Report reasons for a negative histogram sample count.

base/metrics/persistent_sample_map.cc

Index: base/metrics/persistent_sample_map.cc
diff --git a/base/metrics/persistent_sample_map.cc b/base/metrics/persistent_sample_map.cc
index 1cc425423c787dc4575ceaeed0640c8337ac59d0..131e47b68a0d33eee92a44c99bc12c2c67594514 100644
--- a/base/metrics/persistent_sample_map.cc
+++ b/base/metrics/persistent_sample_map.cc
@@ -6,6 +6,7 @@
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/metrics/persistent_histogram_allocator.h"
 #include "base/stl_util.h"
 
@@ -16,6 +17,12 @@ typedef HistogramBase::Sample Sample;
 
 namespace {
 
+enum NegativeSampleReason {
+  PERSISTENT_SPARSE_HAVE_LOGGED_BUT_NOT_SAMPLE,
+  PERSISTENT_SPARSE_SAMPLE_LESS_THAN_LOGGED,
+  MAX_NEGATIVE_SAMPLE_REASONS
+};
+
 // An iterator for going through a PersistentSampleMap. The logic here is
 // identical to that of SampleMapIterator but with different data structures.
 // Changes here likely need to be duplicated there.
@@ -181,11 +188,39 @@ bool PersistentSampleMap::AddSubtractImpl(SampleCountIterator* iter,
   Count count;
   for (; !iter->Done(); iter->Next()) {
     iter->Get(&min, &max, &count);
+    if (count == 0)
+      continue;
     if (min + 1 != max)
       return false;  // SparseHistogram only supports bucket with size 1.
 
+#if 0  // TODO(bcwhite) Re-enable efficient version after crbug.com/682680.
     *GetOrCreateSampleCountStorage(min) +=
         (op == HistogramSamples::ADD) ? count : -count;
+#else
+    if (op == HistogramSamples::ADD) {
+      *GetOrCreateSampleCountStorage(min) += count;
+    } else {
+      // Subtract is used only for determining deltas when reporting which
+      // means that it's in the "logged" iterator. It should have an active
+      // sample record and thus there is no need to try to create one.
+      NegativeSampleReason reason = MAX_NEGATIVE_SAMPLE_REASONS;
+      Count* bucket = GetSampleCountStorage(min);
+      if (bucket == nullptr) {
+        reason = PERSISTENT_SPARSE_HAVE_LOGGED_BUT_NOT_SAMPLE;
+      } else {
+        if (*bucket < count) {
+          reason = PERSISTENT_SPARSE_SAMPLE_LESS_THAN_LOGGED;
+          *bucket = 0;
+        } else {
+          *bucket -= count;
+        }
+      }
+      if (reason != MAX_NEGATIVE_SAMPLE_REASONS) {
+        UMA_HISTOGRAM_ENUMERATION("UMA.NegativeSamples.Reason", reason,
+                                  MAX_NEGATIVE_SAMPLE_REASONS);
+      }
+    }
+#endif
   }
   return true;
 }