//
// Copyright 2019 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#ifndef ABSL_FLAGS_INTERNAL_FLAG_H_
#define ABSL_FLAGS_INTERNAL_FLAG_H_
#include <atomic>
#include <cstring>
#include "absl/base/thread_annotations.h"
#include "absl/flags/internal/commandlineflag.h"
#include "absl/flags/internal/registry.h"
#include "absl/memory/memory.h"
#include "absl/strings/str_cat.h"
#include "absl/synchronization/mutex.h"
namespace absl {
namespace flags_internal {
constexpr int64_t AtomicInit() { return 0xababababababababll; }
template <typename T>
class Flag;
template <typename T>
class FlagState : public flags_internal::FlagStateInterface {
public:
FlagState(Flag<T>* flag, T&& cur, bool modified, bool on_command_line,
int64_t counter)
: flag_(flag),
cur_value_(std::move(cur)),
modified_(modified),
on_command_line_(on_command_line),
counter_(counter) {}
~FlagState() override = default;
private:
friend class Flag<T>;
// Restores the flag to the saved state.
void Restore() const override;
// Flag and saved flag data.
Flag<T>* flag_;
T cur_value_;
bool modified_;
bool on_command_line_;
int64_t counter_;
};
// This is help argument for absl::Flag encapsulating the string literal pointer
// or pointer to function generating it as well as enum descriminating two
// cases.
using HelpGenFunc = std::string (*)();
union FlagHelpSrc {
constexpr explicit FlagHelpSrc(const char* help_msg) : literal(help_msg) {}
constexpr explicit FlagHelpSrc(HelpGenFunc help_gen) : gen_func(help_gen) {}
const char* literal;
HelpGenFunc gen_func;
};
enum class FlagHelpSrcKind : int8_t { kLiteral, kGenFunc };
struct HelpInitArg {
FlagHelpSrc source;
FlagHelpSrcKind kind;
};
// HelpConstexprWrap is used by struct AbslFlagHelpGenFor##name generated by
// ABSL_FLAG macro. It is only used to silence the compiler in the case where
// help message expression is not constexpr and does not have type const char*.
// If help message expression is indeed constexpr const char* HelpConstexprWrap
// is just a trivial identity function.
template <typename T>
const char* HelpConstexprWrap(const T&) {
return nullptr;
}
constexpr const char* HelpConstexprWrap(const char* p) { return p; }
constexpr const char* HelpConstexprWrap(char* p) { return p; }
// These two HelpArg overloads allows us to select at compile time one of two
// way to pass Help argument to absl::Flag. We'll be passing
// AbslFlagHelpGenFor##name as T and integer 0 as a single argument to prefer
// first overload if possible. If T::Const is evaluatable on constexpr
// context (see non template int parameter below) we'll choose first overload.
// In this case the help message expression is immediately evaluated and is used
// to construct the absl::Flag. No additionl code is generated by ABSL_FLAG.
// Otherwise SFINAE kicks in and first overload is dropped from the
// consideration, in which case the second overload will be used. The second
// overload does not attempt to evaluate the help message expression
// immediately and instead delays the evaluation by returing the function
// pointer (&T::NonConst) genering the help message when necessary. This is
// evaluatable in constexpr context, but the cost is an extra function being
// generated in the ABSL_FLAG code.
template <typename T, int = (T::Const(), 1)>
constexpr flags_internal::HelpInitArg HelpArg(int) {
return {flags_internal::FlagHelpSrc(T::Const()),
flags_internal::FlagHelpSrcKind::kLiteral};
}
template <typename T>
constexpr flags_internal::HelpInitArg HelpArg(char) {
return {flags_internal::FlagHelpSrc(&T::NonConst),
flags_internal::FlagHelpSrcKind::kGenFunc};
}
// Signature for the function generating the initial flag value (usually
// based on default value supplied in flag's definition)
using FlagDfltGenFunc = void* (*)();
union FlagDefaultSrc {
constexpr explicit FlagDefaultSrc(FlagDfltGenFunc gen_func_arg)
: gen_func(gen_func_arg) {}
void* dynamic_value;
FlagDfltGenFunc gen_func;
};
enum class FlagDefaultSrcKind : int8_t { kDynamicValue, kGenFunc };
// Signature for the mutation callback used by watched Flags
// The callback is noexcept.
// TODO(rogeeff): add noexcept after C++17 support is added.
using FlagCallback = void (*)();
struct DynValueDeleter {
void operator()(void* ptr) const { Delete(op, ptr); }
const FlagOpFn op;
};
// The class encapsulates the Flag's data and safe access to it.
class FlagImpl {
public:
constexpr FlagImpl(const flags_internal::FlagOpFn op,
const flags_internal::FlagMarshallingOpFn marshalling_op,
const flags_internal::FlagDfltGenFunc default_value_gen,
const HelpInitArg help)
: op_(op),
marshalling_op_(marshalling_op),
help_(help.source),
help_source_kind_(help.kind),
def_kind_(flags_internal::FlagDefaultSrcKind::kGenFunc),
default_src_(default_value_gen) {}
// Forces destruction of the Flag's data.
void Destroy() const;
// Constant access methods
std::string Help() const;
bool IsModified() const ABSL_LOCKS_EXCLUDED(*DataGuard());
bool IsSpecifiedOnCommandLine() const ABSL_LOCKS_EXCLUDED(*DataGuard());
std::string DefaultValue() const ABSL_LOCKS_EXCLUDED(*DataGuard());
std::string CurrentValue() const ABSL_LOCKS_EXCLUDED(*DataGuard());
void Read(const CommandLineFlag& flag, void* dst,
const flags_internal::FlagOpFn dst_op) const
ABSL_LOCKS_EXCLUDED(*DataGuard());
// Attempts to parse supplied `value` std::string.
bool TryParse(const CommandLineFlag& flag, void* dst, absl::string_view value,
std::string* err) const
ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
template <typename T>
bool AtomicGet(T* v) const {
const int64_t r = atomic_.load(std::memory_order_acquire);
if (r != flags_internal::AtomicInit()) {
std::memcpy(v, &r, sizeof(T));
return true;
}
return false;
}
// Mutating access methods
void Write(const CommandLineFlag& flag, const void* src,
const flags_internal::FlagOpFn src_op)
ABSL_LOCKS_EXCLUDED(*DataGuard());
bool SetFromString(const CommandLineFlag& flag, absl::string_view value,
FlagSettingMode set_mode, ValueSource source,
std::string* err) ABSL_LOCKS_EXCLUDED(*DataGuard());
// If possible, updates copy of the Flag's value that is stored in an
// atomic word.
void StoreAtomic() ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
// Interfaces to operate on callbacks.
void SetCallback(const flags_internal::FlagCallback mutation_callback)
ABSL_LOCKS_EXCLUDED(*DataGuard());
void InvokeCallback() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
// Interfaces to save/restore mutable flag data
template <typename T>
std::unique_ptr<flags_internal::FlagStateInterface> SaveState(
Flag<T>* flag) const ABSL_LOCKS_EXCLUDED(*DataGuard()) {
T&& cur_value = flag->Get();
absl::MutexLock l(DataGuard());
return absl::make_unique<flags_internal::FlagState<T>>(
flag, std::move(cur_value), modified_, on_command_line_, counter_);
}
bool RestoreState(const CommandLineFlag& flag, const void* value,
bool modified, bool on_command_line, int64_t counter)
ABSL_LOCKS_EXCLUDED(*DataGuard());
// Value validation interfaces.
void CheckDefaultValueParsingRoundtrip(const CommandLineFlag& flag) const
ABSL_LOCKS_EXCLUDED(*DataGuard());
bool ValidateInputValue(absl::string_view value) const
ABSL_LOCKS_EXCLUDED(*DataGuard());
private:
// Lazy initialization of the Flag's data.
void Init();
// Ensures that the lazily initialized data is initialized,
// and returns pointer to the mutex guarding flags data.
absl::Mutex* DataGuard() const ABSL_LOCK_RETURNED(locks_->primary_mu);
// Returns heap allocated value of type T initialized with default value.
std::unique_ptr<void, DynValueDeleter> MakeInitValue() const
ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
// Immutable Flag's data.
const FlagOpFn op_; // Type-specific handler.
const FlagMarshallingOpFn marshalling_op_; // Marshalling ops handler.
const FlagHelpSrc help_; // Help message literal or function to generate it.
// Indicates if help message was supplied as literal or generator func.
const FlagHelpSrcKind help_source_kind_;
// Mutable Flag's data. (guarded by DataGuard()).
// Indicates that locks_ and cur_ fields have been lazily initialized.
std::atomic<bool> inited_{false};
// Has flag value been modified?
bool modified_ ABSL_GUARDED_BY(*DataGuard()) = false;
// Specified on command line.
bool on_command_line_ ABSL_GUARDED_BY(*DataGuard()) = false;
// If def_kind_ == kDynamicValue, default_src_ holds a dynamically allocated
// value.
FlagDefaultSrcKind def_kind_ ABSL_GUARDED_BY(*DataGuard());
// Either a pointer to the function generating the default value based on the
// value specified in ABSL_FLAG or pointer to the dynamically set default
// value via SetCommandLineOptionWithMode. def_kind_ is used to distinguish
// these two cases.
FlagDefaultSrc default_src_ ABSL_GUARDED_BY(*DataGuard());
// Lazily initialized pointer to current value
void* cur_ ABSL_GUARDED_BY(*DataGuard()) = nullptr;
// Mutation counter
int64_t counter_ ABSL_GUARDED_BY(*DataGuard()) = 0;
// For some types, a copy of the current value is kept in an atomically
// accessible field.
std::atomic<int64_t> atomic_{flags_internal::AtomicInit()};
// Mutation callback
FlagCallback callback_ = nullptr;
// Lazily initialized mutexes for this flag value. We cannot inline a
// SpinLock or Mutex here because those have non-constexpr constructors and
// so would prevent constant initialization of this type.
// TODO(rogeeff): fix it once Mutex has constexpr constructor
// The following struct contains the locks in a CommandLineFlag struct.
// They are in a separate struct that is lazily allocated to avoid problems
// with static initialization and to avoid multiple allocations.
struct CommandLineFlagLocks {
absl::Mutex primary_mu; // protects several fields in CommandLineFlag
absl::Mutex callback_mu; // used to serialize callbacks
};
CommandLineFlagLocks* locks_ = nullptr; // locks, laziliy allocated.
};
// This is "unspecified" implementation of absl::Flag<T> type.
template <typename T>
class Flag final : public flags_internal::CommandLineFlag {
public:
constexpr Flag(const char* name, const flags_internal::HelpInitArg help,
const char* filename,
const flags_internal::FlagMarshallingOpFn marshalling_op,
const flags_internal::FlagDfltGenFunc initial_value_gen)
: flags_internal::CommandLineFlag(name, filename),
impl_(&flags_internal::FlagOps<T>, marshalling_op, initial_value_gen,
help) {}
T Get() const {
// See implementation notes in CommandLineFlag::Get().
union U {
T value;
U() {}
~U() { value.~T(); }
};
U u;
impl_.Read(*this, &u.value, &flags_internal::FlagOps<T>);
return std::move(u.value);
}
bool AtomicGet(T* v) const { return impl_.AtomicGet(v); }
void Set(const T& v) { impl_.Write(*this, &v, &flags_internal::FlagOps<T>); }
void SetCallback(const flags_internal::FlagCallback mutation_callback) {
impl_.SetCallback(mutation_callback);
}
// CommandLineFlag interface
std::string Help() const override { return impl_.Help(); }
bool IsModified() const override { return impl_.IsModified(); }
bool IsSpecifiedOnCommandLine() const override {
return impl_.IsSpecifiedOnCommandLine();
}
std::string DefaultValue() const override { return impl_.DefaultValue(); }
std::string CurrentValue() const override { return impl_.CurrentValue(); }
bool ValidateInputValue(absl::string_view value) const override {
return impl_.ValidateInputValue(value);
}
// Interfaces to save and restore flags to/from persistent state.
// Returns current flag state or nullptr if flag does not support
// saving and restoring a state.
std::unique_ptr<flags_internal::FlagStateInterface> SaveState() override {
return impl_.SaveState(this);
}
// Restores the flag state to the supplied state object. If there is
// nothing to restore returns false. Otherwise returns true.
bool RestoreState(const flags_internal::FlagState<T>& flag_state) {
return impl_.RestoreState(*this, &flag_state.cur_value_,
flag_state.modified_, flag_state.on_command_line_,
flag_state.counter_);
}
bool SetFromString(absl::string_view value,
flags_internal::FlagSettingMode set_mode,
flags_internal::ValueSource source,
std::string* error) override {
return impl_.SetFromString(*this, value, set_mode, source, error);
}
void CheckDefaultValueParsingRoundtrip() const override {
impl_.CheckDefaultValueParsingRoundtrip(*this);
}
private:
friend class FlagState<T>;
void Destroy() const override { impl_.Destroy(); }
void Read(void* dst) const override {
impl_.Read(*this, dst, &flags_internal::FlagOps<T>);
}
flags_internal::FlagOpFn TypeId() const override {
return &flags_internal::FlagOps<T>;
}
// Flag's data
FlagImpl impl_;
};
template <typename T>
inline void FlagState<T>::Restore() const {
if (flag_->RestoreState(*this)) {
ABSL_INTERNAL_LOG(INFO,
absl::StrCat("Restore saved value of ", flag_->Name(),
" to: ", flag_->CurrentValue()));
}
}
// This class facilitates Flag object registration and tail expression-based
// flag definition, for example:
// ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher);
template <typename T, bool do_register>
class FlagRegistrar {
public:
explicit FlagRegistrar(Flag<T>* flag) : flag_(flag) {
if (do_register) flags_internal::RegisterCommandLineFlag(flag_);
}
FlagRegistrar& OnUpdate(flags_internal::FlagCallback cb) && {
flag_->SetCallback(cb);
return *this;
}
// Make the registrar "die" gracefully as a bool on a line where registration
// happens. Registrar objects are intended to live only as temporary.
operator bool() const { return true; } // NOLINT
private:
Flag<T>* flag_; // Flag being registered (not owned).
};
// This struct and corresponding overload to MakeDefaultValue are used to
// facilitate usage of {} as default value in ABSL_FLAG macro.
struct EmptyBraces {};
template <typename T>
T* MakeFromDefaultValue(T t) {
return new T(std::move(t));
}
template <typename T>
T* MakeFromDefaultValue(EmptyBraces) {
return new T;
}
} // namespace flags_internal
} // namespace absl
#endif // ABSL_FLAGS_INTERNAL_FLAG_H_