Remove InitOnce from the port API.

This is not an API-breaking change, because it reduces the API that the
leveldb embedder must implement. The project will build just fine
against ports that still implement InitOnce.

C++11 guarantees thread-safe initialization of static variables inside
functions. This is a more restricted form of std::call_once or
pthread_once_t (e.g., single call site), so the compiler might be able
to generate better code [1]. Equally important, having less code in
port_example.h makes it easier to port to other platforms.

Due to the change above, this CL introduces a new approach for storing
the singleton BytewiseComparatorImpl instance returned by
BytewiseComparator(). The new approach avoids a dynamic memory
allocation, which eliminates the false positive from LeakSanitizer
reported in https://github.com/google/leveldb/issues/200

[1] https://stackoverflow.com/a/27206650/

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212348004

CMakeLists.txt

@@ -149,6 +149,7 @@ target_sources(leveldb
     "${PROJECT_SOURCE_DIR}/util/logging.cc"
     "${PROJECT_SOURCE_DIR}/util/logging.h"
     "${PROJECT_SOURCE_DIR}/util/mutexlock.h"
+    "${PROJECT_SOURCE_DIR}/util/no_destructor.h"
     "${PROJECT_SOURCE_DIR}/util/options.cc"
     "${PROJECT_SOURCE_DIR}/util/random.h"
     "${PROJECT_SOURCE_DIR}/util/status.cc"
@@ -278,6 +279,7 @@ if(LEVELDB_BUILD_TESTS)
 
   leveldb_test("${PROJECT_SOURCE_DIR}/util/env_test.cc")
   leveldb_test("${PROJECT_SOURCE_DIR}/util/status_test.cc")
+  leveldb_test("${PROJECT_SOURCE_DIR}/util/no_destructor_test.cc")
 
   if(NOT BUILD_SHARED_LIBS)
     leveldb_test("${PROJECT_SOURCE_DIR}/db/autocompact_test.cc")

port/port_example.h

@@ -62,16 +62,6 @@ class CondVar {
   void SignallAll();
 };
 
-// Thread-safe initialization.
-// Used as follows:
-//      static port::OnceType init_control = LEVELDB_ONCE_INIT;
-//      static void Initializer() { ... do something ...; }
-//      ...
-//      port::InitOnce(&init_control, &Initializer);
-typedef intptr_t OnceType;
-#define LEVELDB_ONCE_INIT 0
-void InitOnce(port::OnceType*, void (*initializer)());
-
 // A type that holds a pointer that can be read or written atomically
 // (i.e., without word-tearing.)
 class AtomicPointer {

port/port_stdcxx.h

@@ -84,14 +84,6 @@ class CondVar {
   Mutex* const mu_;
 };
 
-using OnceType = std::once_flag;
-#define LEVELDB_ONCE_INIT {}
-
-// Thinly wraps std::call_once.
-inline void InitOnce(OnceType* once, void (*initializer)()) {
-  std::call_once(*once, *initializer);
-}
-
 inline bool Snappy_Compress(const char* input, size_t length,
                             ::std::string* output) {
 #if HAVE_SNAPPY

util/comparator.cc

@@ -3,11 +3,13 @@
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
 
 #include <algorithm>
-#include <stdint.h>
+#include <cstdint>
+#include <string>
+
 #include "leveldb/comparator.h"
 #include "leveldb/slice.h"
-#include "port/port.h"
 #include "util/logging.h"
+#include "util/no_destructor.h"
 
 namespace leveldb {
 
@@ -66,16 +68,9 @@ class BytewiseComparatorImpl : public Comparator {
 };
 }  // namespace
 
-static port::OnceType once = LEVELDB_ONCE_INIT;
-static const Comparator* bytewise;
-
-static void InitModule() {
-  bytewise = new BytewiseComparatorImpl;
-}
-
 const Comparator* BytewiseComparator() {
-  port::InitOnce(&once, InitModule);
-  return bytewise;
+  static NoDestructor<BytewiseComparatorImpl> singleton;
+  return singleton.get();
 }
 
 }  // namespace leveldb

util/no_destructor.h

@@ -0,0 +1,47 @@
+// Copyright (c) 2018 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#ifndef STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_
+#define STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_
+
+#include <type_traits>
+#include <utility>
+
+namespace leveldb {
+
+// Wraps an instance whose destructor is never called.
+//
+// This is intended for use with function-level static variables.
+template<typename InstanceType>
+class NoDestructor {
+ public:
+  template <typename... ConstructorArgTypes>
+  explicit NoDestructor(ConstructorArgTypes&&... constructor_args) {
+    static_assert(sizeof(instance_storage_) >= sizeof(InstanceType),
+                  "instance_storage_ is not large enough to hold the instance");
+    static_assert(
+        alignof(decltype(instance_storage_)) >= alignof(InstanceType),
+        "instance_storage_ does not meet the instance's alignment requirement");
+    new (&instance_storage_) InstanceType(
+        std::forward<ConstructorArgTypes>(constructor_args)...);
+  }
+
+  ~NoDestructor() = default;
+
+  NoDestructor(const NoDestructor&) = delete;
+  NoDestructor& operator=(const NoDestructor&) = delete;
+
+  InstanceType* get() {
+    return reinterpret_cast<InstanceType*>(&instance_storage_);
+  }
+
+ private:
+  typename
+      std::aligned_storage<sizeof(InstanceType), alignof(InstanceType)>::type
+      instance_storage_;
+};
+
+}  // namespace leveldb
+
+#endif  // STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_

util/no_destructor_test.cc

@@ -0,0 +1,49 @@
+// Copyright (c) 2018 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#include <cstdint>
+#include <cstdlib>
+#include <utility>
+
+#include "util/no_destructor.h"
+#include "util/testharness.h"
+
+namespace leveldb {
+
+namespace {
+
+struct DoNotDestruct {
+ public:
+  DoNotDestruct(uint32_t a, uint64_t b) : a(a), b(b) {}
+  ~DoNotDestruct() { std::abort(); }
+
+  // Used to check constructor argument forwarding.
+  uint32_t a;
+  uint64_t b;
+};
+
+constexpr const uint32_t kGoldenA = 0xdeadbeef;
+constexpr const uint64_t kGoldenB = 0xaabbccddeeffaabb;
+
+}  // namespace
+
+class NoDestructorTest { };
+
+TEST(NoDestructorTest, StackInstance) {
+  NoDestructor<DoNotDestruct> instance(kGoldenA, kGoldenB);
+  ASSERT_EQ(kGoldenA, instance.get()->a);
+  ASSERT_EQ(kGoldenB, instance.get()->b);
+}
+
+TEST(NoDestructorTest, StaticInstance) {
+  static NoDestructor<DoNotDestruct> instance(kGoldenA, kGoldenB);
+  ASSERT_EQ(kGoldenA, instance.get()->a);
+  ASSERT_EQ(kGoldenB, instance.get()->b);
+}
+
+}  // namespace leveldb
+
+int main(int argc, char** argv) {
+  return leveldb::test::RunAllTests();
+}