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// Copyright 2024 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_SYNCHRONIZATION_CANCELABLE_EVENT_H_
#define BASE_SYNCHRONIZATION_CANCELABLE_EVENT_H_
#include "base/base_export.h"
#include "base/compiler_specific.h"
#include "base/time/time.h"
#if BUILDFLAG(IS_WIN)
#include <windows.h>
#elif BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_ANDROID) || BUILDFLAG(IS_CHROMEOS)
#include <semaphore.h>
#else
#include "base/synchronization/waitable_event.h"
#endif
namespace base {
// A CancelableEvent functions as a 0-1 semaphore. It does not start signaled.
// It must not be signaled twice.
//
// Cancel() can only succeed on Windows, Linux, ChromeOS, and Android.
class BASE_EXPORT CancelableEvent {
public:
CancelableEvent();
~CancelableEvent();
// Puts the event in the signaled state. Causes the thread blocked on
// Wait() (if there is one) to be woken up.
void Signal();
// Cancels a signal, if possible. Returns whether canceling a signal was
// successful or not. On success, no thread will wake up. On failure, either
// no signal was sent in the first place, or a waiting thread already consumed
// the signal.
[[nodiscard]] bool Cancel();
// Waits for this event to be Signal()ed until `wait_delta` has elapsed
// (real-time; ignores time overrides). Returns true if Signal() occurs or
// false if `wait_delta` elapses without a Signal().
//
// TimedWait() can synchronise its own destruction.
NOT_TAIL_CALLED bool TimedWait(TimeDelta wait_delta);
void Wait() { TimedWait(TimeDelta::Max()); }
#if BUILDFLAG(IS_WIN)
using NativeHandle = HANDLE;
#elif BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS) || BUILDFLAG(IS_ANDROID)
using NativeHandle = sem_t;
#else
using NativeHandle = WaitableEvent;
#endif
// Declares that this CancelableEvent will only ever be used by a thread that
// is idle at the bottom of its stack and waiting for work (in particular, it
// is not synchronously waiting on this event before resuming ongoing
// work). This is useful to avoid telling base-internals that this thread is
// "blocked" when it's merely idle and ready to do work. As such, this is only
// expected to be used by thread and thread pool impls. In such cases
// wakeup.flow events aren't emitted on `Signal`/`Wait`, because threading
// implementations are responsible for emitting the cause of their wakeup from
// idle.
void declare_only_used_while_idle() { only_used_while_idle_ = true; }
private:
void SignalImpl();
bool CancelImpl();
bool TimedWaitImpl(TimeDelta wait_delta);
bool only_used_while_idle_ = false;
NativeHandle native_handle_;
};
} // namespace base
#endif // BASE_SYNCHRONIZATION_CANCELABLE_EVENT_H_
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