Add constinit global string holder classes

src/base/strings/BUILD.bazel

@@ -75,6 +75,58 @@ mozc_cc_test(
     ],
 )
 
+mozc_cc_library(
+    name = "const_init_immutable_string",
+    hdrs = ["const_init_immutable_string.h"],
+    defines = mozc_select(
+        client = [],
+        default = ["MOZC_NO_ATOMIC_FLAG_WAIT"],
+    ),
+    deps = ["//base/strings/internal:const_init_string_helpers"] + mozc_select(
+        client = [],
+        default = [
+            "@com_google_absl//absl/base:core_headers",
+            "@com_google_absl//absl/synchronization",
+        ],
+    ),
+)
+
+mozc_cc_test(
+    name = "const_init_immutable_string_test",
+    size = "small",
+    srcs = ["const_init_immutable_string_test.cc"],
+    deps = [
+        ":const_init_immutable_string",
+        "//testing:gunit_main",
+    ],
+)
+
+mozc_cc_library(
+    name = "const_init_mutable_string",
+    hdrs = ["const_init_mutable_string.h"],
+    defines = mozc_select(
+        client = [],
+        default = ["MOZC_NO_ATOMIC_FLAG_WAIT"],
+    ),
+    deps = ["//base/strings/internal:const_init_string_helpers"] + mozc_select(
+        client = [],
+        default = [
+            "@com_google_absl//absl/base:core_headers",
+            "@com_google_absl//absl/synchronization",
+        ],
+    ),
+)
+
+mozc_cc_test(
+    name = "const_init_mutable_string_test",
+    size = "small",
+    srcs = ["const_init_mutable_string_test.cc"],
+    deps = [
+        ":const_init_mutable_string",
+        "//testing:gunit_main",
+    ],
+)
+
 mozc_cc_library(
     name = "japanese",
     srcs = [

src/base/strings/const_init_immutable_string.h

@@ -0,0 +1,235 @@
+// Copyright 2010-2021, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef MOZC_BASE_STRINGS_CONST_INIT_IMMUTABLE_STRING_H_
+#define MOZC_BASE_STRINGS_CONST_INIT_IMMUTABLE_STRING_H_
+
+#include <array>
+#include <atomic>
+#include <cstddef>
+#include <memory>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+#if defined(MOZC_NO_ATOMIC_FLAG_WAIT)
+#include "absl/base/const_init.h"
+#include "absl/synchronization/mutex.h"
+#endif  // defined(MOZC_NO_ATOMIC_FLAG_WAIT)
+
+#include "base/strings/internal/const_init_string_helpers.h"
+
+namespace mozc {
+
+// A utility class to deal with constant global strings whose value is known
+// only at runtime. It has the following capabilities:
+//
+// 1. It allows the library users to lazily initialize the string by calling
+//    GetOrInit() only after it becomes ready, e.g. only after dependent
+//    modules are fully loaded.
+// 2. It is thread-safe, meaning that multiple threads can call GetOrInit()
+//    concurrently without causing data race, with an assumption that the
+//    idempotent_initializer is thread-safe and *idempotent* (i.e., it always
+//    returns the same value when called multiple times).
+// 3. It guarantees that the string is null-terminated.
+// 4. It accepts the constinit keyword.
+//
+// Synchronization model:
+// ---------------------
+// The initializer is invoked without any class-held state. This avoids lock
+// order inversion when the initializer triggers foreign code that acquires
+// its own locks -- e.g. a LoadLibrary() call from within DllMain on Windows,
+// which Microsoft documents as a classic deadlock pattern:
+//
+//   https://learn.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-best-practices#deadlocks-caused-by-lock-order-inversion
+//
+// Racing threads may each invoke the initializer; per the IdempotentInitializer
+// contract that is acceptable.
+//
+// Once the string is published it cannot be replaced. See
+// ConstInitMutableString for a variant that additionally supports thread-safe
+// Set().
+//
+// Cavets on destruction:
+// ---------------------
+//   * Without MOZC_NO_ATOMIC_FLAG_WAIT, this class is guaranteed to be
+//     trivially destructible and can be safely used in the global scope without
+//     worrying about destruction order at exit.
+//   * With MOZC_NO_ATOMIC_FLAG_WAIT (e.g. server-side configurations where
+//     std::atomic_flag::wait is not allowed), it falls back to absl::Mutex,
+//     which offers better integration with TSAN. In this case it is no longer
+//     trivially destructible, which means atexit-style destructions will happen
+//     for globally instantiated objects. As explained in
+//     absl/base/const_init.h, strictly speaking this can fall into undefined
+//     behavior and we are just relying on known toolchain-specific behaviors
+//     that are not guaranteed by the standard.
+//
+// Storage model:
+// -------------
+//   * Values that fit in fixed_array_size characters (including the terminating
+//     NUL) live in an inline array.
+//   * Larger values live in a heap buffer. When Set() replaces a heap-backed
+//     value, the prior heap allocation is freed before Set() returns.
+//   * The currently-installed heap buffer (if any) is intentionally leaked at
+//     process exit.
+#if !defined(MOZC_NO_ATOMIC_FLAG_WAIT)
+
+// Trivially-destructible variant using `std::atomic_flag::wait`.
+template <size_t fixed_array_size,
+          const_init_string_internal::SupportedChar CharT = char>
+class ConstInitImmutableString {
+ public:
+  using StringT = std::basic_string<CharT>;
+  using StringViewT = std::basic_string_view<CharT>;
+
+  // A data initializer that is guaranteed to return the same value no matter
+  // how many times it is called. It must also be reentrant and safe to call
+  // from multiple threads concurrently.
+  using IdempotentInitializer = std::add_pointer_t<StringT()>;
+
+  ConstInitImmutableString() = delete;
+  ~ConstInitImmutableString() = default;
+
+  consteval explicit ConstInitImmutableString(IdempotentInitializer init)
+      : idempotent_initializer_(init) {}
+
+  [[nodiscard]] StringViewT GetOrInit() {
+    // Fast path: latch observed. Synchronization is on init_done_; the acquire
+    // on test() already publishes all writes prior to the publisher's
+    // release on init_done_.test_and_set(), so result_ptr_ can can be loaded
+    // with relaxed ordering.
+    if (init_done_.test(std::memory_order::acquire)) [[likely]] {
+      return StringViewT(result_ptr_.load(std::memory_order::relaxed),
+                         result_size_);
+    }
+    // Invoke the initializer and stage any heap fallback *outside* of
+    // init_started_. Holding init_started_ across foreign code would introduce
+    // the Loader-Lock-style lock-order-inversion deadlock described above;
+    // staging outside also lets a throwing initializer on one thread leave the
+    // instance recoverable for other threads.
+    const StringT value = idempotent_initializer_();
+    std::unique_ptr<CharT[]> heap_fallback =
+        const_init_string_internal::StageHeapFallback<CharT>(value,
+                                                             std::size(value_));
+    if (!init_started_.test_and_set(std::memory_order::acquire)) {
+      // Won the publish race. Commit the staged value.
+      CharT* dest = const_init_string_internal::CommitStagedValue<CharT>(
+          value, heap_fallback, value_.data());
+      result_size_ = value.size();
+      result_ptr_.store(dest, std::memory_order::relaxed);
+      init_done_.test_and_set(std::memory_order::release);
+      init_done_.notify_all();
+      return StringViewT(dest, value.size());
+    }
+    // Lost the publish race. Block at the OS level until the winner publishes.
+    init_done_.wait(false, std::memory_order::acquire);
+    return StringViewT(result_ptr_.load(std::memory_order::relaxed),
+                       result_size_);
+  }
+
+ private:
+  // Hot fields: all three are touched on the fast path. Keep them together at
+  // the front so they share a cache line regardless of `fixed_array_size`.
+  std::atomic_flag init_done_ = {};
+  std::atomic<const CharT*> result_ptr_ = nullptr;
+  size_t result_size_ = 0;
+  // Cold fields: only the slow init path touches these.
+  const IdempotentInitializer idempotent_initializer_;
+  std::atomic_flag init_started_ = {};
+  std::array<CharT, fixed_array_size> value_ = {};
+};
+
+static_assert(
+    std::is_trivially_destructible_v<ConstInitImmutableString<1, char>>);
+static_assert(
+    std::is_trivially_destructible_v<ConstInitImmutableString<1, wchar_t>>);
+
+#else  // !defined(MOZC_NO_ATOMIC_FLAG_WAIT)
+
+// absl::Mutex-based variant, which offers better integration with TSAN by
+// giving up trivial destructibility and relying on toolchain-specific behaviors
+// that are not guaranteed by the standard.
+template <size_t fixed_array_size,
+          const_init_string_internal::SupportedChar CharT = char>
+class ConstInitImmutableString {
+ public:
+  using StringT = std::basic_string<CharT>;
+  using StringViewT = std::basic_string_view<CharT>;
+
+  // A data initializer that is guaranteed to return the same value no matter
+  // how many times it is called. It must also be reentrant and safe to call
+  // from multiple threads concurrently.
+  using IdempotentInitializer = std::add_pointer_t<StringT()>;
+
+  ConstInitImmutableString() = delete;
+  ~ConstInitImmutableString() = default;
+
+  consteval explicit ConstInitImmutableString(IdempotentInitializer init)
+      : idempotent_initializer_(init) {}
+
+  [[nodiscard]] StringViewT GetOrInit() {
+    // Fast path: publication observed (no mutex_ involvement).
+    if (const CharT* p = result_ptr_.load(std::memory_order::acquire))
+        [[likely]] {
+      return StringViewT(p, result_size_);
+    }
+    // Invoke the initializer and stage any heap fallback *outside* of mutex_.
+    // See the class comment for the Loader-Lock-style lock-order-inversion
+    // hazard this avoids.
+    const StringT value = idempotent_initializer_();
+    std::unique_ptr<CharT[]> heap_fallback =
+        const_init_string_internal::StageHeapFallback<CharT>(value,
+                                                             std::size(value_));
+    absl::MutexLock l(mutex_);
+    if (const CharT* p = result_ptr_.load(std::memory_order::relaxed)) {
+      return StringViewT(p, result_size_);
+    }
+    CharT* dest = const_init_string_internal::CommitStagedValue<CharT>(
+        value, heap_fallback, value_.data());
+    result_size_ = value.size();
+    result_ptr_.store(dest, std::memory_order::release);
+    return StringViewT(dest, value.size());
+  }
+
+ private:
+  // Hot fields: all two are touched on the fast path. Keep them together at
+  // the front so they share a cache line regardless of `fixed_array_size`.
+  std::atomic<const CharT*> result_ptr_ = nullptr;
+  size_t result_size_ = 0;
+  // Cold fields: only the slow init path touches these.
+  const IdempotentInitializer idempotent_initializer_;
+  absl::Mutex mutex_{absl::kConstInit};
+  std::array<CharT, fixed_array_size> value_ = {};
+};
+
+#endif  // !defined(MOZC_NO_ATOMIC_FLAG_WAIT)
+
+}  // namespace mozc
+
+#endif  // MOZC_BASE_STRINGS_CONST_INIT_IMMUTABLE_STRING_H_

src/base/strings/const_init_immutable_string_test.cc

@@ -0,0 +1,78 @@
+// Copyright 2010-2021, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "base/strings/const_init_immutable_string.h"
+
+#include <string>
+#include <string_view>
+
+#include "testing/gunit.h"
+
+namespace mozc::strings {
+namespace {
+
+constinit ConstInitImmutableString<256> g_value_simple_init(
+    []() -> std::string { return "Mozc"; });
+
+TEST(ConstInitImmutableStringTest, SimpleInit) {
+  EXPECT_EQ(g_value_simple_init.GetOrInit(), "Mozc");
+  EXPECT_EQ(g_value_simple_init.GetOrInit(), "Mozc");
+}
+
+TEST(ConstInitImmutableStringTest, NullTermination) {
+  const auto value = g_value_simple_init.GetOrInit();
+  EXPECT_EQ(value.data()[value.size()], '\0');
+}
+
+constexpr std::string_view kLongValue =
+    "this string is longer than the inline buffer";
+
+constinit ConstInitImmutableString<8> g_value_overflow([]() -> std::string {
+  return std::string("this string is longer than the inline buffer");
+});
+
+TEST(ConstInitImmutableStringTest, HeapFallback) {
+  const auto value = g_value_overflow.GetOrInit();
+  EXPECT_EQ(value, kLongValue);
+  EXPECT_EQ(value.data()[value.size()], '\0');
+  // Repeated calls return the same pointer (stable storage).
+  EXPECT_EQ(g_value_overflow.GetOrInit().data(), value.data());
+}
+
+constinit ConstInitImmutableString<256, wchar_t> g_value_wide(
+    []() -> std::wstring { return L"WideMozc"; });
+
+TEST(ConstInitImmutableStringTest, WideChar) {
+  EXPECT_EQ(g_value_wide.GetOrInit(), std::wstring_view(L"WideMozc"));
+  const auto value = g_value_wide.GetOrInit();
+  EXPECT_EQ(value.data()[value.size()], L'\0');
+}
+
+}  // namespace
+}  // namespace mozc::strings