Fix overflow when logging MaxInt32 to a sparse histogram.

base/metrics/histogram_samples.h

Index: base/metrics/histogram_samples.h
diff --git a/base/metrics/histogram_samples.h b/base/metrics/histogram_samples.h
index 0e24c4f160e7a2a29f83eefc554efbf2dffc80f4..9a4829ae84a594f54f78ca2054b571ca7bfd46da 100644
--- a/base/metrics/histogram_samples.h
+++ b/base/metrics/histogram_samples.h
@@ -198,9 +198,12 @@ class BASE_EXPORT SampleCountIterator {
 
   // Get the sample and count at current position.
   // |min| |max| and |count| can be NULL if the value is not of interest.
+  // Note: |max| is int64_t because histograms support logged values in the
+  // full int32_t range and bucket max is exclusive, so it needs to support
+  // values up to MAXINT32+1.

[Ilya Sherman, 2017/05/02 21:37:49]

Could you add a compile-time assertion somewhere that the max-value for the
recorded-to bucket is less than the max-possible value for the |max| param? 
We've been talking about possibly extending histograms to report 64-bit values,
and I'm a bit worried that we'd accidentally regress this were anyone to
undertake implementing that transition.

[Alexei Svitkine (slow), 2017/05/03 15:21:53]

Done.

   // Requires: !Done();
   virtual void Get(HistogramBase::Sample* min,
-                   HistogramBase::Sample* max,
+                   int64_t* max,
                    HistogramBase::Count* count) const = 0;
 
   // Get the index of current histogram bucket.
@@ -212,10 +215,10 @@ class BASE_EXPORT SampleCountIterator {
 class BASE_EXPORT SingleSampleIterator : public SampleCountIterator {
  public:
   SingleSampleIterator(HistogramBase::Sample min,
-                       HistogramBase::Sample max,
+                       int64_t max,
                        HistogramBase::Count count);
   SingleSampleIterator(HistogramBase::Sample min,
-                       HistogramBase::Sample max,
+                       int64_t max,
                        HistogramBase::Count count,
                        size_t bucket_index);
   ~SingleSampleIterator() override;
@@ -224,7 +227,7 @@ class BASE_EXPORT SingleSampleIterator : public SampleCountIterator {
   bool Done() const override;
   void Next() override;
   void Get(HistogramBase::Sample* min,
-           HistogramBase::Sample* max,
+           int64_t* max,
            HistogramBase::Count* count) const override;
 
   // SampleVector uses predefined buckets so iterator can return bucket index.
@@ -233,7 +236,7 @@ class BASE_EXPORT SingleSampleIterator : public SampleCountIterator {
  private:
   // Information about the single value to return.
   const HistogramBase::Sample min_;
-  const HistogramBase::Sample max_;
+  const int64_t max_;
   const size_t bucket_index_;
   HistogramBase::Count count_;
 };