Abseil Common Libraries (C++) (grcp 依赖)
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244 lines
7.5 KiB
244 lines
7.5 KiB
// |
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// Copyright 2019 The Abseil Authors. |
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// |
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// Licensed under the Apache License, Version 2.0 (the "License"); |
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// you may not use this file except in compliance with the License. |
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// You may obtain a copy of the License at |
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// |
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// https://www.apache.org/licenses/LICENSE-2.0 |
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// |
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// Unless required by applicable law or agreed to in writing, software |
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// distributed under the License is distributed on an "AS IS" BASIS, |
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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// See the License for the specific language governing permissions and |
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// limitations under the License. |
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#ifndef ABSL_FLAGS_INTERNAL_FLAG_H_ |
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#define ABSL_FLAGS_INTERNAL_FLAG_H_ |
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#include <cstring> |
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#include "absl/flags/internal/commandlineflag.h" |
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#include "absl/flags/internal/registry.h" |
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#include "absl/memory/memory.h" |
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#include "absl/strings/str_cat.h" |
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namespace absl { |
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namespace flags_internal { |
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constexpr int64_t AtomicInit() { return 0xababababababababll; } |
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template <typename T> |
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class Flag; |
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template <typename T> |
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class FlagState : public flags_internal::FlagStateInterface { |
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public: |
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FlagState(Flag<T>* flag, T&& cur, bool modified, bool on_command_line, |
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int64_t counter) |
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: flag_(flag), |
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cur_value_(std::move(cur)), |
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modified_(modified), |
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on_command_line_(on_command_line), |
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counter_(counter) {} |
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~FlagState() override = default; |
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private: |
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friend class Flag<T>; |
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// Restores the flag to the saved state. |
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void Restore() const override; |
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// Flag and saved flag data. |
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Flag<T>* flag_; |
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T cur_value_; |
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bool modified_; |
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bool on_command_line_; |
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int64_t counter_; |
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}; |
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// Signature for the mutation callback used by watched Flags |
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// The callback is noexcept. |
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// TODO(rogeeff): add noexcept after C++17 support is added. |
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using FlagCallback = void (*)(); |
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void InvokeCallback(absl::Mutex* primary_mu, absl::Mutex* callback_mu, |
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FlagCallback cb) ABSL_EXCLUSIVE_LOCKS_REQUIRED(primary_mu); |
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// This is "unspecified" implementation of absl::Flag<T> type. |
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template <typename T> |
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class Flag final : public flags_internal::CommandLineFlag { |
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public: |
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constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen, |
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const char* filename, |
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const flags_internal::FlagMarshallingOpFn marshalling_op, |
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const flags_internal::InitialValGenFunc initial_value_gen) |
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: flags_internal::CommandLineFlag( |
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name, flags_internal::HelpText::FromFunctionPointer(help_gen), |
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filename, &flags_internal::FlagOps<T>, marshalling_op, |
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initial_value_gen, |
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/*def=*/nullptr, |
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/*cur=*/nullptr), |
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atomic_(flags_internal::AtomicInit()), |
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callback_(nullptr) {} |
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T Get() const { |
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// Implementation notes: |
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// |
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// We are wrapping a union around the value of `T` to serve three purposes: |
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// |
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// 1. `U.value` has correct size and alignment for a value of type `T` |
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// 2. The `U.value` constructor is not invoked since U's constructor does |
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// not |
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// do it explicitly. |
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// 3. The `U.value` destructor is invoked since U's destructor does it |
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// explicitly. This makes `U` a kind of RAII wrapper around non default |
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// constructible value of T, which is destructed when we leave the |
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// scope. We do need to destroy U.value, which is constructed by |
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// CommandLineFlag::Read even though we left it in a moved-from state |
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// after std::move. |
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// |
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// All of this serves to avoid requiring `T` being default constructible. |
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union U { |
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T value; |
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U() {} |
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~U() { value.~T(); } |
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}; |
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U u; |
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Read(&u.value, &flags_internal::FlagOps<T>); |
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return std::move(u.value); |
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} |
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bool AtomicGet(T* v) const { |
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const int64_t r = atomic_.load(std::memory_order_acquire); |
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if (r != flags_internal::AtomicInit()) { |
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std::memcpy(v, &r, sizeof(T)); |
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return true; |
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} |
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return false; |
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} |
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void Set(const T& v) { Write(&v, &flags_internal::FlagOps<T>); } |
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void SetCallback(const flags_internal::FlagCallback mutation_callback) { |
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absl::MutexLock l(InitFlagIfNecessary()); |
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callback_ = mutation_callback; |
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InvokeCallback(); |
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} |
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private: |
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friend class FlagState<T>; |
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void Destroy() const override { |
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// Values are heap allocated for Abseil Flags. |
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if (cur_) Delete(op_, cur_); |
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if (def_) Delete(op_, def_); |
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delete locks_; |
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} |
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void StoreAtomic() override { |
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if (sizeof(T) <= sizeof(int64_t)) { |
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int64_t t = 0; |
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std::memcpy(&t, cur_, (std::min)(sizeof(T), sizeof(int64_t))); |
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atomic_.store(t, std::memory_order_release); |
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} |
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} |
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// Interfaces to save and restore flags to/from persistent state. |
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// Returns current flag state or nullptr if flag does not support |
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// saving and restoring a state. |
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std::unique_ptr<flags_internal::FlagStateInterface> SaveState() override { |
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T curr_value = Get(); |
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absl::MutexLock l(InitFlagIfNecessary()); |
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return absl::make_unique<flags_internal::FlagState<T>>( |
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this, std::move(curr_value), modified_, on_command_line_, counter_); |
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} |
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// Restores the flag state to the supplied state object. If there is |
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// nothing to restore returns false. Otherwise returns true. |
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bool RestoreState(const flags_internal::FlagState<T>& flag_state) { |
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if (MutationCounter() == flag_state.counter_) return false; |
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Set(flag_state.cur_value_); |
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// Race condition here? This should disappear once we move the rest of the |
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// flag's data into Flag's internals. |
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absl::MutexLock l(InitFlagIfNecessary()); |
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modified_ = flag_state.modified_; |
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on_command_line_ = flag_state.on_command_line_; |
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return true; |
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} |
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// Interfaces to overate on callbacks. |
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void InvokeCallback() override |
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ABSL_EXCLUSIVE_LOCKS_REQUIRED(locks_->primary_mu) { |
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flags_internal::InvokeCallback(&locks_->primary_mu, &locks_->callback_mu, |
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callback_); |
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} |
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// Flag's data |
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// For some types, a copy of the current value is kept in an atomically |
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// accessible field. |
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std::atomic<int64_t> atomic_; |
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FlagCallback callback_; // Mutation callback |
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}; |
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template <typename T> |
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inline void FlagState<T>::Restore() const { |
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if (flag_->RestoreState(*this)) { |
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ABSL_INTERNAL_LOG(INFO, |
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absl::StrCat("Restore saved value of ", flag_->Name(), |
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" to: ", flag_->CurrentValue())); |
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} |
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} |
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// This class facilitates Flag object registration and tail expression-based |
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// flag definition, for example: |
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// ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher); |
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template <typename T, bool do_register> |
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class FlagRegistrar { |
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public: |
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explicit FlagRegistrar(Flag<T>* flag) : flag_(flag) { |
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if (do_register) flags_internal::RegisterCommandLineFlag(flag_); |
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} |
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FlagRegistrar& OnUpdate(flags_internal::FlagCallback cb) && { |
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flag_->SetCallback(cb); |
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return *this; |
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} |
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// Make the registrar "die" gracefully as a bool on a line where registration |
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// happens. Registrar objects are intended to live only as temporary. |
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operator bool() const { return true; } // NOLINT |
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private: |
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Flag<T>* flag_; // Flag being registered (not owned). |
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}; |
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// This struct and corresponding overload to MakeDefaultValue are used to |
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// facilitate usage of {} as default value in ABSL_FLAG macro. |
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struct EmptyBraces {}; |
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template <typename T> |
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T* MakeFromDefaultValue(T t) { |
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return new T(std::move(t)); |
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} |
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template <typename T> |
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T* MakeFromDefaultValue(EmptyBraces) { |
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return new T; |
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} |
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} // namespace flags_internal |
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} // namespace absl |
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#endif // ABSL_FLAGS_INTERNAL_FLAG_H_
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