Monitor Add for negative results.

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 51cc0c709d8b0d9087c75d64c512b06e434f6ba7..f931fd579c3ef71edebc1f8a81fc133f35afd7d2 100644
--- a/base/metrics/persistent_sample_map.cc
+++ b/base/metrics/persistent_sample_map.cc
@@ -20,6 +20,9 @@ namespace {
 enum NegativeSampleReason {
   PERSISTENT_SPARSE_HAVE_LOGGED_BUT_NOT_SAMPLE,
   PERSISTENT_SPARSE_SAMPLE_LESS_THAN_LOGGED,
+  PERSISTENT_SPARSE_ADDED_NEGATIVE_COUNT,
+  PERSISTENT_SPARSE_ADD_WENT_NEGATIVE,
+  PERSISTENT_SPARSE_ADD_OVERFLOW,
   MAX_NEGATIVE_SAMPLE_REASONS
 };
 
@@ -197,28 +200,41 @@ bool PersistentSampleMap::AddSubtractImpl(SampleCountIterator* iter,
     *GetOrCreateSampleCountStorage(min) +=
         (op == HistogramSamples::ADD) ? count : -count;
 #else
+    NegativeSampleReason reason = MAX_NEGATIVE_SAMPLE_REASONS;
     if (op == HistogramSamples::ADD) {
-      *GetOrCreateSampleCountStorage(min) += count;
+      // Add should generally be adding only positive values.
+      Count* local_count_ptr = GetOrCreateSampleCountStorage(min);
+      if (count < 0) {
+        reason = PERSISTENT_SPARSE_ADDED_NEGATIVE_COUNT;
+        if (*local_count_ptr < -count) {
+          reason = PERSISTENT_SPARSE_ADD_WENT_NEGATIVE;
+          *local_count_ptr = 0;
+        }
+      } else {
+        *local_count_ptr += count;
+        if (*local_count_ptr < 0)
+          reason = PERSISTENT_SPARSE_ADD_OVERFLOW;
+      }
     } 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) {
+      Count* local_count_ptr = GetSampleCountStorage(min);
+      if (local_count_ptr == nullptr) {
         reason = PERSISTENT_SPARSE_HAVE_LOGGED_BUT_NOT_SAMPLE;
       } else {
-        if (*bucket < count) {
+        if (*local_count_ptr < count) {
           reason = PERSISTENT_SPARSE_SAMPLE_LESS_THAN_LOGGED;
-          *bucket = 0;
+          *local_count_ptr = 0;
         } else {
-          *bucket -= count;
+          *local_count_ptr -= count;
         }
       }
-      if (reason != MAX_NEGATIVE_SAMPLE_REASONS) {
-        UMA_HISTOGRAM_ENUMERATION("UMA.NegativeSamples.Reason", reason,
-                                  MAX_NEGATIVE_SAMPLE_REASONS);
-      }
+    }
+    if (reason != MAX_NEGATIVE_SAMPLE_REASONS) {
+      NOTREACHED();
+      UMA_HISTOGRAM_ENUMERATION("UMA.NegativeSamples.Reason", reason,
+                                MAX_NEGATIVE_SAMPLE_REASONS);
     }
 #endif
   }