absl/flags/internal/flag.cc

//
// 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.

#include "absl/flags/internal/flag.h"

#include <stddef.h>
#include <stdint.h>
#include <string.h>

#include <atomic>
#include <memory>
#include <string>
#include <vector>

#include "absl/base/attributes.h"
#include "absl/base/config.h"
#include "absl/base/const_init.h"
#include "absl/base/optimization.h"
#include "absl/flags/internal/commandlineflag.h"
#include "absl/flags/usage_config.h"
#include "absl/strings/str_cat.h"
#include "absl/strings/string_view.h"
#include "absl/synchronization/mutex.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace flags_internal {

// The help message indicating that the commandline flag has been
// 'stripped'. It will not show up when doing "-help" and its
// variants. The flag is stripped if ABSL_FLAGS_STRIP_HELP is set to 1
// before including absl/flags/flag.h
const char kStrippedFlagHelp[] = "\001\002\003\004 (unknown) \004\003\002\001";

namespace {

// Currently we only validate flag values for user-defined flag types.
bool ShouldValidateFlagValue(FlagOpFn flag_type_id) {
#define DONT_VALIDATE(T) \
  if (flag_type_id == &flags_internal::FlagOps<T>) return false;
  ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(DONT_VALIDATE)
#undef DONT_VALIDATE

  return true;
}

// RAII helper used to temporarily unlock and relock `absl::Mutex`.
// This is used when we need to ensure that locks are released while
// invoking user supplied callbacks and then reacquired, since callbacks may
// need to acquire these locks themselves.
class MutexRelock {
 public:
  explicit MutexRelock(absl::Mutex* mu) : mu_(mu) { mu_->Unlock(); }
  ~MutexRelock() { mu_->Lock(); }

  MutexRelock(const MutexRelock&) = delete;
  MutexRelock& operator=(const MutexRelock&) = delete;

 private:
  absl::Mutex* mu_;
};

// This global lock guards the initialization and destruction of data_guard_,
// which is used to guard the other Flag data.
ABSL_CONST_INIT static absl::Mutex flag_mutex_lifetime_guard(absl::kConstInit);

}  // namespace

void FlagImpl::Init() {
  {
    absl::MutexLock lock(&flag_mutex_lifetime_guard);

    // Must initialize data guard for this flag.
    if (!is_data_guard_inited_) {
      new (&data_guard_) absl::Mutex;
      is_data_guard_inited_ = true;
    }
  }

  absl::MutexLock lock(reinterpret_cast<absl::Mutex*>(&data_guard_));

  if (cur_ != nullptr) {
    inited_.store(true, std::memory_order_release);
  } else {
    // Need to initialize cur field.
    cur_ = MakeInitValue().release();
    StoreAtomic();
    inited_.store(true, std::memory_order_release);
  }
}

// Ensures that the lazily initialized data is initialized,
// and returns pointer to the mutex guarding flags data.
absl::Mutex* FlagImpl::DataGuard() const {
  if (ABSL_PREDICT_FALSE(!inited_.load(std::memory_order_acquire))) {
    const_cast<FlagImpl*>(this)->Init();
  }

  // data_guard_ is initialized.
  return reinterpret_cast<absl::Mutex*>(&data_guard_);
}

void FlagImpl::Destroy() {
  {
    absl::MutexLock l(DataGuard());

    // Values are heap allocated for Abseil Flags.
    if (cur_) Delete(op_, cur_);

    // Release the dynamically allocated default value if any.
    if (def_kind_ == FlagDefaultSrcKind::kDynamicValue) {
      Delete(op_, default_src_.dynamic_value);
    }

    // If this flag has an assigned callback, release callback data.
    if (callback_data_) delete callback_data_;
  }

  absl::MutexLock l(&flag_mutex_lifetime_guard);
  DataGuard()->~Mutex();
  is_data_guard_inited_ = false;
}

std::unique_ptr<void, DynValueDeleter> FlagImpl::MakeInitValue() const {
  void* res = nullptr;
  if (def_kind_ == FlagDefaultSrcKind::kDynamicValue) {
    res = Clone(op_, default_src_.dynamic_value);
  } else {
    res = (*default_src_.gen_func)();
  }
  return {res, DynValueDeleter{op_}};
}

absl::string_view FlagImpl::Name() const { return name_; }

std::string FlagImpl::Filename() const {
  return flags_internal::GetUsageConfig().normalize_filename(filename_);
}

std::string FlagImpl::Help() const {
  return help_source_kind_ == FlagHelpSrcKind::kLiteral ? help_.literal
                                                        : help_.gen_func();
}

bool FlagImpl::IsModified() const {
  absl::MutexLock l(DataGuard());
  return modified_;
}

bool FlagImpl::IsSpecifiedOnCommandLine() const {
  absl::MutexLock l(DataGuard());
  return on_command_line_;
}

std::string FlagImpl::DefaultValue() const {
  absl::MutexLock l(DataGuard());

  auto obj = MakeInitValue();
  return Unparse(marshalling_op_, obj.get());
}

std::string FlagImpl::CurrentValue() const {
  absl::MutexLock l(DataGuard());

  return Unparse(marshalling_op_, cur_);
}

void FlagImpl::SetCallback(
    const flags_internal::FlagCallback mutation_callback) {
  absl::MutexLock l(DataGuard());

  if (callback_data_ == nullptr) {
    callback_data_ = new CallbackData;
  }
  callback_data_->func = mutation_callback;

  InvokeCallback();
}

void FlagImpl::InvokeCallback() const {
  if (!callback_data_) return;

  // Make a copy of the C-style function pointer that we are about to invoke
  // before we release the lock guarding it.
  FlagCallback cb = callback_data_->func;

  // If the flag has a mutation callback this function invokes it. While the
  // 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.
  MutexRelock relock(DataGuard());
  absl::MutexLock lock(&callback_data_->guard);
  cb();
}

bool FlagImpl::RestoreState(const void* value, bool modified,
                            bool on_command_line, int64_t counter) {
  {
    absl::MutexLock l(DataGuard());

    if (counter_ == counter) return false;
  }

  Write(value, op_);

  {
    absl::MutexLock l(DataGuard());

    modified_ = modified;
    on_command_line_ = on_command_line;
  }

  return true;
}

// Attempts to parse supplied `value` string using parsing routine in the `flag`
// argument. If parsing successful, this function replaces the dst with newly
// parsed value. In case if any error is encountered in either step, the error
// message is stored in 'err'
bool FlagImpl::TryParse(void** dst, absl::string_view value,
                        std::string* err) const {
  auto tentative_value = MakeInitValue();

  std::string parse_err;
  if (!Parse(marshalling_op_, value, tentative_value.get(), &parse_err)) {
    absl::string_view err_sep = parse_err.empty() ? "" : "; ";
    *err = absl::StrCat("Illegal value '", value, "' specified for flag '",
                        Name(), "'", err_sep, parse_err);
    return false;
  }

  void* old_val = *dst;
  *dst = tentative_value.release();
  tentative_value.reset(old_val);

  return true;
}

void FlagImpl::Read(void* dst, const flags_internal::FlagOpFn dst_op) const {
  absl::ReaderMutexLock l(DataGuard());

  // `dst_op` is the unmarshaling operation corresponding to the declaration
  // visibile at the call site. `op` is the Flag's defined unmarshalling
  // operation. They must match for this operation to be well-defined.
  if (ABSL_PREDICT_FALSE(dst_op != op_)) {
    ABSL_INTERNAL_LOG(
        ERROR,
        absl::StrCat("Flag '", Name(),
                     "' is defined as one type and declared as another"));
  }
  CopyConstruct(op_, cur_, dst);
}

void FlagImpl::StoreAtomic() {
  size_t data_size = Sizeof(op_);

  if (data_size <= sizeof(int64_t)) {
    int64_t t = 0;
    std::memcpy(&t, cur_, data_size);
    atomics_.small_atomic.store(t, std::memory_order_release);
  }
#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
  else if (data_size <= sizeof(FlagsInternalTwoWordsType)) {
    FlagsInternalTwoWordsType t{0, 0};
    std::memcpy(&t, cur_, data_size);
    atomics_.big_atomic.store(t, std::memory_order_release);
  }
#endif
}

void FlagImpl::Write(const void* src, const flags_internal::FlagOpFn src_op) {
  absl::MutexLock l(DataGuard());

  // `src_op` is the marshalling operation corresponding to the declaration
  // visible at the call site. `op` is the Flag's defined marshalling operation.
  // They must match for this operation to be well-defined.
  if (ABSL_PREDICT_FALSE(src_op != op_)) {
    ABSL_INTERNAL_LOG(
        ERROR,
        absl::StrCat("Flag '", Name(),
                     "' is defined as one type and declared as another"));
  }

  if (ShouldValidateFlagValue(op_)) {
    void* obj = Clone(op_, src);
    std::string ignored_error;
    std::string src_as_str = Unparse(marshalling_op_, src);
    if (!Parse(marshalling_op_, src_as_str, obj, &ignored_error)) {
      ABSL_INTERNAL_LOG(ERROR, absl::StrCat("Attempt to set flag '", Name(),
                                            "' to invalid value ", src_as_str));
    }
    Delete(op_, obj);
  }

  modified_ = true;
  counter_++;
  Copy(op_, src, cur_);

  StoreAtomic();
  InvokeCallback();
}

// Sets the value of the flag based on specified string `value`. If the flag
// was successfully set to new value, it returns true. Otherwise, sets `err`
// 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 FlagImpl::SetFromString(absl::string_view value, FlagSettingMode set_mode,
                             ValueSource source, std::string* err) {
  absl::MutexLock l(DataGuard());

  switch (set_mode) {
    case SET_FLAGS_VALUE: {
      // set or modify the flag's value
      if (!TryParse(&cur_, value, err)) return false;
      modified_ = true;
      counter_++;
      StoreAtomic();
      InvokeCallback();

      if (source == kCommandLine) {
        on_command_line_ = true;
      }
      break;
    }
    case SET_FLAG_IF_DEFAULT: {
      // set the flag's value, but only if it hasn't been set by someone else
      if (!modified_) {
        if (!TryParse(&cur_, value, err)) return false;
        modified_ = true;
        counter_++;
        StoreAtomic();
        InvokeCallback();
      } else {
        // TODO(rogeeff): review and fix this semantic. Currently we do not fail
        // in this case if flag is modified. This is misleading since the flag's
        // value is not updated even though we return true.
        // *err = absl::StrCat(Name(), " is already set to ",
        //                     CurrentValue(), "\n");
        // return false;
        return true;
      }
      break;
    }
    case SET_FLAGS_DEFAULT: {
      if (def_kind_ == FlagDefaultSrcKind::kDynamicValue) {
        if (!TryParse(&default_src_.dynamic_value, value, err)) {
          return false;
        }
      } else {
        void* new_default_val = nullptr;
        if (!TryParse(&new_default_val, value, err)) {
          return false;
        }

        default_src_.dynamic_value = new_default_val;
        def_kind_ = FlagDefaultSrcKind::kDynamicValue;
      }

      if (!modified_) {
        // Need to set both default value *and* current, in this case
        Copy(op_, default_src_.dynamic_value, cur_);
        StoreAtomic();
        InvokeCallback();
      }
      break;
    }
  }

  return true;
}

void FlagImpl::CheckDefaultValueParsingRoundtrip() const {
  std::string v = DefaultValue();

  absl::MutexLock lock(DataGuard());

  auto dst = MakeInitValue();
  std::string error;
  if (!flags_internal::Parse(marshalling_op_, v, dst.get(), &error)) {
    ABSL_INTERNAL_LOG(
        FATAL,
        absl::StrCat("Flag ", Name(), " (from ", Filename(),
                     "): std::string form of default value '", v,
                     "' could not be parsed; error=", error));
  }

  // We do not compare dst to def since parsing/unparsing may make
  // small changes, e.g., precision loss for floating point types.
}

bool FlagImpl::ValidateInputValue(absl::string_view value) const {
  absl::MutexLock l(DataGuard());

  auto obj = MakeInitValue();
  std::string ignored_error;
  return flags_internal::Parse(marshalling_op_, value, obj.get(),
                               &ignored_error);
}

}  // namespace flags_internal
ABSL_NAMESPACE_END
}  // namespace absl