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..f61e53c164fc9961b277cb495b230abda5fb76fd 100644
--- a/base/metrics/persistent_sample_map.cc
+++ b/base/metrics/persistent_sample_map.cc
@@ -131,7 +131,7 @@ Count PersistentSampleMap::GetCount(Sample value) const {
 Count PersistentSampleMap::TotalCount() const {
   // Have to override "const" in order to make sure all samples have been
   // loaded before trying to iterate over the map.
-  const_cast<PersistentSampleMap*>(this)->ImportSamples(kAllSamples);
+  const_cast<PersistentSampleMap*>(this)->ImportSamples(-1, true);
 
   Count count = 0;
   for (const auto& entry : sample_counts_) {
@@ -143,7 +143,7 @@ Count PersistentSampleMap::TotalCount() const {
 std::unique_ptr<SampleCountIterator> PersistentSampleMap::Iterator() const {
   // Have to override "const" in order to make sure all samples have been
   // loaded before trying to iterate over the map.
-  const_cast<PersistentSampleMap*>(this)->ImportSamples(kAllSamples);
+  const_cast<PersistentSampleMap*>(this)->ImportSamples(-1, true);
   return WrapUnique(new PersistentSampleMapIterator(sample_counts_));
 }
 
@@ -204,15 +204,13 @@ 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())
     return it->second;
 
   // Import any new samples from persistent memory looking for the value.
-  return ImportSamples(value);
+  return ImportSamples(value, false);
 }
 
 Count* PersistentSampleMap::GetOrCreateSampleCountStorage(Sample value) {
@@ -242,12 +240,14 @@ Count* PersistentSampleMap::GetOrCreateSampleCountStorage(Sample value) {
   // Thread-safety within a process where multiple threads use the same
   // histogram object is delegated to the controlling histogram object which,
   // for sparse histograms, is a lock object.
-  count_pointer = ImportSamples(value);
+  count_pointer = ImportSamples(value, false);
   DCHECK(count_pointer);
   return count_pointer;
 }
 
-Count* PersistentSampleMap::ImportSamples(Sample until_value) {
+Count* PersistentSampleMap::ImportSamples(Sample until_value,
+                                          bool import_everything) {
+  Count* found_count = nullptr;
   PersistentMemoryAllocator::Reference ref;
   while ((ref = records_->GetNext()) != 0) {
     SampleRecord* record =
@@ -259,9 +259,8 @@ 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 +268,19 @@ 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 and, if so, keep a pointer
+    // to return later. Stop here unless everything is being imported.
+    // Because race conditions can cause multiple records for a single value,
+    // be sure to return the first one found.
+    if (record->value == until_value) {
+      if (!found_count)
+        found_count = &record->count;
+      if (!import_everything)
+        break;
+    }
   }
 
-  return nullptr;
+  return found_count;
 }
 
 }  // namespace base