cppcoro/include/cppcoro/shared_task.hpp at master · OSDI/cppcoro · GitHub

Repository navigation

History

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

328

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346

347

348

349

350

351

352

353

354

355

356

357

358

359

360

361

362

363

364

365

366

367

368

369

370

371

372

373

374

375

376

377

378

379

380

381

382

383

384

385

386

387

388

389

390

391

392

393

394

395

396

397

398

399

400

401

402

403

404

405

406

407

408

409

410

411

412

413

414

415

416

417

418

419

420

421

422

423

424

425

426

427

428

429

430

431

432

433

434

435

436

437

438

439

440

441

442

443

444

445

446

447

448

449

450

451

452

453

454

455

456

457

458

459

460

461

462

463

464

465

466

467

468

469

470

471

472

473

474

475

476

477

478

479

480

481

482

483

484

485

486

487

488

489

490

491

492

493

494

495

496

497

498

499

500

501

502

503

504

505

506

507

508

509

510

511

512

513

514

515

516

517

518

519

520

521

522

523

524

525

526

527

528

529

530

531

532

533

534

535

536

537

538

539

540

541

542

543

544

545

546

547

548

549

550

551

552

553

554

555

556

557

558

559

560

561

562

563

564

565

566

567

568

569

570

571

572

573

574

575

576

577

///////////////////////////////////////////////////////////////////////////////

// Copyright (c) Lewis Baker

// Licenced under MIT license. See LICENSE.txt for details.

///////////////////////////////////////////////////////////////////////////////

#ifndef CPPCORO_SHARED_LAZY_TASK_HPP_INCLUDED

#define CPPCORO_SHARED_LAZY_TASK_HPP_INCLUDED

#include <cppcoro/config.hpp>

#include <cppcoro/broken_promise.hpp>

#include <cppcoro/task.hpp>

#include <cppcoro/detail/continuation.hpp>

#include <atomic>

#include <exception>

#include <utility>

#include <type_traits>

#include <experimental/coroutine>

namespace cppcoro

{

template<typename T>

class shared_task;

namespace detail

{

struct shared_task_waiter

{

continuation m_continuation;

shared_task_waiter* m_next;

};

class shared_task_promise_base

{

public:

shared_task_promise_base() noexcept

: m_waiters(&this->m_waiters)

, m_refCount(2)

, m_exception(nullptr)

{}

auto initial_suspend() noexcept

{

return std::experimental::suspend_always{};

}

auto final_suspend() noexcept

{

struct awaitable

{

shared_task_promise_base& m_promise;

awaitable(shared_task_promise_base& promise) noexcept

: m_promise(promise)

{}

bool await_ready() const noexcept

{

return m_promise.m_refCount.load(std::memory_order_acquire) == 1;

}

bool await_suspend(std::experimental::coroutine_handle<>) noexcept

{

return m_promise.m_refCount.fetch_sub(1, std::memory_order_acq_rel) > 1;

}

void await_resume() noexcept

{}

};

// Exchange operation needs to be 'release' so that subsequent awaiters have

// visibility of the result. Also needs to be 'acquire' so we have visibility

// of writes to the waiters list.

void* const valueReadyValue = this;

void* waiters = m_waiters.exchange(valueReadyValue, std::memory_order_acq_rel);

if (waiters != nullptr)

{

shared_task_waiter* waiter = static_cast<shared_task_waiter*>(waiters);

{

// Read the m_next pointer before resuming the coroutine

// since resuming the coroutine may destroy the shared_task_waiter value.

auto* next = waiter->m_next;

waiter->m_continuation.resume();

waiter = next;

} while (waiter != nullptr);

}

return awaitable{ *this };

}

void unhandled_exception() noexcept

{

// No point capturing exception if no more references to the task.

if (m_refCount.load(std::memory_order_relaxed) > 1)

{

m_exception = std::current_exception();

}

bool is_ready() const noexcept

{

const void* const valueReadyValue = this;

return m_waiters.load(std::memory_order_acquire) == valueReadyValue;

}

void add_ref() noexcept

{

m_refCount.fetch_add(1, std::memory_order_relaxed);

}

/// Decrement the reference count.

///

/// \return

/// true if successfully detached, false if this was the last

/// reference to the coroutine, in which case the caller must

/// call destroy() on the coroutine handle.

bool try_detach() noexcept

{

return m_refCount.fetch_sub(1, std::memory_order_acq_rel) > 1;

}

/// Try to enqueue a waiter to the list of waiters.

///

/// \param waiter

/// Pointer to the state from the waiter object.

/// Must have waiter->m_coroutine member populated with the coroutine

/// handle of the awaiting coroutine.

///

/// \param coroutine

/// Coroutine handle for this promise object.

///

/// \return

/// true if the waiter was successfully queued, in which case

/// waiter->m_coroutine will be resumed when the task completes.

/// false if the coroutine was already completed and the awaiting

/// coroutine can continue without suspending.

bool try_await(shared_task_waiter* waiter, std::experimental::coroutine_handle<> coroutine)

{

void* const valueReadyValue = this;

void* const notStartedValue = &this->m_waiters;

constexpr void* startedNoWaitersValue = static_cast<shared_task_waiter*>(nullptr);

// NOTE: If the coroutine is not yet started then the first waiter

// will start the coroutine before enqueuing itself up to the list

// of suspended waiters waiting for completion. We split this into

// two steps to allow the first awaiter to return without suspending.

// This avoids recursively resuming the first waiter inside the call to

// coroutine.resume() in the case that the coroutine completes

// synchronously, which could otherwise lead to stack-overflow if

// the awaiting coroutine awaited many synchronously-completing

// tasks in a row.

// Start the coroutine if not already started.

void* oldWaiters = m_waiters.load(std::memory_order_acquire);

if (oldWaiters == notStartedValue &&

m_waiters.compare_exchange_strong(oldWaiters, startedNoWaitersValue, std::memory_order_relaxed))

{

// Start the task executing.

coroutine.resume();

oldWaiters = m_waiters.load(std::memory_order_acquire);

}

// Enqueue the waiter into the list of waiting coroutines.

{

if (oldWaiters == valueReadyValue)

{

// Coroutine already completed, don't suspend.

return false;

}

waiter->m_next = static_cast<shared_task_waiter*>(oldWaiters);

} while (!m_waiters.compare_exchange_weak(

oldWaiters,

static_cast<void*>(waiter),

std::memory_order_release,

std::memory_order_acquire));

return true;

}

protected:

bool completed_with_unhandled_exception()

{

return m_exception != nullptr;

}

void rethrow_if_unhandled_exception()

{

if (m_exception != nullptr)

{

std::rethrow_exception(m_exception);

}

private:

std::atomic<std::uint32_t> m_refCount;

// Value is either

// - nullptr - indicates started, no waiters

// - this - indicates value is ready

// - &this->m_waiters - indicates coroutine not started

// - other - pointer to head item in linked-list of waiters.

// values are of type 'cppcoro::shared_task_waiter'.

// indicates that the coroutine has been started.

std::atomic<void*> m_waiters;

std::exception_ptr m_exception;

};

template<typename T>

class shared_task_promise : public shared_task_promise_base

{

public:

shared_task_promise() noexcept = default;

~shared_task_promise()

{

if (this->is_ready() && !this->completed_with_unhandled_exception())

{

reinterpret_cast<T*>(&m_valueStorage)->~T();

}

shared_task<T> get_return_object() noexcept;

template<

typename VALUE,

typename = std::enable_if_t<std::is_convertible_v<VALUE&&, T>>>

void return_value(VALUE&& value)

noexcept(std::is_nothrow_constructible_v<T, VALUE&&>)

{

new (&m_valueStorage) T(std::forward<VALUE>(value));

}

T& result()

{

this->rethrow_if_unhandled_exception();

return *reinterpret_cast<T*>(&m_valueStorage);

}

private:

// Not using std::aligned_storage here due to bug in MSVC 2015 Update 2

// that means it doesn't work for types with alignof(T) > 8.

// See MS-Connect bug #2658635.

alignas(T) char m_valueStorage[sizeof(T)];

};

template<>

class shared_task_promise<void> : public shared_task_promise_base

{

public:

shared_task_promise() noexcept = default;

shared_task<void> get_return_object() noexcept;

void return_void() noexcept

{}

void result()

{

this->rethrow_if_unhandled_exception();

}

};

template<typename T>

class shared_task_promise<T&> : public shared_task_promise_base

{

public:

shared_task_promise() noexcept = default;

shared_task<T&> get_return_object() noexcept;

void return_value(T& value) noexcept

{

m_value = std::addressof(value);

}

T& result()

{

this->rethrow_if_unhandled_exception();

return *m_value;

}

private:

T* m_value;

};

}

template<typename T = void>

class shared_task

{

public:

using promise_type = detail::shared_task_promise<T>;

using value_type = T;

private:

struct awaitable_base

{

std::experimental::coroutine_handle<promise_type> m_coroutine;

detail::shared_task_waiter m_waiter;

awaitable_base(std::experimental::coroutine_handle<promise_type> coroutine) noexcept

: m_coroutine(coroutine)

{}

bool await_ready() const noexcept

{

return !m_coroutine || m_coroutine.promise().is_ready();

}

bool await_suspend(std::experimental::coroutine_handle<> awaiter) noexcept

{

m_waiter.m_continuation = detail::continuation{ awaiter };

return m_coroutine.promise().try_await(&m_waiter, m_coroutine);

}

};

public:

shared_task() noexcept

: m_coroutine(nullptr)

{}

explicit shared_task(std::experimental::coroutine_handle<promise_type> coroutine)

: m_coroutine(coroutine)

{

// Don't increment the ref-count here since it has already been

// initialised to 2 (one for shared_task and one for coroutine)

// in the shared_task_promise constructor.

}

shared_task(shared_task&& other) noexcept

: m_coroutine(other.m_coroutine)

{

other.m_coroutine = nullptr;

}

shared_task(const shared_task& other) noexcept

: m_coroutine(other.m_coroutine)

{

if (m_coroutine)

{

m_coroutine.promise().add_ref();

}

~shared_task()

{

destroy();

}

shared_task& operator=(shared_task&& other) noexcept

{

if (&other != this)

{

destroy();

m_coroutine = other.m_coroutine;

other.m_coroutine = nullptr;

}

return *this;

}

shared_task& operator=(const shared_task& other) noexcept

{

if (m_coroutine != other.m_coroutine)

{

destroy();

m_coroutine = other.m_coroutine;

if (m_coroutine)

{

m_coroutine.promise().add_ref();

}

return *this;

}

void swap(shared_task& other) noexcept

{

std::swap(m_coroutine, other.m_coroutine);

}

/// \brief

/// Query if the task result is complete.

///

/// Awaiting a task that is ready will not block.

bool is_ready() const noexcept

{

return !m_coroutine || m_coroutine.promise().is_ready();

}

auto operator co_await() const noexcept

{

struct awaitable : awaitable_base

{

using awaitable_base::awaitable_base;

decltype(auto) await_resume()

{

if (!this->m_coroutine)

{

throw broken_promise{};

}

return this->m_coroutine.promise().result();

}

};

return awaitable{ m_coroutine };

}

/// \brief

/// Returns an awaitable that will await completion of the task without

/// attempting to retrieve the result.

auto when_ready() const noexcept

{

struct awaitable : awaitable_base

{

using awaitable_base::awaitable_base;

void await_resume() const noexcept {}

};

return awaitable{ m_coroutine };

}

auto get_starter() const noexcept

{

struct starter

{

public:

explicit starter(std::experimental::coroutine_handle<promise_type> coroutine)

: m_coroutine(coroutine)

{}

void start(detail::continuation c) noexcept

{

m_waiter.m_continuation = c;

if (!m_coroutine ||

m_coroutine.promise().is_ready() ||

!m_coroutine.promise().try_await(&m_waiter, m_coroutine))

{

// Task completed synchronously, resume immediately.

c.resume();

}

private:

std::experimental::coroutine_handle<promise_type> m_coroutine;

detail::shared_task_waiter m_waiter;

};

return starter{ m_coroutine };

}

private:

template<typename U>

friend bool operator==(const shared_task<U>&, const shared_task<U>&) noexcept;

void destroy() noexcept

{

if (m_coroutine)

{

if (!m_coroutine.promise().try_detach())

{

m_coroutine.destroy();

}

std::experimental::coroutine_handle<promise_type> m_coroutine;

};

template<typename T>

bool operator==(const shared_task<T>& lhs, const shared_task<T>& rhs) noexcept

{

return lhs.m_coroutine == rhs.m_coroutine;

}

template<typename T>

bool operator!=(const shared_task<T>& lhs, const shared_task<T>& rhs) noexcept

{

return !(lhs == rhs);

}

template<typename T>

void swap(shared_task<T>& a, shared_task<T>& b) noexcept

{

a.swap(b);

}

namespace detail

{

template<typename T>

shared_task<T> shared_task_promise<T>::get_return_object() noexcept

{

return shared_task<T>{

std::experimental::coroutine_handle<shared_task_promise>::from_promise(*this)

};

}

template<typename T>

shared_task<T&> shared_task_promise<T&>::get_return_object() noexcept

{

return shared_task<T&>{

std::experimental::coroutine_handle<shared_task_promise>::from_promise(*this)

};

}

inline shared_task<void> shared_task_promise<void>::get_return_object() noexcept

{

return shared_task<void>{

std::experimental::coroutine_handle<shared_task_promise>::from_promise(*this)

};

}

template<typename T>

shared_task<T> make_shared_task(task<T> t)

{

co_return co_await std::move(t);

}

#if defined(_MSC_VER) && _MSC_FULL_VER <= 191025019 || CPPCORO_COMPILER_CLANG

// HACK: Workaround for broken MSVC that doesn't execute <expr> in 'co_return <expr>;'.

inline shared_task<void> make_shared_task(task<void> t)

{

co_await t;

}

#endif

// Note: We yield a task<> when applying fmap() operator to a shared_task<> since

// it's not necessarily the case that because the source task was shared that the

// result will be used in a shared context. So we choose to return a task<> which

// generally has less overhead than a shared_task<>.

template<typename FUNC, typename T>

task<std::result_of_t<FUNC&&(T&)>> fmap(FUNC func, shared_task<T> task)

{

co_return std::invoke(std::move(func), co_await std::move(task));

}

template<typename FUNC>

task<std::result_of_t<FUNC&&()>> fmap(FUNC func, shared_task<void> task)

{

co_await task;

co_return std::invoke(std::move(func));

}

#endif

cppcoro/include/cppcoro/shared_task.hpp at master · OSDI/cppcoro · GitHub

Repository navigation

Expand file tree

Breadcrumbs

shared_task.hpp

Latest commit

History

Breadcrumbs

shared_task.hpp

File metadata and controls