helper for it, since you use it on line 364 too.

[bcwhite, 2016/02/11 16:42:38]

Done.

     RecordCreateHistogramResult(CREATE_HISTOGRAM_INVALID_COUNTS_ARRAY);
     NOTREACHED();
     return nullptr;
   }
+  HistogramBase::AtomicCount* logged_data =

[Alexei Svitkine (slow), 2016/02/09 19:46:51]

Add a comment mentioning the second half of the counts data is the logged data.

[bcwhite, 2016/02/11 16:42:38]

Done.

+      counts_data + histogram_data.bucket_count;
 
   std::string name(histogram_data_ptr->name);
   HistogramBase* histogram = nullptr;
@@ -238,6 +241,7 @@ HistogramBase* CreatePersistentHistogram(
           histogram_data.maximum,
           ranges,
           counts_data,
+          logged_data,
           histogram_data.bucket_count,
           &histogram_data_ptr->samples_metadata);
       DCHECK(histogram);
@@ -249,6 +253,7 @@ HistogramBase* CreatePersistentHistogram(
           histogram_data.maximum,
           ranges,
           counts_data,
+          logged_data,
           histogram_data.bucket_count,
           &histogram_data_ptr->samples_metadata);
       DCHECK(histogram);
@@ -258,6 +263,7 @@ HistogramBase* CreatePersistentHistogram(
           name,
           ranges,
           counts_data,
+          logged_data,
           &histogram_data_ptr->samples_metadata);
       DCHECK(histogram);
       break;
@@ -266,6 +272,7 @@ HistogramBase* CreatePersistentHistogram(
           name,
           ranges,
           counts_data,
+          logged_data,
           histogram_data.bucket_count,
           &histogram_data_ptr->samples_metadata);
       DCHECK(histogram);
@@ -351,7 +358,10 @@ HistogramBase* AllocatePersistentHistogram(
     NOTREACHED();
     return nullptr;
   }
-  size_t counts_bytes = bucket_count * sizeof(HistogramBase::AtomicCount);
+
+  // Allocate persistent memory for 2x bucket counts (one for "live" and one
+  // for "logged") and memory for bucket ranges.
+  size_t counts_bytes = 2 * bucket_count * sizeof(HistogramBase::AtomicCount);
   size_t ranges_bytes = (bucket_count + 1) * sizeof(HistogramBase::Sample);
   PersistentMemoryAllocator::Reference ranges_ref =
       allocator->Allocate(ranges_bytes, kTypeIdRangesArray);