base/synchronization/waitable_event_posix.cc

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/synchronization/waitable_event.h"

#include <stddef.h>

#include <limits>
#include <optional>
#include <vector>

#include "base/check_op.h"
#include "base/memory/stack_allocated.h"
#include "base/ranges/algorithm.h"
#include "base/synchronization/condition_variable.h"
#include "base/synchronization/lock.h"
#include "base/threading/scoped_blocking_call.h"
#include "base/threading/thread_restrictions.h"
#include "base/time/time.h"
#include "base/time/time_override.h"

// -----------------------------------------------------------------------------
// A WaitableEvent on POSIX is implemented as a wait-list. Currently we don't
// support cross-process events (where one process can signal an event which
// others are waiting on). Because of this, we can avoid having one thread per
// listener in several cases.
//
// The WaitableEvent maintains a list of waiters, protected by a lock. Each
// waiter is either an async wait, in which case we have a Task and the
// MessageLoop to run it on, or a blocking wait, in which case we have the
// condition variable to signal.
//
// Waiting involves grabbing the lock and adding oneself to the wait list. Async
// waits can be canceled, which means grabbing the lock and removing oneself
// from the list.
//
// Waiting on multiple events is handled by adding a single, synchronous wait to
// the wait-list of many events. An event passes a pointer to itself when
// firing a waiter and so we can store that pointer to find out which event
// triggered.
// -----------------------------------------------------------------------------

namespace base {

// -----------------------------------------------------------------------------
// This is just an abstract base class for waking the two types of waiters
// -----------------------------------------------------------------------------
WaitableEvent::WaitableEvent(ResetPolicy reset_policy,
                             InitialState initial_state)
    : kernel_(new WaitableEventKernel(reset_policy, initial_state)) {}

void WaitableEvent::Reset() {
  base::AutoLock locked(kernel_->lock_);
  kernel_->signaled_ = false;
}

void WaitableEvent::SignalImpl() {
  base::AutoLock locked(kernel_->lock_);

  if (kernel_->signaled_)
    return;

  if (kernel_->manual_reset_) {
    SignalAll();
    kernel_->signaled_ = true;
  } else {
    // In the case of auto reset, if no waiters were woken, we remain
    // signaled.
    if (!SignalOne())
      kernel_->signaled_ = true;
  }
}

bool WaitableEvent::IsSignaled() {
  base::AutoLock locked(kernel_->lock_);

  const bool result = kernel_->signaled_;
  if (result && !kernel_->manual_reset_)
    kernel_->signaled_ = false;
  return result;
}

// -----------------------------------------------------------------------------
// Synchronous waits

// -----------------------------------------------------------------------------
// This is a synchronous waiter. The thread is waiting on the given condition
// variable and the fired flag in this object.
// -----------------------------------------------------------------------------
class SyncWaiter : public WaitableEvent::Waiter {
  STACK_ALLOCATED();

 public:
  SyncWaiter()
      : fired_(false), signaling_event_(nullptr), lock_(), cv_(&lock_) {}

  bool Fire(WaitableEvent* signaling_event) override {
    base::AutoLock locked(lock_);

    if (fired_)
      return false;

    fired_ = true;
    signaling_event_ = signaling_event;

    cv_.Broadcast();

    // Unlike AsyncWaiter objects, SyncWaiter objects are stack-allocated on
    // the blocking thread's stack.  There is no |delete this;| in Fire.  The
    // SyncWaiter object is destroyed when it goes out of scope.

    return true;
  }

  WaitableEvent* signaling_event() const {
    return signaling_event_;
  }

  // ---------------------------------------------------------------------------
  // These waiters are always stack allocated and don't delete themselves. Thus
  // there's no problem and the ABA tag is the same as the object pointer.
  // ---------------------------------------------------------------------------
  bool Compare(void* tag) override { return this == tag; }

  // ---------------------------------------------------------------------------
  // Called with lock held.
  // ---------------------------------------------------------------------------
  bool fired() const {
    return fired_;
  }

  // ---------------------------------------------------------------------------
  // During a TimedWait, we need a way to make sure that an auto-reset
  // WaitableEvent doesn't think that this event has been signaled between
  // unlocking it and removing it from the wait-list. Called with lock held.
  // ---------------------------------------------------------------------------
  void Disable() {
    fired_ = true;
  }

  base::Lock* lock() {
    return &lock_;
  }

  base::ConditionVariable* cv() {
    return &cv_;
  }

 private:
  bool fired_;
  WaitableEvent* signaling_event_;  // The WaitableEvent which woke us
  base::Lock lock_;
  base::ConditionVariable cv_;
};

bool WaitableEvent::TimedWaitImpl(TimeDelta wait_delta) {
  kernel_->lock_.Acquire();
  if (kernel_->signaled_) {
    if (!kernel_->manual_reset_) {
      // In this case we were signaled when we had no waiters. Now that
      // someone has waited upon us, we can automatically reset.
      kernel_->signaled_ = false;
    }

    kernel_->lock_.Release();
    return true;
  }

  SyncWaiter sw;
  if (only_used_while_idle_) {
    sw.cv()->declare_only_used_while_idle();
  }
  sw.lock()->Acquire();

  Enqueue(&sw);
  kernel_->lock_.Release();
  // We are violating locking order here by holding the SyncWaiter lock but not
  // the WaitableEvent lock. However, this is safe because we don't lock |lock_|
  // again before unlocking it.

  // TimeTicks takes care of overflow but we special case is_max() nonetheless
  // to avoid invoking TimeTicksNowIgnoringOverride() unnecessarily (same for
  // the increment step of the for loop if the condition variable returns
  // early). Ref: https://crbug.com/910524#c7
  const TimeTicks end_time =
      wait_delta.is_max() ? TimeTicks::Max()
                          : subtle::TimeTicksNowIgnoringOverride() + wait_delta;
  for (TimeDelta remaining = wait_delta; remaining.is_positive() && !sw.fired();
       remaining = end_time.is_max()
                       ? TimeDelta::Max()
                       : end_time - subtle::TimeTicksNowIgnoringOverride()) {
    if (end_time.is_max())
      sw.cv()->Wait();
    else
      sw.cv()->TimedWait(remaining);
  }

  // Get the SyncWaiter signaled state before releasing the lock.
  const bool return_value = sw.fired();

  // We can't acquire |lock_| before releasing the SyncWaiter lock (because of
  // locking order), however, in between the two a signal could be fired and
  // |sw| would accept it, however we will still return false, so the signal
  // would be lost on an auto-reset WaitableEvent. Thus we call Disable which
  // makes sw::Fire return false.
  sw.Disable();
  sw.lock()->Release();

  // This is a bug that has been enshrined in the interface of WaitableEvent
  // now: |Dequeue| is called even when |sw.fired()| is true, even though it'll
  // always return false in that case. However, taking the lock ensures that
  // |Signal| has completed before we return and means that a WaitableEvent can
  // synchronise its own destruction.
  kernel_->lock_.Acquire();
  kernel_->Dequeue(&sw, &sw);
  kernel_->lock_.Release();

  return return_value;
}

// -----------------------------------------------------------------------------
// Synchronous waiting on multiple objects.

static bool  // StrictWeakOrdering
cmp_fst_addr(const std::pair<WaitableEvent*, unsigned> &a,
             const std::pair<WaitableEvent*, unsigned> &b) {
  return a.first < b.first;
}

// static
// NO_THREAD_SAFETY_ANALYSIS: Complex control flow.
size_t WaitableEvent::WaitManyImpl(WaitableEvent** raw_waitables,
                                   size_t count) NO_THREAD_SAFETY_ANALYSIS {
  // We need to acquire the locks in a globally consistent order. Thus we sort
  // the array of waitables by address. We actually sort a pairs so that we can
  // map back to the original index values later.
  std::vector<std::pair<WaitableEvent*, size_t> > waitables;
  waitables.reserve(count);
  for (size_t i = 0; i < count; ++i)
    waitables.push_back(std::make_pair(raw_waitables[i], i));

  DCHECK_EQ(count, waitables.size());

  ranges::sort(waitables, cmp_fst_addr);

  // The set of waitables must be distinct. Since we have just sorted by
  // address, we can check this cheaply by comparing pairs of consecutive
  // elements.
  for (size_t i = 0; i < waitables.size() - 1; ++i) {
    DCHECK(waitables[i].first != waitables[i+1].first);
  }

  SyncWaiter sw;

  const size_t r = EnqueueMany(&waitables[0], count, &sw);
  if (r < count) {
    // One of the events is already signaled. The SyncWaiter has not been
    // enqueued anywhere.
    return waitables[r].second;
  }

  // At this point, we hold the locks on all the WaitableEvents and we have
  // enqueued our waiter in them all.
  sw.lock()->Acquire();
    // Release the WaitableEvent locks in the reverse order
    for (size_t i = 0; i < count; ++i) {
      waitables[count - (1 + i)].first->kernel_->lock_.Release();
    }

    for (;;) {
      if (sw.fired())
        break;

      sw.cv()->Wait();
    }
  sw.lock()->Release();

  // The address of the WaitableEvent which fired is stored in the SyncWaiter.
  WaitableEvent *const signaled_event = sw.signaling_event();
  // This will store the index of the raw_waitables which fired.
  size_t signaled_index = 0;

  // Take the locks of each WaitableEvent in turn (except the signaled one) and
  // remove our SyncWaiter from the wait-list
  for (size_t i = 0; i < count; ++i) {
    if (raw_waitables[i] != signaled_event) {
      raw_waitables[i]->kernel_->lock_.Acquire();
        // There's no possible ABA issue with the address of the SyncWaiter here
        // because it lives on the stack. Thus the tag value is just the pointer
        // value again.
        raw_waitables[i]->kernel_->Dequeue(&sw, &sw);
      raw_waitables[i]->kernel_->lock_.Release();
    } else {
      // By taking this lock here we ensure that |Signal| has completed by the
      // time we return, because |Signal| holds this lock. This matches the
      // behaviour of |Wait| and |TimedWait|.
      raw_waitables[i]->kernel_->lock_.Acquire();
      raw_waitables[i]->kernel_->lock_.Release();
      signaled_index = i;
    }
  }

  return signaled_index;
}

// -----------------------------------------------------------------------------
// If return value == count:
//   The locks of the WaitableEvents have been taken in order and the Waiter has
//   been enqueued in the wait-list of each. None of the WaitableEvents are
//   currently signaled
// else:
//   None of the WaitableEvent locks are held. The Waiter has not been enqueued
//   in any of them and the return value is the index of the WaitableEvent which
//   was signaled with the lowest input index from the original WaitMany call.
// -----------------------------------------------------------------------------
// static
// NO_THREAD_SAFETY_ANALYSIS: Complex control flow.
size_t WaitableEvent::EnqueueMany(std::pair<WaitableEvent*, size_t>* waitables,
                                  size_t count,
                                  Waiter* waiter) NO_THREAD_SAFETY_ANALYSIS {
  size_t winner = count;
  size_t winner_index = count;
  for (size_t i = 0; i < count; ++i) {
    auto& kernel = waitables[i].first->kernel_;
    kernel->lock_.Acquire();
    if (kernel->signaled_ && waitables[i].second < winner) {
      winner = waitables[i].second;
      winner_index = i;
    }
  }

  // No events signaled. All locks acquired. Enqueue the Waiter on all of them
  // and return.
  if (winner == count) {
    for (size_t i = 0; i < count; ++i)
      waitables[i].first->Enqueue(waiter);
    return count;
  }

  // Unlock in reverse order and possibly clear the chosen winner's signal
  // before returning its index.
  for (auto* w = waitables + count - 1; w >= waitables; --w) {
    auto& kernel = w->first->kernel_;
    if (w->second == winner) {
      if (!kernel->manual_reset_)
        kernel->signaled_ = false;
    }
    kernel->lock_.Release();
  }

  return winner_index;
}

// -----------------------------------------------------------------------------


// -----------------------------------------------------------------------------
// Private functions...

WaitableEvent::WaitableEventKernel::WaitableEventKernel(
    ResetPolicy reset_policy,
    InitialState initial_state)
    : manual_reset_(reset_policy == ResetPolicy::MANUAL),
      signaled_(initial_state == InitialState::SIGNALED) {}

WaitableEvent::WaitableEventKernel::~WaitableEventKernel() = default;

// -----------------------------------------------------------------------------
// Wake all waiting waiters. Called with lock held.
// -----------------------------------------------------------------------------
bool WaitableEvent::SignalAll() {
  bool signaled_at_least_one = false;

  for (Waiter* i : kernel_->waiters_) {
    if (i->Fire(this))
      signaled_at_least_one = true;
  }

  kernel_->waiters_.clear();
  return signaled_at_least_one;
}

// ---------------------------------------------------------------------------
// Try to wake a single waiter. Return true if one was woken. Called with lock
// held.
// ---------------------------------------------------------------------------
bool WaitableEvent::SignalOne() {
  for (;;) {
    if (kernel_->waiters_.empty())
      return false;

    const bool r = (*kernel_->waiters_.begin())->Fire(this);
    kernel_->waiters_.pop_front();
    if (r)
      return true;
  }
}

// -----------------------------------------------------------------------------
// Add a waiter to the list of those waiting. Called with lock held.
// -----------------------------------------------------------------------------
void WaitableEvent::Enqueue(Waiter* waiter) {
  kernel_->waiters_.push_back(waiter);
}

// -----------------------------------------------------------------------------
// Remove a waiter from the list of those waiting. Return true if the waiter was
// actually removed. Called with lock held.
// -----------------------------------------------------------------------------
bool WaitableEvent::WaitableEventKernel::Dequeue(Waiter* waiter, void* tag) {
  for (auto i = waiters_.begin(); i != waiters_.end(); ++i) {
    if (*i == waiter && (*i)->Compare(tag)) {
      waiters_.erase(i);
      return true;
    }
  }

  return false;
}

// -----------------------------------------------------------------------------

}  // namespace base