about summary refs log tree commit diff
path: root/absl/flags/internal/flag.h
blob: ce0ccf2d31d25dd90016eb039f464a13a3d7dd8e (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
//
// 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_;
};

// 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 (*)();

void InvokeCallback(absl::Mutex* primary_mu, absl::Mutex* callback_mu,
                    FlagCallback cb) ABSL_EXCLUSIVE_LOCKS_REQUIRED(primary_mu);

// 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::InitialValGenFunc initial_value_gen)
      : op_(op),
        marshalling_op_(marshalling_op),
        initial_value_gen_(initial_value_gen) {}

  // Forces destruction of the Flag's data.
  void Destroy() const;

  // Constant access methods
  bool IsModified() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
  bool IsSpecifiedOnCommandLine() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
  std::string DefaultValue() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
  std::string CurrentValue() const ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
  void Read(const CommandLineFlag& flag, void* dst,
            const flags_internal::FlagOpFn dst_op) const
      ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
  // 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(locks_->primary_mu);
  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(locks_->primary_mu);
  bool SetFromString(const CommandLineFlag& flag, absl::string_view value,
                     FlagSettingMode set_mode, ValueSource source,
                     std::string* err) ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
  // If possible, updates copy of the Flag's value that is stored in an
  // atomic word.
  void StoreAtomic() ABSL_EXCLUSIVE_LOCKS_REQUIRED(locks_->primary_mu);

  // Interfaces to operate on callbacks.
  void SetCallback(const flags_internal::FlagCallback mutation_callback)
      ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
  void InvokeCallback() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(locks_->primary_mu);

  // Interfaces to save/restore mutable flag data
  template <typename T>
  std::unique_ptr<flags_internal::FlagStateInterface> SaveState(
      Flag<T>* flag) const ABSL_LOCKS_EXCLUDED(locks_->primary_mu) {
    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(locks_->primary_mu);

  // Value validation interfaces.
  void CheckDefaultValueParsingRoundtrip(const CommandLineFlag& flag) const
      ABSL_LOCKS_EXCLUDED(locks_->primary_mu);
  bool ValidateInputValue(absl::string_view value) const
      ABSL_LOCKS_EXCLUDED(locks_->primary_mu);

 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);

  // Immutable Flag's data.
  const FlagOpFn op_;                          // Type-specific handler
  const FlagMarshallingOpFn marshalling_op_;   // Marshalling ops handler
  const InitialValGenFunc initial_value_gen_;  // Makes flag's initial value

  // Mutable Flag's data. (guarded by locks_->primary_mu).
  // Indicates that locks_, cur_ and def_ fields have been lazily initialized.
  std::atomic<bool> inited_{false};
  // Has flag value been modified?
  bool modified_ ABSL_GUARDED_BY(locks_->primary_mu) = false;
  // Specified on command line.
  bool on_command_line_ ABSL_GUARDED_BY(locks_->primary_mu) = false;
  // Lazily initialized pointer to default value
  void* def_ ABSL_GUARDED_BY(locks_->primary_mu) = nullptr;
  // Lazily initialized pointer to current value
  void* cur_ ABSL_GUARDED_BY(locks_->primary_mu) = nullptr;
  // Mutation counter
  int64_t counter_ ABSL_GUARDED_BY(locks_->primary_mu) = 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::HelpGenFunc help_gen,
                 const char* filename,
                 const flags_internal::FlagMarshallingOpFn marshalling_op,
                 const flags_internal::InitialValGenFunc initial_value_gen)
      : flags_internal::CommandLineFlag(
            name, flags_internal::HelpText::FromFunctionPointer(help_gen),
            filename),
        impl_(&flags_internal::FlagOps<T>, marshalling_op, initial_value_gen) {}

  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
  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_