Support negative sample values in PersistentHistogramMap.

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 cc20f45412bec2160f62dfb7aa9380e6b9a4caca..9857b97aa3a5b5bb3bb314beb535632fda6173f8 100644
--- a/base/metrics/persistent_sample_map.cc
+++ b/base/metrics/persistent_sample_map.cc
@@ -204,8 +204,6 @@ bool PersistentSampleMap::AddSubtractImpl(SampleCountIterator* iter,
 }
 
 Count* PersistentSampleMap::GetSampleCountStorage(Sample value) {
-  DCHECK_LE(0, value);
-
   // If |value| is already in the map, just return that.
   auto it = sample_counts_.find(value);
   if (it != sample_counts_.end())
@@ -248,6 +246,7 @@ Count* PersistentSampleMap::GetOrCreateSampleCountStorage(Sample value) {
 }
 
 Count* PersistentSampleMap::ImportSamples(Sample until_value) {
+  Count* found_count = nullptr;
   PersistentMemoryAllocator::Reference ref;
   while ((ref = records_->GetNext()) != 0) {
     SampleRecord* record =
@@ -259,9 +258,8 @@ Count* PersistentSampleMap::ImportSamples(Sample until_value) {
 
     // Check if the record's value is already known.
     if (!ContainsKey(sample_counts_, record->value)) {
-      // No: Add it to map of known values if the value is valid.
-      if (record->value >= 0)
-        sample_counts_[record->value] = &record->count;
+      // No: Add it to map of known values.
+      sample_counts_[record->value] = &record->count;
     } else {
       // Yes: Ignore it; it's a duplicate caused by a race condition -- see
       // code & comment in GetOrCreateSampleCountStorage() for details.
@@ -269,12 +267,21 @@ Count* PersistentSampleMap::ImportSamples(Sample until_value) {
       DCHECK_EQ(0, record->count);
     }
 
-    // Stop if it's the value being searched for.
-    if (record->value == until_value)
-      return &record->count;
+    // Check if it's the value being searched for. There is a small chance
+    // that some histogram is actually using the "kAllSamples" value. In
+    // that case, keep the pointer for later but continue loading all the
+    // rest of the samples, too. And because race conditions can cause
+    // multiple records for a single value, be sure to only return the
+    // first one found.
+    if (record->value == until_value) {
+      if (!found_count)
+        found_count = &record->count;
+      if (until_value != kAllSamples)
+        break;
+    }
   }
 
-  return nullptr;
+  return found_count;
 }
 
 }  // namespace base