commandlineflag.h

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//
// 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_