components/metrics: Add runtime memory leak detector

components/metrics/leak_detector/leak_analyzer.cc

Index: components/metrics/leak_detector/leak_analyzer.cc
diff --git a/components/metrics/leak_detector/leak_analyzer.cc b/components/metrics/leak_detector/leak_analyzer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..4b2686ab69eb293e2545385682e30191cb4f5457
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+++ b/components/metrics/leak_detector/leak_analyzer.cc
@@ -0,0 +1,130 @@
+// Copyright 2015 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.
+
+#include "components/metrics/leak_detector/leak_analyzer.h"
+
+#include <set>
+
+namespace metrics {
+namespace leak_detector {
+
+namespace {
+
+using RankedEntry = RankedList::Entry;
+
+// Increase suspicion scores by this much each time an entry is suspected as
+// being a leak.
+const int kSuspicionScoreIncrease = 1;
+
+}  // namespace
+
+void LeakAnalyzer::AddSample(RankedList ranked_list) {

[Alexei Svitkine (slow), 2015/11/13 20:51:27]

Why is this taking RankedList by value? Either it should be a const ref or a
pointer.

[Simon Que, 2015/11/13 23:44:57]

This is meant to use move semantics, to avoid unnecessary copying when assigning
to |ranked_entries_|. I've updated the comment in the header file to reflect
this usage.

+  // Save the ranked entries from the previous call.
+  prev_ranked_entries_ = ranked_entries_.Pass();
+
+  // Save the current entries.
+  ranked_entries_ = ranked_list.Pass();
+
+  RankedList ranked_deltas(ranking_size_);
+  for (const RankedEntry& entry : ranked_entries_) {
+    // Determine what count was recorded for this value last time.
+    uint32_t prev_count = 0;
+    if (GetPreviousCountForValue(entry.value, &prev_count))
+      ranked_deltas.Add(entry.value, entry.count - prev_count);
+  }
+
+  AnalyzeDeltas(ranked_deltas);
+}
+
+void LeakAnalyzer::AnalyzeDeltas(const RankedList& ranked_deltas) {
+  bool found_drop = false;
+  RankedList::const_iterator drop_position = ranked_deltas.end();
+
+  if (ranked_deltas.size() > 1) {

[Alexei Svitkine (slow), 2015/11/13 20:51:27]

Nit: !.empty()

[Simon Que, 2015/11/13 23:44:57]

This is checking that there is more than 1 element.

+    RankedList::const_iterator entry_iter = ranked_deltas.begin();
+    RankedList::const_iterator next_entry_iter = ranked_deltas.begin();
+    ++next_entry_iter;
+
+    // If the first entry is 0, that means all deltas are 0 or negative. Do
+    // not treat this as a suspicion of leaks; just quit.
+    if (entry_iter->count > 0) {
+      while (next_entry_iter != ranked_deltas.end()) {
+        const RankedEntry& entry = *entry_iter;
+        const RankedEntry& next_entry = *next_entry_iter;
+
+        // Find the first major drop in values (i.e. by 50% or more).
+        if (entry.count > next_entry.count * 2) {
+          found_drop = true;
+          drop_position = next_entry_iter;
+          break;
+        }
+        ++entry_iter;
+        ++next_entry_iter;
+      }
+    }
+  }
+
+  // All leak values before the drop are suspected during this analysis.
+  std::set<ValueType,
+           std::less<ValueType>,
+           Allocator<ValueType>> current_suspects;
+  if (found_drop) {
+    for (RankedList::const_iterator ranked_list_iter = ranked_deltas.begin();
+         ranked_list_iter != drop_position;
+         ++ranked_list_iter) {
+      current_suspects.insert(ranked_list_iter->value);
+    }
+  }
+
+  // Reset the score to 0 for all previously suspected leak values that did
+  // not get suspected this time.
+  auto iter = suspected_histogram_.begin();
+  while (iter != suspected_histogram_.end()) {
+    const ValueType& value = iter->first;
+    // Erase entries whose suspicion score reaches 0.
+    auto erase_iter = iter++;
+    if (current_suspects.find(value) == current_suspects.end())
+      suspected_histogram_.erase(erase_iter);
+  }
+
+  // For currently suspected values, increase the leak score.
+  for (const ValueType& value : current_suspects) {
+    auto histogram_iter = suspected_histogram_.find(value);
+    if (histogram_iter != suspected_histogram_.end()) {
+      histogram_iter->second += kSuspicionScoreIncrease;
+    } else if (suspected_histogram_.size() < ranking_size_) {
+      // Create a new entry if it didn't already exist.
+      suspected_histogram_[value] = kSuspicionScoreIncrease;
+    }
+  }
+
+  // Now check the leak suspicion scores. Make sure to erase the suspected
+  // leaks from the previous call.
+  suspected_leaks_.clear();
+  for (const auto& entry : suspected_histogram_) {
+    if (suspected_leaks_.size() > ranking_size_)
+      break;
+
+    // Only report suspected values that have accumulated a suspicion score.
+    // This is achieved by maintaining suspicion for several cycles, with few
+    // skips.
+    if (entry.second >= score_threshold_)
+      suspected_leaks_.emplace_back(entry.first);
+  }
+}
+
+bool LeakAnalyzer::GetPreviousCountForValue(const ValueType& value,
+                                            uint32_t* count) const {
+  // Determine what count was recorded for this value last time.
+  for (const RankedEntry& entry : prev_ranked_entries_) {
+    if (entry.value == value) {
+      *count = entry.count;
+      return true;
+    }
+  }
+  return false;
+}
+
+}  // namespace leak_detector
+}  // namespace metrics