From c334673e96ce73cbf1a693c7c85b1450fcd3571c Mon Sep 17 00:00:00 2001
From: Ben Chan <benchan@chromium.org>
Date: Fri, 2 Nov 2018 23:07:01 -0700
Subject: [PATCH] libchrome: add base::NoDestructor<T>

CL:869351 introduces base::NoDestructor<T>, which is preferred in new
code as a drop-in replacement for a function scoped static T* or T& that
is dynamically initialized, and a global base::LazyInstance<T>.

This CL patches libchrome to pull in base/no_destructor.h at r599267, so
that we can migrate existing Chrome OS code to use base::NoDestructor<T>
before the next libchrome uprev.

BUG=None
TEST=`emerge-$BOARD librchrome`

Change-Id: I791a70e10da6318ea81eaaec869ba4702361289e
---
 base/no_destructor.h | 98 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)
 create mode 100644 base/no_destructor.h

diff --git base/no_destructor.h base/no_destructor.h
new file mode 100644
index 0000000..21cfef8
--- /dev/null
+++ base/no_destructor.h
@@ -0,0 +1,98 @@
+// Copyright 2018 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 BASE_NO_DESTRUCTOR_H_
+#define BASE_NO_DESTRUCTOR_H_
+
+#include <new>
+#include <utility>
+
+namespace base {
+
+// A wrapper that makes it easy to create an object of type T with static
+// storage duration that:
+// - is only constructed on first access
+// - never invokes the destructor
+// in order to satisfy the styleguide ban on global constructors and
+// destructors.
+//
+// Runtime constant example:
+// const std::string& GetLineSeparator() {
+//  // Forwards to std::string(size_t, char, const Allocator&) constructor.
+//   static const base::NoDestructor<std::string> s(5, '-');
+//   return *s;
+// }
+//
+// More complex initialization with a lambda:
+// const std::string& GetSessionNonce() {
+//   static const base::NoDestructor<std::string> nonce([] {
+//     std::string s(16);
+//     crypto::RandString(s.data(), s.size());
+//     return s;
+//   }());
+//   return *nonce;
+// }
+//
+// NoDestructor<T> stores the object inline, so it also avoids a pointer
+// indirection and a malloc. Also note that since C++11 static local variable
+// initialization is thread-safe and so is this pattern. Code should prefer to
+// use NoDestructor<T> over:
+// - A function scoped static T* or T& that is dynamically initialized.
+// - A global base::LazyInstance<T>.
+//
+// Note that since the destructor is never run, this *will* leak memory if used
+// as a stack or member variable. Furthermore, a NoDestructor<T> should never
+// have global scope as that may require a static initializer.
+template <typename T>
+class NoDestructor {
+ public:
+  // Not constexpr; just write static constexpr T x = ...; if the value should
+  // be a constexpr.
+  template <typename... Args>
+  explicit NoDestructor(Args&&... args) {
+    new (storage_) T(std::forward<Args>(args)...);
+  }
+
+  // Allows copy and move construction of the contained type, to allow
+  // construction from an initializer list, e.g. for std::vector.
+  explicit NoDestructor(const T& x) { new (storage_) T(x); }
+  explicit NoDestructor(T&& x) { new (storage_) T(std::move(x)); }
+
+  NoDestructor(const NoDestructor&) = delete;
+  NoDestructor& operator=(const NoDestructor&) = delete;
+
+  ~NoDestructor() = default;
+
+  const T& operator*() const { return *get(); }
+  T& operator*() { return *get(); }
+
+  const T* operator->() const { return get(); }
+  T* operator->() { return get(); }
+
+  const T* get() const { return reinterpret_cast<const T*>(storage_); }
+  T* get() { return reinterpret_cast<T*>(storage_); }
+
+ private:
+  alignas(T) char storage_[sizeof(T)];
+
+#if defined(LEAK_SANITIZER)
+  // TODO(https://crbug.com/812277): This is a hack to work around the fact
+  // that LSan doesn't seem to treat NoDestructor as a root for reachability
+  // analysis. This means that code like this:
+  //   static base::NoDestructor<std::vector<int>> v({1, 2, 3});
+  // is considered a leak. Using the standard leak sanitizer annotations to
+  // suppress leaks doesn't work: std::vector is implicitly constructed before
+  // calling the base::NoDestructor constructor.
+  //
+  // Unfortunately, I haven't been able to demonstrate this issue in simpler
+  // reproductions: until that's resolved, hold an explicit pointer to the
+  // placement-new'd object in leak sanitizer mode to help LSan realize that
+  // objects allocated by the contained type are still reachable.
+  T* storage_ptr_ = reinterpret_cast<T*>(storage_);
+#endif  // defined(LEAK_SANITIZER)
+};
+
+}  // namespace base
+
+#endif  // BASE_NO_DESTRUCTOR_H_
-- 
2.19.1.930.g4563a0d9d0-goog