Disallow non-sparse histograms with too many buckets.

base/metrics/histogram.cc

 // Copyright (c) 2012 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"
@@ skipping 382 matching lines @@
     std::swap(*minimum, *maximum);
   }
   if (*maximum == *minimum) {
     check_okay = false;
     *maximum = *minimum + 1;
   }
   if (*bucket_count < 3) {
     check_okay = false;
     *bucket_count = 3;
   }
+  // Very high bucket counts are wasteful. Use a sparse histogram instead.
+  // Value of 10002 equals a user-supplied value of 10k + 2 overflow buckets.
+  constexpr uint32_t kMaxBucketCount = 10002;

[bcwhite, 2017/05/25 19:21:41]

Even 10k sounds really, really large.

[Alexei Svitkine (slow), 2017/05/25 19:24:04]

Yeah agreed. There was a unit test that was using 10k so I picked a limit that
doesn't require other changes for now. We can reduce it later - in code reviews
I usually push back on anything 100 in size or more when I see it. :)

+  if (*bucket_count > kMaxBucketCount) {
+    check_okay = false;
+    *bucket_count = kMaxBucketCount;
+  }
   if (*bucket_count > static_cast<uint32_t>(*maximum - *minimum + 2)) {
     check_okay = false;
     *bucket_count = static_cast<uint32_t>(*maximum - *minimum + 2);
   }
 
   if (!check_okay) {
     UMA_HISTOGRAM_SPARSE_SLOWLY("Histogram.BadConstructionArguments",
                                 static_cast<Sample>(HashMetricName(name)));
   }
 
@@ skipping 797 matching lines @@
     Sample sample = custom_ranges[i];
     if (sample < 0 || sample > HistogramBase::kSampleType_MAX - 1)
       return false;
     if (sample != 0)
       has_valid_range = true;
   }
   return has_valid_range;
 }
 
 }  // namespace base