//
// 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_COMMANDLINEFLAG_H_
#define ABSL_FLAGS_INTERNAL_COMMANDLINEFLAG_H_
#include <atomic>
#include "absl/base/macros.h"
#include "absl/flags/marshalling.h"
#include "absl/synchronization/mutex.h"
#include "absl/types/optional.h"
namespace absl {
namespace flags_internal {
// Type-specific operations, eg., parsing, copying, etc. are provided
// by function specific to that type with a signature matching FlagOpFn.
enum FlagOp {
kDelete,
kClone,
kCopy,
kCopyConstruct,
kSizeof,
kParse,
kUnparse
};
using FlagOpFn = void* (*)(FlagOp, const void*, void*);
using FlagMarshallingOpFn = void* (*)(FlagOp, const void*, void*, void*);
// Options that control SetCommandLineOptionWithMode.
enum FlagSettingMode {
// update the flag's value unconditionally (can call this multiple times).
SET_FLAGS_VALUE,
// update the flag's value, but *only if* it has not yet been updated
// with SET_FLAGS_VALUE, SET_FLAG_IF_DEFAULT, or "FLAGS_xxx = nondef".
SET_FLAG_IF_DEFAULT,
// set the flag's default value to this. If the flag has not been updated
// yet (via SET_FLAGS_VALUE, SET_FLAG_IF_DEFAULT, or "FLAGS_xxx = nondef")
// change the flag's current value to the new default value as well.
SET_FLAGS_DEFAULT
};
// Options that control SetFromString: Source of a value.
enum ValueSource {
// Flag is being set by value specified on a command line.
kCommandLine,
// Flag is being set by value specified in the code.
kProgrammaticChange,
};
// Signature for the help generation function used as an argument for the
// absl::Flag constructor.
using HelpGenFunc = std::string (*)();
// Signature for the function generating the initial flag value based (usually
// based on default value supplied in flag's definition)
using InitialValGenFunc = void* (*)();
// 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 (*)();
extern const char kStrippedFlagHelp[];
// The per-type function
template <typename T>
void* FlagOps(FlagOp op, const void* v1, void* v2) {
switch (op) {
case kDelete:
delete static_cast<const T*>(v1);
return nullptr;
case kClone:
return new T(*static_cast<const T*>(v1));
case kCopy:
*static_cast<T*>(v2) = *static_cast<const T*>(v1);
return nullptr;
case kCopyConstruct:
new (v2) T(*static_cast<const T*>(v1));
return nullptr;
case kSizeof:
return reinterpret_cast<void*>(sizeof(T));
default:
return nullptr;
}
}
template <typename T>
void* FlagMarshallingOps(FlagOp op, const void* v1, void* v2, void* v3) {
switch (op) {
case kParse: {
// initialize the temporary instance of type T based on current value in
// destination (which is going to be flag's default value).
T temp(*static_cast<T*>(v2));
if (!absl::ParseFlag<T>(*static_cast<const absl::string_view*>(v1), &temp,
static_cast<std::string*>(v3))) {
return nullptr;
}
*static_cast<T*>(v2) = std::move(temp);
return v2;
}
case kUnparse:
*static_cast<std::string*>(v2) =
absl::UnparseFlag<T>(*static_cast<const T*>(v1));
return nullptr;
default:
return nullptr;
}
}
// Functions that invoke flag-type-specific operations.
inline void Delete(FlagOpFn op, const void* obj) {
op(flags_internal::kDelete, obj, nullptr);
}
inline void* Clone(FlagOpFn op, const void* obj) {
return op(flags_internal::kClone, obj, nullptr);
}
inline void Copy(FlagOpFn op, const void* src, void* dst) {
op(flags_internal::kCopy, src, dst);
}
inline void CopyConstruct(FlagOpFn op, const void* src, void* dst) {
op(flags_internal::kCopyConstruct, src, dst);
}
inline bool Parse(FlagMarshallingOpFn op, absl::string_view text, void* dst,
std::string* error) {
return op(flags_internal::kParse, &text, dst, error) != nullptr;
}
inline std::string Unparse(FlagMarshallingOpFn op, const void* val) {
std::string result;
op(flags_internal::kUnparse, val, &result, nullptr);
return result;
}
inline size_t Sizeof(FlagOpFn op) {
// This sequence of casts reverses the sequence from base::internal::FlagOps()
return static_cast<size_t>(reinterpret_cast<intptr_t>(
op(flags_internal::kSizeof, nullptr, nullptr)));
}
// 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
};
// Holds either a pointer to help text or a function which produces it. This is
// needed for supporting both static initialization of Flags while supporting
// the legacy registration framework. We can't use absl::variant<const char*,
// const char*(*)()> since anybody passing 0 or nullptr in to a CommandLineFlag
// would find an ambiguity.
class HelpText {
public:
static constexpr HelpText FromFunctionPointer(const HelpGenFunc fn) {
return HelpText(fn, nullptr);
}
static constexpr HelpText FromStaticCString(const char* msg) {
return HelpText(nullptr, msg);
}
std::string GetHelpText() const;
HelpText() = delete;
HelpText(const HelpText&) = default;
HelpText(HelpText&&) = default;
private:
explicit constexpr HelpText(const HelpGenFunc fn, const char* msg)
: help_function_(fn), help_message_(msg) {}
HelpGenFunc help_function_;
const char* help_message_;
};
// Holds all information for a flag.
struct CommandLineFlag {
constexpr CommandLineFlag(
const char* name_arg, HelpText help_text, const char* filename_arg,
const flags_internal::FlagOpFn op_arg,
const flags_internal::FlagMarshallingOpFn marshalling_op_arg,
const flags_internal::InitialValGenFunc initial_value_gen,
const bool retired_arg, void* def_arg, void* cur_arg)
: name(name_arg),
help(help_text),
filename(filename_arg),
op(op_arg),
marshalling_op(marshalling_op_arg),
make_init_value(initial_value_gen),
retired(retired_arg),
inited(false),
modified(false),
on_command_line(false),
validator(nullptr),
callback(nullptr),
def(def_arg),
cur(cur_arg),
counter(0),
atomic(kAtomicInit),
locks(nullptr) {}
// Not copyable/assignable.
CommandLineFlag(const CommandLineFlag&) = delete;
CommandLineFlag& operator=(const CommandLineFlag&) = delete;
absl::string_view Name() const { return name; }
std::string Help() const { return help.GetHelpText(); }
bool IsRetired() const { return this->retired; }
bool IsSpecifiedOnCommandLine() const { return on_command_line; }
// Returns true iff this is a handle to an Abseil Flag.
bool IsAbseilFlag() const {
// Set to null for V1 flags
return this->make_init_value != nullptr;
}
absl::string_view Typename() const;
std::string Filename() const;
std::string DefaultValue() const;
std::string CurrentValue() const;
// Return true iff flag has type T.
template <typename T>
inline bool IsOfType() const {
return this->op == &flags_internal::FlagOps<T>;
}
// Attempts to retrieve the flag value. Returns value on success,
// absl::nullopt otherwise.
template <typename T>
absl::optional<T> Get() {
if (IsRetired() || flags_internal::FlagOps<T> != this->op)
return absl::nullopt;
T res;
Read(&res, flags_internal::FlagOps<T>);
return res;
}
void SetCallback(const flags_internal::FlagCallback mutation_callback);
// Sets the value of the flag based on specified std::string `value`. If the flag
// was successfully set to new value, it returns true. Otherwise, sets `error`
// to indicate the error, leaves the flag unchanged, and returns false. There
// are three ways to set the flag's value:
// * Update the current flag value
// * Update the flag's default value
// * Update the current flag value if it was never set before
// The mode is selected based on `set_mode` parameter.
bool SetFromString(absl::string_view value,
flags_internal::FlagSettingMode set_mode,
flags_internal::ValueSource source, std::string* error);
// Constant configuration for a particular flag.
private:
const char* const name;
const HelpText help;
const char* const filename;
public:
const FlagOpFn op; // Type-specific handler
const FlagMarshallingOpFn marshalling_op; // Marshalling ops handler
const InitialValGenFunc make_init_value; // Makes initial value for the flag
const bool retired; // Is the flag retired?
std::atomic<bool> inited; // fields have been lazily initialized
// Mutable state (guarded by locks->primary_mu).
bool modified; // Has flag value been modified?
bool on_command_line; // Specified on command line.
bool (*validator)(); // Validator function, or nullptr
FlagCallback callback; // Mutation callback, or nullptr
void* def; // Lazily initialized pointer to default value
void* cur; // Lazily initialized pointer to current value
int64_t counter; // Mutation counter
// For some types, a copy of the current value is kept in an atomically
// accessible field.
static const int64_t kAtomicInit = 0xababababababababll;
std::atomic<int64_t> atomic;
// 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
struct CommandLineFlagLocks* locks; // locks, laziliy allocated.
// copy construct new value of flag's type in a memory referenced by dst
// based on current flag's value
void Read(void* dst, const flags_internal::FlagOpFn dst_op) const;
// updates flag's value to *src (locked)
void Write(const void* src, const flags_internal::FlagOpFn src_op);
ABSL_DEPRECATED(
"temporary until FlagName call sites are migrated and validator API is "
"changed")
const char* NameAsCString() const { return name; }
private:
friend class FlagRegistry;
};
// Ensure that the lazily initialized fields of *flag have been initialized,
// and return &flag->locks->primary_mu.
absl::Mutex* InitFlagIfNecessary(CommandLineFlag* flag);
// Update any copy of the flag value that is stored in an atomic word.
// In addition if flag has a mutation callback this function invokes it. While
// callback is being invoked the primary flag's mutex is unlocked and it is
// re-locked back after call to callback is completed. Callback invocation is
// guarded by flag's secondary mutex instead which prevents concurrent callback
// invocation. Note that it is possible for other thread to grab the primary
// lock and update flag's value at any time during the callback invocation.
// This is by design. Callback can get a value of the flag if necessary, but it
// might be different from the value initiated the callback and it also can be
// different by the time the callback invocation is completed.
// Requires that *primary_lock be held in exclusive mode; it may be released
// and reacquired by the implementation.
void UpdateCopy(CommandLineFlag* flag, absl::Mutex* primary_lock);
// Return true iff flag value was changed via direct-access.
bool ChangedDirectly(CommandLineFlag* flag, const void* a, const void* b);
// Direct-access flags can be modified without going through the
// flag API. Detect such changes and updated the modified bit.
void UpdateModifiedBit(CommandLineFlag* flag);
// Invoke the flag validators for old flags.
// TODO(rogeeff): implement proper validators for Abseil Flags
bool Validate(CommandLineFlag* flag, const void* value);
// This macro is the "source of truth" for the list of supported flag types we
// expect to perform lock free operations on. Specifically it generates code,
// a one argument macro operating on a type, supplied as a macro argument, for
// each type in the list.
#define ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(A) \
A(bool) \
A(short) \
A(unsigned short) \
A(int) \
A(unsigned int) \
A(long) \
A(unsigned long) \
A(long long) \
A(unsigned long long) \
A(double) \
A(float)
} // namespace flags_internal
} // namespace absl
#endif // ABSL_FLAGS_INTERNAL_COMMANDLINEFLAG_H_
