Add out-of-process memory logging stream parsing.

chrome/profiling/stack.h

+// Copyright 2017 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.
+
+#ifndef CHROME_PROFILING_STACK_H_
+#define CHROME_PROFILING_STACK_H_
+
+#include <functional>
+#include <vector>
+
+#include "base/macros.h"
+#include "chrome/profiling/address.h"
+
+namespace profiling {
+
+// Holds a move-only stack backtrace and a precomputed hash.
+//
+// This is immutable since we assume it can be read from multiple threads
+// without locking.
+//
+// This class has a ref_count member which is used by the allocation tracker
+// to track references to the stack. The reference counting is managed
+// externally. Tracking live objects with a global atom list in a threadsafe
+// manner is much more difficult if this class derives from RefCount.
+class Stack {

[awong, 2017/06/19 21:51:27]

nit: rename Backtrace.

Stack looks like a data structure with push/pop to me.

Also explain why this is different from base::debug::StackTrace

[brettw, 2017/06/19 23:29:46]

Done.

+ public:
+  explicit Stack(std::vector<Address>&& a);

[awong, 2017/06/19 21:51:27]

If this is move-only, don't you need a DisallowCopyAndASsign?

[brettw, 2017/06/19 23:29:46]

It's not move-only, but I guess I can make it like that.

+  ~Stack();
+
+  bool operator==(const Stack& other) const;
+  bool operator!=(const Stack& other) const;
+
+  const std::vector<Address>& addrs() const;
+
+  size_t hash() const { return hash_; }

[awong, 2017/06/19 21:51:27]

Is this actually a "hash()" as in an unstable value for use in a hashtable that
you are memoizing?  Or is it really a fingerprint() that we're going to
(conceptually) expect to be identifying of this particular back trace?

If it's the latter, this should be called fingerprint(), preferable
fingerprint_algorithm_name().

FWIW, I think fingerprint_sha1 or finterprint_murmur would be more useful than
generic hash as an API.

[brettw, 2017/06/19 23:29:46]

Changed to fingerprint.

+
+ private:
+  friend class StackStorage;  // Only StackStorage can do ref counting.
+
+  // The reference counting is not threadsafe. it's assumed the StackStorage is
+  // the only class accessing this, and it's done inside a lock.
+  void AddRef() const { ref_count_++; }
+  bool Release() const {  // Returns whether the result is non-zero.
+    return !!(--ref_count_);
+  }
+
+  std::vector<Address> addrs_;
+  size_t hash_;
+  mutable int ref_count_ = 0;
+};
+
+}  // namespace profiling
+
+namespace std {
+
+template <>
+struct hash<profiling::Stack> {
+  using argument_type = profiling::Stack;
+  using result_type = size_t;
+  result_type operator()(const argument_type& s) const { return s.hash(); }
+};
+
+}  // namespace std
+
+#endif  // CHROME_PROFILING_STACK_H_