Crash when we notice a corruption of the histogram range-vector...

base/metrics/histogram.cc

 // Copyright (c) 2010 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.
 
 // Histogram is an object that aggregates statistics, and can summarize them in
 // various forms, including ASCII graphical, HTML, and numerically (as a
 // vector of numbers corresponding to each of the aggregating buckets).
 // See header file for details and examples.
 
 #include "base/metrics/histogram.h"
 
 #include <math.h>
 
 #include <algorithm>
 #include <string>
 
 #include "base/logging.h"
 #include "base/pickle.h"
 #include "base/stringprintf.h"
 #include "base/synchronization/lock.h"
 
 namespace base {
 
+// Static table of checksums for all possible 8 bit bytes.
+const uint32 Histogram::kCrcTable[256] = {0x0, 0x77073096L, 0xee0e612cL,
+0x990951baL, 0x76dc419L, 0x706af48fL, 0xe963a535L, 0x9e6495a3L, 0xedb8832L,
+0x79dcb8a4L, 0xe0d5e91eL, 0x97d2d988L, 0x9b64c2bL, 0x7eb17cbdL, 0xe7b82d07L,
+0x90bf1d91L, 0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL, 0x1adad47dL,
+0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L, 0x136c9856L, 0x646ba8c0L, 0xfd62f97aL,
+0x8a65c9ecL, 0x14015c4fL, 0x63066cd9L, 0xfa0f3d63L, 0x8d080df5L, 0x3b6e20c8L,
+0x4c69105eL, 0xd56041e4L, 0xa2677172L, 0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL,
+0xa50ab56bL, 0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L, 0x32d86ce3L,
+0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L, 0x26d930acL, 0x51de003aL, 0xc8d75180L,
+0xbfd06116L, 0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L, 0xb8bda50fL, 0x2802b89eL,
+0x5f058808L, 0xc60cd9b2L, 0xb10be924L, 0x2f6f7c87L, 0x58684c11L, 0xc1611dabL,
+0xb6662d3dL, 0x76dc4190L, 0x1db7106L, 0x98d220bcL, 0xefd5102aL, 0x71b18589L,
+0x6b6b51fL, 0x9fbfe4a5L, 0xe8b8d433L, 0x7807c9a2L, 0xf00f934L, 0x9609a88eL,
+0xe10e9818L, 0x7f6a0dbbL, 0x86d3d2dL, 0x91646c97L, 0xe6635c01L, 0x6b6b51f4L,
+0x1c6c6162L, 0x856530d8L, 0xf262004eL, 0x6c0695edL, 0x1b01a57bL, 0x8208f4c1L,
+0xf50fc457L, 0x65b0d9c6L, 0x12b7e950L, 0x8bbeb8eaL, 0xfcb9887cL, 0x62dd1ddfL,
+0x15da2d49L, 0x8cd37cf3L, 0xfbd44c65L, 0x4db26158L, 0x3ab551ceL, 0xa3bc0074L,
+0xd4bb30e2L, 0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL, 0x4369e96aL,
+0x346ed9fcL, 0xad678846L, 0xda60b8d0L, 0x44042d73L, 0x33031de5L, 0xaa0a4c5fL,
+0xdd0d7cc9L, 0x5005713cL, 0x270241aaL, 0xbe0b1010L, 0xc90c2086L, 0x5768b525L,
+0x206f85b3L, 0xb966d409L, 0xce61e49fL, 0x5edef90eL, 0x29d9c998L, 0xb0d09822L,
+0xc7d7a8b4L, 0x59b33d17L, 0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL, 0xedb88320L,
+0x9abfb3b6L, 0x3b6e20cL, 0x74b1d29aL, 0xead54739L, 0x9dd277afL, 0x4db2615L,
+0x73dc1683L, 0xe3630b12L, 0x94643b84L, 0xd6d6a3eL, 0x7a6a5aa8L, 0xe40ecf0bL,
+0x9309ff9dL, 0xa00ae27L, 0x7d079eb1L, 0xf00f9344L, 0x8708a3d2L, 0x1e01f268L,
+0x6906c2feL, 0xf762575dL, 0x806567cbL, 0x196c3671L, 0x6e6b06e7L, 0xfed41b76L,
+0x89d32be0L, 0x10da7a5aL, 0x67dd4accL, 0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L,
+0x60b08ed5L, 0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L, 0xd1bb67f1L,
+0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL, 0xd80d2bdaL, 0xaf0a1b4cL, 0x36034af6L,
+0x41047a60L, 0xdf60efc3L, 0xa867df55L, 0x316e8eefL, 0x4669be79L, 0xcb61b38cL,
+0xbc66831aL, 0x256fd2a0L, 0x5268e236L, 0xcc0c7795L, 0xbb0b4703L, 0x220216b9L,
+0x5505262fL, 0xc5ba3bbeL, 0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L, 0xc2d7ffa7L,
+0xb5d0cf31L, 0x2cd99e8bL, 0x5bdeae1dL, 0x9b64c2b0L, 0xec63f226L, 0x756aa39cL,
+0x26d930aL, 0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x5005713L, 0x95bf4a82L,
+0xe2b87a14L, 0x7bb12baeL, 0xcb61b38L, 0x92d28e9bL, 0xe5d5be0dL, 0x7cdcefb7L,
+0xbdbdf21L, 0x86d3d2d4L, 0xf1d4e242L, 0x68ddb3f8L, 0x1fda836eL, 0x81be16cdL,
+0xf6b9265bL, 0x6fb077e1L, 0x18b74777L, 0x88085ae6L, 0xff0f6a70L, 0x66063bcaL,
+0x11010b5cL, 0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L, 0xa00ae278L,
+0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L, 0xa7672661L, 0xd06016f7L, 0x4969474dL,
+0x3e6e77dbL, 0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L, 0x37d83bf0L, 0xa9bcae53L,
+0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L, 0xbdbdf21cL, 0xcabac28aL, 0x53b39330L,
+0x24b4a3a6L, 0xbad03605L, 0xcdd70693L, 0x54de5729L, 0x23d967bfL, 0xb3667a2eL,
+0xc4614ab8L, 0x5d681b02L, 0x2a6f2b94L, 0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL,
+0x2d02ef8dL,
+};
+
 typedef Histogram::Count Count;
 
 // static
 const size_t Histogram::kBucketCount_MAX = 10000u;
 
 scoped_refptr<Histogram> Histogram::FactoryGet(const std::string& name,
     Sample minimum, Sample maximum, size_t bucket_count, Flags flags) {
   scoped_refptr<Histogram> histogram(NULL);
 
   // Defensive code.
   if (minimum < 1)
     minimum = 1;
   if (maximum > kSampleType_MAX - 1)
     maximum = kSampleType_MAX - 1;
 
   if (!StatisticsRecorder::FindHistogram(name, &histogram)) {
     histogram = new Histogram(name, minimum, maximum, bucket_count);
-    StatisticsRecorder::FindHistogram(name, &histogram);
+    histogram->InitializeBucketRange();
+    StatisticsRecorder::Register(&histogram);
   }
 
   DCHECK_EQ(HISTOGRAM, histogram->histogram_type());
   DCHECK(histogram->HasConstructorArguments(minimum, maximum, bucket_count));
   histogram->SetFlags(flags);
   return histogram;
 }
 
 scoped_refptr<Histogram> Histogram::FactoryTimeGet(const std::string& name,
                                                    TimeDelta minimum,
@@ skipping 102 matching lines @@
 // static
 std::string Histogram::SerializeHistogramInfo(const Histogram& histogram,
                                               const SampleSet& snapshot) {
   DCHECK_NE(NOT_VALID_IN_RENDERER, histogram.histogram_type());
 
   Pickle pickle;
   pickle.WriteString(histogram.histogram_name());
   pickle.WriteInt(histogram.declared_min());
   pickle.WriteInt(histogram.declared_max());
   pickle.WriteSize(histogram.bucket_count());
-  pickle.WriteInt(histogram.range_checksum());
+  pickle.WriteUInt32(histogram.range_checksum());
   pickle.WriteInt(histogram.histogram_type());
   pickle.WriteInt(histogram.flags());
 
   snapshot.Serialize(&pickle);
   return std::string(static_cast<const char*>(pickle.data()), pickle.size());
 }
 
 // static
 bool Histogram::DeserializeHistogramInfo(const std::string& histogram_info) {
   if (histogram_info.empty()) {
       return false;
   }
 
   Pickle pickle(histogram_info.data(),
                 static_cast<int>(histogram_info.size()));
   std::string histogram_name;
   int declared_min;
   int declared_max;
   size_t bucket_count;
-  int range_checksum;
+  uint32 range_checksum;
   int histogram_type;
   int pickle_flags;
   SampleSet sample;
 
   void* iter = NULL;
   if (!pickle.ReadString(&iter, &histogram_name) ||
       !pickle.ReadInt(&iter, &declared_min) ||
       !pickle.ReadInt(&iter, &declared_max) ||
       !pickle.ReadSize(&iter, &bucket_count) ||
-      !pickle.ReadInt(&iter, &range_checksum) ||
+      !pickle.ReadUInt32(&iter, &range_checksum) ||
       !pickle.ReadInt(&iter, &histogram_type) ||
       !pickle.ReadInt(&iter, &pickle_flags) ||
       !sample.Histogram::SampleSet::Deserialize(&iter, pickle)) {
     LOG(ERROR) << "Pickle error decoding Histogram: " << histogram_name;
     return false;
   }
   DCHECK(pickle_flags & kIPCSerializationSourceFlag);
   // Since these fields may have come from an untrusted renderer, do additional
   // checks above and beyond those in Histogram::Initialize()
   if (declared_max <= 0 || declared_min <= 0 || declared_max < declared_min ||
@@ skipping 40 matching lines @@
 }
 
 //------------------------------------------------------------------------------
 // Methods for the validating a sample and a related histogram.
 //------------------------------------------------------------------------------
 
 Histogram::Inconsistencies Histogram::FindCorruption(
     const SampleSet& snapshot) const {
   int inconsistencies = NO_INCONSISTENCIES;
   Sample previous_range = -1;  // Bottom range is always 0.
-  Sample checksum = 0;
   int64 count = 0;
   for (size_t index = 0; index < bucket_count(); ++index) {
     count += snapshot.counts(index);
     int new_range = ranges(index);
-    checksum += new_range;
     if (previous_range >= new_range)
       inconsistencies |= BUCKET_ORDER_ERROR;
     previous_range = new_range;
   }
 
-  if (checksum != range_checksum_)
+  if (!HasValidRangeChecksum())
     inconsistencies |= RANGE_CHECKSUM_ERROR;
 
   int64 delta64 = snapshot.redundant_count() - count;
   if (delta64 != 0) {
     int delta = static_cast<int>(delta64);
     if (delta != delta64)
       delta = INT_MAX;  // Flag all giant errors as INT_MAX.
     // Since snapshots of histograms are taken asynchronously relative to
     // sampling (and snapped from different threads), it is pretty likely that
     // we'll catch a redundant count that doesn't match the sample count.  We
@@ skipping 44 matching lines @@
 }
 
 bool Histogram::HasConstructorTimeDeltaArguments(TimeDelta minimum,
                                                  TimeDelta maximum,
                                                  size_t bucket_count) {
   return ((minimum.InMilliseconds() == declared_min_) &&
           (maximum.InMilliseconds() == declared_max_) &&
           (bucket_count == bucket_count_));
 }
 
+bool Histogram::HasValidRangeChecksum() const {
+  return CalculateRangeChecksum() == range_checksum_;
+}
+
 Histogram::Histogram(const std::string& name, Sample minimum,
                      Sample maximum, size_t bucket_count)
   : histogram_name_(name),
     declared_min_(minimum),
     declared_max_(maximum),
     bucket_count_(bucket_count),
     flags_(kNoFlags),
     ranges_(bucket_count + 1, 0),
     range_checksum_(0),
     sample_() {
@@ skipping 15 matching lines @@
 
 Histogram::~Histogram() {
   if (StatisticsRecorder::dump_on_exit()) {
     std::string output;
     WriteAscii(true, "\n", &output);
     LOG(INFO) << output;
   }
 
   // Just to make sure most derived class did this properly...
   DCHECK(ValidateBucketRanges());
-  DCHECK(HasValidRangeChecksum());
 }
 
 bool Histogram::PrintEmptyBucket(size_t index) const {
   return true;
 }
 
 // Calculate what range of values are held in each bucket.
 // We have to be careful that we don't pick a ratio between starting points in
 // consecutive buckets that is sooo small, that the integer bounds are the same
 // (effectively making one bucket get no values).  We need to avoid:
@@ skipping 94 matching lines @@
 bool Histogram::ValidateBucketRanges() const {
   // Standard assertions that all bucket ranges should satisfy.
   DCHECK_EQ(bucket_count_ + 1, ranges_.size());
   DCHECK_EQ(0, ranges_[0]);
   DCHECK_EQ(declared_min(), ranges_[1]);
   DCHECK_EQ(declared_max(), ranges_[bucket_count_ - 1]);
   DCHECK_EQ(kSampleType_MAX, ranges_[bucket_count_]);
   return true;
 }
 
+uint32 Histogram::CalculateRangeChecksum() const {
+  DCHECK_EQ(ranges_.size(), bucket_count() + 1);
+  uint32 checksum = static_cast<uint32>(ranges_.size());  // Seed checksum.
+  for (size_t index = 0; index < bucket_count(); ++index)
+    checksum = Crc32(checksum, ranges(index));
+  return checksum;
+}
+
 void Histogram::Initialize() {
   sample_.Resize(*this);
   if (declared_min_ < 1)
     declared_min_ = 1;
   if (declared_max_ > kSampleType_MAX - 1)
     declared_max_ = kSampleType_MAX - 1;
   DCHECK_LE(declared_min_, declared_max_);
   DCHECK_GT(bucket_count_, 1u);
   CHECK_LT(bucket_count_, kBucketCount_MAX);
   size_t maximal_bucket_count = declared_max_ - declared_min_ + 2;
   DCHECK_LE(bucket_count_, maximal_bucket_count);
   DCHECK_EQ(0, ranges_[0]);
   ranges_[bucket_count_] = kSampleType_MAX;
-  InitializeBucketRange();
-  DCHECK(ValidateBucketRanges());
-  StatisticsRecorder::Register(this);
 }
 
-bool Histogram::HasValidRangeChecksum() const {
-  return CalculateRangeChecksum() == range_checksum_;
-}
-
-Histogram::Sample Histogram::CalculateRangeChecksum() const {
-  DCHECK_EQ(ranges_.size(), bucket_count() + 1);
-  Sample checksum = 0;
-  for (size_t index = 0; index < bucket_count(); ++index) {
-    checksum += ranges(index);
+// We generate the CRC-32 using the low order bits to select whether to XOR in
+// the reversed polynomial 0xedb88320L.  This is nice and simple, and allows us
+// to keep the quotient in a uint32.  Since we're not concerned about the nature
+// of corruptions (i.e., we don't care about bit sequencing, since we are
+// handling memory changes, which are more grotesque) so we don't bother to
+// get the CRC correct for big-endian vs little-ending calculations.  All we
+// need is a nice hash, that tends to depend on all the bits of the sample, with
+// very little chance of changes in one place impacting changes in another
+// place.
+uint32 Histogram::Crc32(uint32 sum, Histogram::Sample range) {
+  const bool kUseRealCrc = true;  // TODO(jar): Switch to false and watch stats.
+    if (kUseRealCrc) {

[Mike Belshe, 2011/02/28 17:19:13]

nit: funky space

+    union {
+      Histogram::Sample range;
+      unsigned char bytes[sizeof(Histogram::Sample)];
+    } converter;
+    converter.range = range;
+    for (size_t i = 0; i < sizeof(converter); ++i)
+      sum = kCrcTable[(sum & 0xff) ^ converter.bytes[i]] ^ (sum >> 8);
+  } else {
+    // Use hash techniques provided in ReallyFastHash, except we don't care
+    // about "avalanching" (which would worsten the hash, and add collisions),
+    // and we don't care about edge cases since we have an even number of bytes.
+    union {
+      Histogram::Sample range;
+      uint16 ints[sizeof(Histogram::Sample) / 2];
+    } converter;
+    DCHECK_EQ(sizeof(Histogram::Sample), sizeof(converter));
+    converter.range = range;
+    sum += converter.ints[0];
+    sum = (sum << 16) ^ sum ^ (static_cast<uint32>(converter.ints[1]) << 11);
+    sum += sum >> 11;
   }
-  return checksum;
+  return sum;
 }
 
 //------------------------------------------------------------------------------
 // Private methods
 
 double Histogram::GetPeakBucketSize(const SampleSet& snapshot) const {
   double max = 0;
   for (size_t i = 0; i < bucket_count() ; ++i) {
     double current_size = GetBucketSize(snapshot.counts(i), i);
     if (current_size > max)
@@ skipping 183 matching lines @@
                                                      size_t bucket_count,
                                                      Flags flags) {
   scoped_refptr<Histogram> histogram(NULL);
 
   if (minimum < 1)
     minimum = 1;
   if (maximum > kSampleType_MAX - 1)
     maximum = kSampleType_MAX - 1;
 
   if (!StatisticsRecorder::FindHistogram(name, &histogram)) {
-    histogram = new LinearHistogram(name, minimum, maximum, bucket_count);
-    StatisticsRecorder::FindHistogram(name, &histogram);
+    LinearHistogram* linear_histogram =
+        new LinearHistogram(name, minimum, maximum, bucket_count);
+    linear_histogram->InitializeBucketRange();
+    histogram = linear_histogram;
+    StatisticsRecorder::Register(&histogram);
   }
 
   DCHECK_EQ(LINEAR_HISTOGRAM, histogram->histogram_type());
   DCHECK(histogram->HasConstructorArguments(minimum, maximum, bucket_count));
   histogram->SetFlags(flags);
   return histogram;
 }
 
 scoped_refptr<Histogram> LinearHistogram::FactoryTimeGet(
     const std::string& name,
@@ skipping 14 matching lines @@
   for (int i =0; descriptions[i].description; ++i) {
     bucket_description_[descriptions[i].sample] = descriptions[i].description;
   }
 }
 
 LinearHistogram::LinearHistogram(const std::string& name,
                                  Sample minimum,
                                  Sample maximum,
                                  size_t bucket_count)
     : Histogram(name, minimum >= 1 ? minimum : 1, maximum, bucket_count) {
-  InitializeBucketRange();
-  DCHECK(ValidateBucketRanges());
 }
 
 LinearHistogram::LinearHistogram(const std::string& name,
                                  TimeDelta minimum,
                                  TimeDelta maximum,
                                  size_t bucket_count)
     : Histogram(name, minimum >= TimeDelta::FromMilliseconds(1) ?
                                  minimum : TimeDelta::FromMilliseconds(1),
                 maximum, bucket_count) {
-  // Do a "better" (different) job at init than a base classes did...
-  InitializeBucketRange();
-  DCHECK(ValidateBucketRanges());
 }
 
 void LinearHistogram::InitializeBucketRange() {
   DCHECK_GT(declared_min(), 0);  // 0 is the underflow bucket here.
   double min = declared_min();
   double max = declared_max();
   size_t i;
   for (i = 1; i < bucket_count(); ++i) {
     double linear_range = (min * (bucket_count() -1 - i) + max * (i - 1)) /
                           (bucket_count() - 2);
@@ skipping 25 matching lines @@
 
 //------------------------------------------------------------------------------
 // This section provides implementation for BooleanHistogram.
 //------------------------------------------------------------------------------
 
 scoped_refptr<Histogram> BooleanHistogram::FactoryGet(const std::string& name,
                                                       Flags flags) {
   scoped_refptr<Histogram> histogram(NULL);
 
   if (!StatisticsRecorder::FindHistogram(name, &histogram)) {
-    histogram = new BooleanHistogram(name);
-    StatisticsRecorder::FindHistogram(name, &histogram);
+    BooleanHistogram* boolean_histogram = new BooleanHistogram(name);
+    boolean_histogram->InitializeBucketRange();
+    histogram = boolean_histogram;
+    StatisticsRecorder::Register(&histogram);
   }
 
   DCHECK_EQ(BOOLEAN_HISTOGRAM, histogram->histogram_type());
   histogram->SetFlags(flags);
   return histogram;
 }
 
 Histogram::ClassType BooleanHistogram::histogram_type() const {
   return BOOLEAN_HISTOGRAM;
 }
@@ skipping 23 matching lines @@
   ranges.erase(std::unique(ranges.begin(), ranges.end()), ranges.end());
   if (ranges.size() <= 1) {
     DCHECK(false);
     // Note that we pushed a 0 in above, so for defensive code....
     ranges.push_back(1);  // Put in some data so we can index to [1].
   }
 
   DCHECK_LT(ranges.back(), kSampleType_MAX);
 
   if (!StatisticsRecorder::FindHistogram(name, &histogram)) {
-    histogram = new CustomHistogram(name, ranges);
-    StatisticsRecorder::FindHistogram(name, &histogram);
+    CustomHistogram* custom_histogram = new CustomHistogram(name, ranges);
+    custom_histogram->InitializeBucketRange(ranges);
+    histogram = custom_histogram;
+    StatisticsRecorder::Register(&histogram);
   }
 
   DCHECK_EQ(histogram->histogram_type(), CUSTOM_HISTOGRAM);
   DCHECK(histogram->HasConstructorArguments(ranges[1], ranges.back(),
                                             ranges.size()));
   histogram->SetFlags(flags);
   return histogram;
 }
 
 Histogram::ClassType CustomHistogram::histogram_type() const {
   return CUSTOM_HISTOGRAM;
 }
 
 CustomHistogram::CustomHistogram(const std::string& name,
                                  const std::vector<Sample>& custom_ranges)
     : Histogram(name, custom_ranges[1], custom_ranges.back(),
                 custom_ranges.size()) {
   DCHECK_GT(custom_ranges.size(), 1u);
   DCHECK_EQ(custom_ranges[0], 0);
-  ranges_vector_ = &custom_ranges;
-  InitializeBucketRange();
-  ranges_vector_ = NULL;
-  DCHECK(ValidateBucketRanges());
 }
 
-void CustomHistogram::InitializeBucketRange() {
-  DCHECK_LE(ranges_vector_->size(), bucket_count());
-  for (size_t index = 0; index < ranges_vector_->size(); ++index)
-    SetBucketRange(index, (*ranges_vector_)[index]);
+void CustomHistogram::InitializeBucketRange(
+    const std::vector<Sample>& custom_ranges) {
+  DCHECK_GT(custom_ranges.size(), 1u);
+  DCHECK_EQ(custom_ranges[0], 0);
+  DCHECK_LE(custom_ranges.size(), bucket_count());
+  for (size_t index = 0; index < custom_ranges.size(); ++index)
+    SetBucketRange(index, custom_ranges[index]);
   ResetRangeChecksum();
 }
 
 double CustomHistogram::GetBucketSize(Count current, size_t i) const {
   return 1;
 }
 
 //------------------------------------------------------------------------------
 // The next section handles global (central) support for all histograms, as well
 // as startup/teardown of this service.
@@ skipping 44 matching lines @@
   base::AutoLock auto_lock(*lock_);
   return NULL != histograms_;
 }
 
 // Note: We can't accept a ref_ptr to |histogram| because we *might* not keep a
 // reference, and we are called while in the Histogram constructor. In that
 // scenario, a ref_ptr would have incremented the ref count when the histogram
 // was passed to us, decremented it when we returned, and the instance would be
 // destroyed before assignment (when value was returned by new).
 // static
-void StatisticsRecorder::Register(Histogram* histogram) {
+void StatisticsRecorder::Register(scoped_refptr<Histogram>* histogram) {
+  DCHECK((*histogram)->HasValidRangeChecksum());
   if (lock_ == NULL)
     return;
   base::AutoLock auto_lock(*lock_);
   if (!histograms_)
     return;
-  const std::string name = histogram->histogram_name();
+  const std::string name = (*histogram)->histogram_name();
+  HistogramMap::iterator it = histograms_->find(name);
   // Avoid overwriting a previous registration.
-  if (histograms_->end() == histograms_->find(name))
-    (*histograms_)[name] = histogram;
+  if (histograms_->end() == it)
+    (*histograms_)[name] = *histogram;
+  else
+    *histogram = it->second;
 }
 
 // static
 void StatisticsRecorder::WriteHTMLGraph(const std::string& query,
                                         std::string* output) {
   if (!IsActive())
     return;
   output->append("<html><head><title>About Histograms");
   if (!query.empty())
     output->append(" - " + query);
@@ skipping 79 matching lines @@
 }
 
 // static
 StatisticsRecorder::HistogramMap* StatisticsRecorder::histograms_ = NULL;
 // static
 base::Lock* StatisticsRecorder::lock_ = NULL;
 // static
 bool StatisticsRecorder::dump_on_exit_ = false;
 
 }  // namespace base