Repo created

TMessagesProj/jni/voip/webrtc/absl/flags/internal/flag.cc

@@ -0,0 +1,710 @@
+//
+// 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 <assert.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+
+#include <array>
+#include <atomic>
+#include <cstring>
+#include <memory>
+#include <string>
+#include <typeinfo>
+#include <vector>
+
+#include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/dynamic_annotations.h"
+#include "absl/base/no_destructor.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/flags/config.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/usage_config.h"
+#include "absl/memory/memory.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(FlagFastTypeId flag_type_id) {
+#define DONT_VALIDATE(T, _) \
+  if (flag_type_id == base_internal::FastTypeId<T>()) return false;
+  ABSL_FLAGS_INTERNAL_SUPPORTED_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 is a freelist of leaked flag values and guard for its access.
+// When we can't guarantee it is safe to reuse the memory for flag values,
+// we move the memory to the freelist where it lives indefinitely, so it can
+// still be safely accessed. This also prevents leak checkers from complaining
+// about the leaked memory that can no longer be accessed through any pointer.
+absl::Mutex* FreelistMutex() {
+  static absl::NoDestructor<absl::Mutex> mutex;
+  return mutex.get();
+}
+ABSL_CONST_INIT std::vector<void*>* s_freelist ABSL_GUARDED_BY(FreelistMutex())
+    ABSL_PT_GUARDED_BY(FreelistMutex()) = nullptr;
+
+void AddToFreelist(void* p) {
+  absl::MutexLock l(FreelistMutex());
+  if (!s_freelist) {
+    s_freelist = new std::vector<void*>;
+  }
+  s_freelist->push_back(p);
+}
+
+}  // namespace
+
+///////////////////////////////////////////////////////////////////////////////
+
+uint64_t NumLeakedFlagValues() {
+  absl::MutexLock l(FreelistMutex());
+  return s_freelist == nullptr ? 0u : s_freelist->size();
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Persistent state of the flag data.
+
+class FlagImpl;
+
+class FlagState : public flags_internal::FlagStateInterface {
+ public:
+  template <typename V>
+  FlagState(FlagImpl& flag_impl, const V& v, bool modified,
+            bool on_command_line, int64_t counter)
+      : flag_impl_(flag_impl),
+        value_(v),
+        modified_(modified),
+        on_command_line_(on_command_line),
+        counter_(counter) {}
+
+  ~FlagState() override {
+    if (flag_impl_.ValueStorageKind() != FlagValueStorageKind::kHeapAllocated &&
+        flag_impl_.ValueStorageKind() != FlagValueStorageKind::kSequenceLocked)
+      return;
+    flags_internal::Delete(flag_impl_.op_, value_.heap_allocated);
+  }
+
+ private:
+  friend class FlagImpl;
+
+  // Restores the flag to the saved state.
+  void Restore() const override {
+    if (!flag_impl_.RestoreState(*this)) return;
+
+    ABSL_INTERNAL_LOG(INFO,
+                      absl::StrCat("Restore saved value of ", flag_impl_.Name(),
+                                   " to: ", flag_impl_.CurrentValue()));
+  }
+
+  // Flag and saved flag data.
+  FlagImpl& flag_impl_;
+  union SavedValue {
+    explicit SavedValue(void* v) : heap_allocated(v) {}
+    explicit SavedValue(int64_t v) : one_word(v) {}
+
+    void* heap_allocated;
+    int64_t one_word;
+  } value_;
+  bool modified_;
+  bool on_command_line_;
+  int64_t counter_;
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag implementation, which does not depend on flag value type.
+
+DynValueDeleter::DynValueDeleter(FlagOpFn op_arg) : op(op_arg) {}
+
+void DynValueDeleter::operator()(void* ptr) const {
+  if (op == nullptr) return;
+
+  Delete(op, ptr);
+}
+
+MaskedPointer::MaskedPointer(ptr_t rhs, bool is_candidate) : ptr_(rhs) {
+  if (is_candidate) {
+    ApplyMask(kUnprotectedReadCandidate);
+  }
+}
+
+bool MaskedPointer::IsUnprotectedReadCandidate() const {
+  return CheckMask(kUnprotectedReadCandidate);
+}
+
+bool MaskedPointer::HasBeenRead() const { return CheckMask(kHasBeenRead); }
+
+void MaskedPointer::Set(FlagOpFn op, const void* src, bool is_candidate) {
+  flags_internal::Copy(op, src, Ptr());
+  if (is_candidate) {
+    ApplyMask(kUnprotectedReadCandidate);
+  }
+}
+void MaskedPointer::MarkAsRead() { ApplyMask(kHasBeenRead); }
+
+void MaskedPointer::ApplyMask(mask_t mask) {
+  ptr_ = reinterpret_cast<ptr_t>(reinterpret_cast<mask_t>(ptr_) | mask);
+}
+bool MaskedPointer::CheckMask(mask_t mask) const {
+  return (reinterpret_cast<mask_t>(ptr_) & mask) != 0;
+}
+
+void FlagImpl::Init() {
+  new (&data_guard_) absl::Mutex;
+
+  auto def_kind = static_cast<FlagDefaultKind>(def_kind_);
+
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      alignas(int64_t) std::array<char, sizeof(int64_t)> buf{};
+      if (def_kind == FlagDefaultKind::kGenFunc) {
+        (*default_value_.gen_func)(buf.data());
+      } else {
+        assert(def_kind != FlagDefaultKind::kDynamicValue);
+        std::memcpy(buf.data(), &default_value_, Sizeof(op_));
+      }
+      if (ValueStorageKind() == FlagValueStorageKind::kValueAndInitBit) {
+        // We presume here the memory layout of FlagValueAndInitBit struct.
+        uint8_t initialized = 1;
+        std::memcpy(buf.data() + Sizeof(op_), &initialized,
+                    sizeof(initialized));
+      }
+      // Type can contain valid uninitialized bits, e.g. padding.
+      ABSL_ANNOTATE_MEMORY_IS_INITIALIZED(buf.data(), buf.size());
+      OneWordValue().store(absl::bit_cast<int64_t>(buf),
+                           std::memory_order_release);
+      break;
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      // For this storage kind the default_value_ always points to gen_func
+      // during initialization.
+      assert(def_kind == FlagDefaultKind::kGenFunc);
+      (*default_value_.gen_func)(AtomicBufferValue());
+      break;
+    }
+    case FlagValueStorageKind::kHeapAllocated:
+      // For this storage kind the default_value_ always points to gen_func
+      // during initialization.
+      assert(def_kind == FlagDefaultKind::kGenFunc);
+      // Flag value initially points to the internal buffer.
+      MaskedPointer ptr_value = PtrStorage().load(std::memory_order_acquire);
+      (*default_value_.gen_func)(ptr_value.Ptr());
+      // Default value is a candidate for an unprotected read.
+      PtrStorage().store(MaskedPointer(ptr_value.Ptr(), true),
+                         std::memory_order_release);
+      break;
+  }
+  seq_lock_.MarkInitialized();
+}
+
+absl::Mutex* FlagImpl::DataGuard() const {
+  absl::call_once(const_cast<FlagImpl*>(this)->init_control_, &FlagImpl::Init,
+                  const_cast<FlagImpl*>(this));
+
+  // data_guard_ is initialized inside Init.
+  return reinterpret_cast<absl::Mutex*>(&data_guard_);
+}
+
+void FlagImpl::AssertValidType(FlagFastTypeId rhs_type_id,
+                               const std::type_info* (*gen_rtti)()) const {
+  FlagFastTypeId lhs_type_id = flags_internal::FastTypeId(op_);
+
+  // `rhs_type_id` is the fast type id corresponding to the declaration
+  // visible at the call site. `lhs_type_id` is the fast type id
+  // corresponding to the type specified in flag definition. They must match
+  //  for this operation to be well-defined.
+  if (ABSL_PREDICT_TRUE(lhs_type_id == rhs_type_id)) return;
+
+  const std::type_info* lhs_runtime_type_id =
+      flags_internal::RuntimeTypeId(op_);
+  const std::type_info* rhs_runtime_type_id = (*gen_rtti)();
+
+  if (lhs_runtime_type_id == rhs_runtime_type_id) return;
+
+#ifdef ABSL_INTERNAL_HAS_RTTI
+  if (*lhs_runtime_type_id == *rhs_runtime_type_id) return;
+#endif
+
+  ABSL_INTERNAL_LOG(
+      FATAL, absl::StrCat("Flag '", Name(),
+                          "' is defined as one type and declared as another"));
+}
+
+std::unique_ptr<void, DynValueDeleter> FlagImpl::MakeInitValue() const {
+  void* res = nullptr;
+  switch (DefaultKind()) {
+    case FlagDefaultKind::kDynamicValue:
+      res = flags_internal::Clone(op_, default_value_.dynamic_value);
+      break;
+    case FlagDefaultKind::kGenFunc:
+      res = flags_internal::Alloc(op_);
+      (*default_value_.gen_func)(res);
+      break;
+    default:
+      res = flags_internal::Clone(op_, &default_value_);
+      break;
+  }
+  return {res, DynValueDeleter{op_}};
+}
+
+void FlagImpl::StoreValue(const void* src, ValueSource source) {
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      // Load the current value to avoid setting 'init' bit manually.
+      int64_t one_word_val = OneWordValue().load(std::memory_order_acquire);
+      std::memcpy(&one_word_val, src, Sizeof(op_));
+      OneWordValue().store(one_word_val, std::memory_order_release);
+      seq_lock_.IncrementModificationCount();
+      break;
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      seq_lock_.Write(AtomicBufferValue(), src, Sizeof(op_));
+      break;
+    }
+    case FlagValueStorageKind::kHeapAllocated:
+      MaskedPointer ptr_value = PtrStorage().load(std::memory_order_acquire);
+
+      if (ptr_value.IsUnprotectedReadCandidate() && ptr_value.HasBeenRead()) {
+        // If current value is a candidate for an unprotected read and if it was
+        // already read at least once, follow up reads (if any) are done without
+        // mutex protection. We can't guarantee it is safe to reuse this memory
+        // since it may have been accessed by another thread concurrently, so
+        // instead we move the memory to a freelist so it can still be safely
+        // accessed, and allocate a new one for the new value.
+        AddToFreelist(ptr_value.Ptr());
+        ptr_value = MaskedPointer(Clone(op_, src), source == kCommandLine);
+      } else {
+        // Current value either was set programmatically or was never read.
+        // We can reuse the memory since all accesses to this value (if any)
+        // were protected by mutex. That said, if a new value comes from command
+        // line it now becomes a candidate for an unprotected read.
+        ptr_value.Set(op_, src, source == kCommandLine);
+      }
+
+      PtrStorage().store(ptr_value, std::memory_order_release);
+      seq_lock_.IncrementModificationCount();
+      break;
+  }
+  modified_ = true;
+  InvokeCallback();
+}
+
+absl::string_view FlagImpl::Name() const { return name_; }
+
+absl::string_view FlagImpl::TypeName() const { return type_name_; }
+
+std::string FlagImpl::Filename() const {
+  return flags_internal::GetUsageConfig().normalize_filename(filename_);
+}
+
+std::string FlagImpl::Help() const {
+  return HelpSourceKind() == FlagHelpKind::kLiteral ? help_.literal
+                                                    : help_.gen_func();
+}
+
+FlagFastTypeId FlagImpl::TypeId() const {
+  return flags_internal::FastTypeId(op_);
+}
+
+int64_t FlagImpl::ModificationCount() const {
+  return seq_lock_.ModificationCount();
+}
+
+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 flags_internal::Unparse(op_, obj.get());
+}
+
+std::string FlagImpl::CurrentValue() const {
+  auto* guard = DataGuard();  // Make sure flag initialized
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      const auto one_word_val =
+          absl::bit_cast<std::array<char, sizeof(int64_t)>>(
+              OneWordValue().load(std::memory_order_acquire));
+      return flags_internal::Unparse(op_, one_word_val.data());
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      std::unique_ptr<void, DynValueDeleter> cloned(flags_internal::Alloc(op_),
+                                                    DynValueDeleter{op_});
+      ReadSequenceLockedData(cloned.get());
+      return flags_internal::Unparse(op_, cloned.get());
+    }
+    case FlagValueStorageKind::kHeapAllocated: {
+      absl::MutexLock l(guard);
+      return flags_internal::Unparse(
+          op_, PtrStorage().load(std::memory_order_acquire).Ptr());
+    }
+  }
+
+  return "";
+}
+
+void FlagImpl::SetCallback(const FlagCallbackFunc mutation_callback) {
+  absl::MutexLock l(DataGuard());
+
+  if (callback_ == nullptr) {
+    callback_ = new FlagCallback;
+  }
+  callback_->func = mutation_callback;
+
+  InvokeCallback();
+}
+
+void FlagImpl::InvokeCallback() const {
+  if (!callback_) return;
+
+  // Make a copy of the C-style function pointer that we are about to invoke
+  // before we release the lock guarding it.
+  FlagCallbackFunc cb = callback_->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_->guard);
+  cb();
+}
+
+std::unique_ptr<FlagStateInterface> FlagImpl::SaveState() {
+  absl::MutexLock l(DataGuard());
+
+  bool modified = modified_;
+  bool on_command_line = on_command_line_;
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      return absl::make_unique<FlagState>(
+          *this, OneWordValue().load(std::memory_order_acquire), modified,
+          on_command_line, ModificationCount());
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      void* cloned = flags_internal::Alloc(op_);
+      // Read is guaranteed to be successful because we hold the lock.
+      bool success =
+          seq_lock_.TryRead(cloned, AtomicBufferValue(), Sizeof(op_));
+      assert(success);
+      static_cast<void>(success);
+      return absl::make_unique<FlagState>(*this, cloned, modified,
+                                          on_command_line, ModificationCount());
+    }
+    case FlagValueStorageKind::kHeapAllocated: {
+      return absl::make_unique<FlagState>(
+          *this,
+          flags_internal::Clone(
+              op_, PtrStorage().load(std::memory_order_acquire).Ptr()),
+          modified, on_command_line, ModificationCount());
+    }
+  }
+  return nullptr;
+}
+
+bool FlagImpl::RestoreState(const FlagState& flag_state) {
+  absl::MutexLock l(DataGuard());
+  if (flag_state.counter_ == ModificationCount()) {
+    return false;
+  }
+
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic:
+      StoreValue(&flag_state.value_.one_word, kProgrammaticChange);
+      break;
+    case FlagValueStorageKind::kSequenceLocked:
+    case FlagValueStorageKind::kHeapAllocated:
+      StoreValue(flag_state.value_.heap_allocated, kProgrammaticChange);
+      break;
+  }
+
+  modified_ = flag_state.modified_;
+  on_command_line_ = flag_state.on_command_line_;
+
+  return true;
+}
+
+template <typename StorageT>
+StorageT* FlagImpl::OffsetValue() const {
+  char* p = reinterpret_cast<char*>(const_cast<FlagImpl*>(this));
+  // The offset is deduced via Flag value type specific op_.
+  ptrdiff_t offset = flags_internal::ValueOffset(op_);
+
+  return reinterpret_cast<StorageT*>(p + offset);
+}
+
+std::atomic<uint64_t>* FlagImpl::AtomicBufferValue() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kSequenceLocked);
+  return OffsetValue<std::atomic<uint64_t>>();
+}
+
+std::atomic<int64_t>& FlagImpl::OneWordValue() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kOneWordAtomic ||
+         ValueStorageKind() == FlagValueStorageKind::kValueAndInitBit);
+  return OffsetValue<FlagOneWordValue>()->value;
+}
+
+std::atomic<MaskedPointer>& FlagImpl::PtrStorage() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kHeapAllocated);
+  return OffsetValue<FlagMaskedPointerValue>()->value;
+}
+
+// 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'
+std::unique_ptr<void, DynValueDeleter> FlagImpl::TryParse(
+    absl::string_view value, std::string& err) const {
+  std::unique_ptr<void, DynValueDeleter> tentative_value = MakeInitValue();
+
+  std::string parse_err;
+  if (!flags_internal::Parse(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 nullptr;
+  }
+
+  return tentative_value;
+}
+
+void FlagImpl::Read(void* dst) const {
+  auto* guard = DataGuard();  // Make sure flag initialized
+  switch (ValueStorageKind()) {
+    case FlagValueStorageKind::kValueAndInitBit:
+    case FlagValueStorageKind::kOneWordAtomic: {
+      const int64_t one_word_val =
+          OneWordValue().load(std::memory_order_acquire);
+      std::memcpy(dst, &one_word_val, Sizeof(op_));
+      break;
+    }
+    case FlagValueStorageKind::kSequenceLocked: {
+      ReadSequenceLockedData(dst);
+      break;
+    }
+    case FlagValueStorageKind::kHeapAllocated: {
+      absl::MutexLock l(guard);
+      MaskedPointer ptr_value = PtrStorage().load(std::memory_order_acquire);
+
+      flags_internal::CopyConstruct(op_, ptr_value.Ptr(), dst);
+
+      // For unprotected read candidates, mark that the value as has been read.
+      if (ptr_value.IsUnprotectedReadCandidate() && !ptr_value.HasBeenRead()) {
+        ptr_value.MarkAsRead();
+        PtrStorage().store(ptr_value, std::memory_order_release);
+      }
+      break;
+    }
+  }
+}
+
+int64_t FlagImpl::ReadOneWord() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kOneWordAtomic ||
+         ValueStorageKind() == FlagValueStorageKind::kValueAndInitBit);
+  auto* guard = DataGuard();  // Make sure flag initialized
+  (void)guard;
+  return OneWordValue().load(std::memory_order_acquire);
+}
+
+bool FlagImpl::ReadOneBool() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kValueAndInitBit);
+  auto* guard = DataGuard();  // Make sure flag initialized
+  (void)guard;
+  return absl::bit_cast<FlagValueAndInitBit<bool>>(
+             OneWordValue().load(std::memory_order_acquire))
+      .value;
+}
+
+void FlagImpl::ReadSequenceLockedData(void* dst) const {
+  size_t size = Sizeof(op_);
+  // Attempt to read using the sequence lock.
+  if (ABSL_PREDICT_TRUE(seq_lock_.TryRead(dst, AtomicBufferValue(), size))) {
+    return;
+  }
+  // We failed due to contention. Acquire the lock to prevent contention
+  // and try again.
+  absl::ReaderMutexLock l(DataGuard());
+  bool success = seq_lock_.TryRead(dst, AtomicBufferValue(), size);
+  assert(success);
+  static_cast<void>(success);
+}
+
+void FlagImpl::Write(const void* src) {
+  absl::MutexLock l(DataGuard());
+
+  if (ShouldValidateFlagValue(flags_internal::FastTypeId(op_))) {
+    std::unique_ptr<void, DynValueDeleter> obj{flags_internal::Clone(op_, src),
+                                               DynValueDeleter{op_}};
+    std::string ignored_error;
+    std::string src_as_str = flags_internal::Unparse(op_, src);
+    if (!flags_internal::Parse(op_, src_as_str, obj.get(), &ignored_error)) {
+      ABSL_INTERNAL_LOG(ERROR, absl::StrCat("Attempt to set flag '", Name(),
+                                            "' to invalid value ", src_as_str));
+    }
+  }
+
+  StoreValue(src, kProgrammaticChange);
+}
+
+// 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::ParseFrom(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
+      auto tentative_value = TryParse(value, err);
+      if (!tentative_value) return false;
+
+      StoreValue(tentative_value.get(), source);
+
+      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_) {
+        // 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;
+      }
+      auto tentative_value = TryParse(value, err);
+      if (!tentative_value) return false;
+
+      StoreValue(tentative_value.get(), source);
+      break;
+    }
+    case SET_FLAGS_DEFAULT: {
+      auto tentative_value = TryParse(value, err);
+      if (!tentative_value) return false;
+
+      if (DefaultKind() == FlagDefaultKind::kDynamicValue) {
+        void* old_value = default_value_.dynamic_value;
+        default_value_.dynamic_value = tentative_value.release();
+        tentative_value.reset(old_value);
+      } else {
+        default_value_.dynamic_value = tentative_value.release();
+        def_kind_ = static_cast<uint8_t>(FlagDefaultKind::kDynamicValue);
+      }
+
+      if (!modified_) {
+        // Need to set both default value *and* current, in this case.
+        StoreValue(default_value_.dynamic_value, source);
+        modified_ = false;
+      }
+      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(op_, v, dst.get(), &error)) {
+    ABSL_INTERNAL_LOG(
+        FATAL,
+        absl::StrCat("Flag ", Name(), " (from ", Filename(),
+                     "): 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(op_, value, obj.get(), &ignored_error);
+}
+
+}  // namespace flags_internal
+ABSL_NAMESPACE_END
+}  // namespace absl