base/message_loop/message_pump_epoll.cc

// Copyright 2022 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/message_loop/message_pump_epoll.h"

#include <sys/epoll.h>
#include <sys/eventfd.h>

#include <cstddef>
#include <cstdint>
#include <utility>

#include "base/auto_reset.h"
#include "base/check_op.h"
#include "base/memory/ref_counted.h"
#include "base/posix/eintr_wrapper.h"
#include "base/ranges/algorithm.h"
#include "base/threading/thread_checker.h"
#include "base/trace_event/base_tracing.h"
#include "third_party/abseil-cpp/absl/types/optional.h"

namespace base {

MessagePumpEpoll::MessagePumpEpoll() {
  epoll_.reset(epoll_create1(/*flags=*/0));
  PCHECK(epoll_.is_valid());

  wake_event_.reset(eventfd(0, EFD_NONBLOCK));
  PCHECK(wake_event_.is_valid());

  epoll_event wake{.events = EPOLLIN, .data = {.ptr = &wake_event_}};
  int rv = epoll_ctl(epoll_.get(), EPOLL_CTL_ADD, wake_event_.get(), &wake);
  PCHECK(rv == 0);
}

MessagePumpEpoll::~MessagePumpEpoll() = default;

bool MessagePumpEpoll::WatchFileDescriptor(int fd,
                                           bool persistent,
                                           int mode,
                                           FdWatchController* controller,
                                           FdWatcher* watcher) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
  TRACE_EVENT("base", "MessagePumpEpoll::WatchFileDescriptor", "fd", fd,
              "persistent", persistent, "watch_read", mode & WATCH_READ,
              "watch_write", mode & WATCH_WRITE);

  const InterestParams params{
      .fd = fd,
      .read = (mode == WATCH_READ || mode == WATCH_READ_WRITE),
      .write = (mode == WATCH_WRITE || mode == WATCH_READ_WRITE),
      .one_shot = !persistent,
  };

  auto [it, is_new_fd_entry] = entries_.emplace(fd, fd);
  EpollEventEntry& entry = it->second;
  scoped_refptr<Interest> existing_interest = controller->epoll_interest();
  if (existing_interest && existing_interest->params().IsEqual(params)) {
    // WatchFileDescriptor() has already been called for this controller at
    // least once before, and as in the most common cases, it is now being
    // called again with the same parameters.
    //
    // We don't need to allocate and register a new Interest in this case, but
    // we can instead reactivate the existing (presumably deactivated,
    // non-persistent) Interest.
    existing_interest->set_active(true);
  } else {
    entry.interests.push_back(controller->AssignEpollInterest(params));
    if (existing_interest) {
      UnregisterInterest(existing_interest);
    }
  }

  if (is_new_fd_entry) {
    AddEpollEvent(entry);
  } else {
    UpdateEpollEvent(entry);
  }

  controller->set_epoll_pump(weak_ptr_factory_.GetWeakPtr());
  controller->set_watcher(watcher);
  return true;
}

void MessagePumpEpoll::Run(Delegate* delegate) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
  RunState run_state(delegate);
  AutoReset<RunState*> auto_reset_run_state(&run_state_, &run_state);
  for (;;) {
    // Do some work and see if the next task is ready right away.
    Delegate::NextWorkInfo next_work_info = delegate->DoWork();
    const bool immediate_work_available = next_work_info.is_immediate();
    if (run_state.should_quit) {
      break;
    }

    // Process any immediately ready IO event, but don't wait for more yet.
    const bool processed_events = WaitForEpollEvents(TimeDelta(), delegate);
    if (run_state.should_quit) {
      break;
    }

    if (immediate_work_available || processed_events) {
      continue;
    }

    const bool did_idle_work = delegate->DoIdleWork();
    if (run_state.should_quit) {
      break;
    }
    if (did_idle_work) {
      continue;
    }

    TimeDelta timeout = TimeDelta::Max();
    DCHECK(!next_work_info.delayed_run_time.is_null());
    if (!next_work_info.delayed_run_time.is_max()) {
      timeout = next_work_info.remaining_delay();
    }
    delegate->BeforeWait();
    WaitForEpollEvents(timeout, delegate);
    if (run_state.should_quit) {
      break;
    }
  }
}

void MessagePumpEpoll::Quit() {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
  DCHECK(run_state_) << "Quit() called outside of Run()";
  run_state_->should_quit = true;
}

void MessagePumpEpoll::ScheduleWork() {
  const uint64_t value = 1;
  ssize_t n = HANDLE_EINTR(write(wake_event_.get(), &value, sizeof(value)));

  // EAGAIN here implies that the write() would overflow of the event counter,
  // which is a condition we can safely ignore. It implies that the event
  // counter is non-zero and therefore readable, which is enough to ensure that
  // any pending wait eventually wakes up.
  DPCHECK(n == sizeof(value) || errno == EAGAIN);
}

void MessagePumpEpoll::ScheduleDelayedWork(
    const Delegate::NextWorkInfo& next_work_info) {
  // Nothing to do. This can only be called from the same thread as Run(), so
  // the pump must be in between waits. The scheduled work therefore will be
  // seen in time for the next wait.
}

void MessagePumpEpoll::AddEpollEvent(EpollEventEntry& entry) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
  const uint32_t events = entry.ComputeActiveEvents();
  epoll_event event{.events = events, .data = {.ptr = &entry}};
  int rv = epoll_ctl(epoll_.get(), EPOLL_CTL_ADD, entry.fd, &event);
  DPCHECK(rv == 0);
  entry.registered_events = events;
}

void MessagePumpEpoll::UpdateEpollEvent(EpollEventEntry& entry) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
  const uint32_t events = entry.ComputeActiveEvents();
  if (events == entry.registered_events && !(events & EPOLLONESHOT)) {
    // Persistent events don't need to be modified if no bits are changing.
    return;
  }
  epoll_event event{.events = events, .data = {.ptr = &entry}};
  int rv = epoll_ctl(epoll_.get(), EPOLL_CTL_MOD, entry.fd, &event);
  DPCHECK(rv == 0);
  entry.registered_events = events;
}

void MessagePumpEpoll::UnregisterInterest(
    const scoped_refptr<Interest>& interest) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);

  const int fd = interest->params().fd;
  auto entry_it = entries_.find(fd);
  DCHECK(entry_it != entries_.end());

  EpollEventEntry& entry = entry_it->second;
  auto& interests = entry.interests;
  auto* it = ranges::find(interests, interest);
  DCHECK(it != interests.end());
  interests.erase(it);

  if (interests.empty()) {
    entries_.erase(entry_it);
    int rv = epoll_ctl(epoll_.get(), EPOLL_CTL_DEL, fd, nullptr);
    DPCHECK(rv == 0);
  } else {
    UpdateEpollEvent(entry);
  }
}

bool MessagePumpEpoll::WaitForEpollEvents(TimeDelta timeout,
                                          Delegate* delegate) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);

  // `timeout` has microsecond resolution, but timeouts accepted by epoll_wait()
  // are integral milliseconds. Round up to the next millisecond.
  // TODO(https://crbug.com/1382894): Consider higher-resolution timeouts.
  const int epoll_timeout =
      timeout.is_max() ? -1
                       : saturated_cast<int>(timeout.InMillisecondsRoundedUp());
  epoll_event events[16];
  const int epoll_result =
      epoll_wait(epoll_.get(), events, std::size(events), epoll_timeout);
  if (epoll_result < 0) {
    DPCHECK(errno == EINTR);
    return false;
  }

  if (epoll_result == 0) {
    return false;
  }

  delegate->BeginNativeWorkBeforeDoWork();
  const base::span<epoll_event> ready_events(events,
                                             static_cast<size_t>(epoll_result));
  for (auto& e : ready_events) {
    if (e.data.ptr == &wake_event_) {
      // Wake-up events are always safe to handle immediately. Unlike other
      // events used by MessagePumpEpoll they also don't point to an
      // EpollEventEntry, so we handle them separately here.
      HandleWakeUp();
      e.data.ptr = nullptr;
      continue;
    }

    // To guard against one of the ready events unregistering and thus
    // invalidating one of the others here, first link each entry to the
    // corresponding epoll_event returned by epoll_wait(). We do this before
    // dispatching any events, and the second pass below will only dispatch an
    // event if its epoll_event data is still valid.
    auto& entry = EpollEventEntry::FromEpollEvent(e);
    DCHECK(!entry.active_event);
    EpollEventEntry::FromEpollEvent(e).active_event = &e;
  }

  for (auto& e : ready_events) {
    if (e.data.ptr) {
      auto& entry = EpollEventEntry::FromEpollEvent(e);
      entry.active_event = nullptr;
      OnEpollEvent(entry, e.events);
    }
  }

  return true;
}

void MessagePumpEpoll::OnEpollEvent(EpollEventEntry& entry, uint32_t events) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);

  const bool readable = (events & EPOLLIN) != 0;
  const bool writable = (events & EPOLLOUT) != 0;

  // Under different circumstances, peer closure may raise both/either EPOLLHUP
  // and/or EPOLLERR. Treat them as equivalent.
  const bool disconnected = (events & (EPOLLHUP | EPOLLERR)) != 0;

  // Copy the set of Interests, since interests may be added to or removed from
  // `entry` during the loop below. This copy is inexpensive in practice
  // because the size of this vector is expected to be very small (<= 2).
  auto interests = entry.interests;

  // Any of these interests' event handlers may destroy any of the others'
  // controllers. Start all of them watching for destruction before we actually
  // dispatch any events.
  for (const auto& interest : interests) {
    interest->WatchForControllerDestruction();
  }

  for (const auto& interest : interests) {
    if (!interest->active()) {
      continue;
    }

    const bool can_read = (readable || disconnected) && interest->params().read;
    const bool can_write = writable && interest->params().write;
    if (!can_read && !can_write) {
      // If this Interest is active but not watching for whichever event was
      // raised here, there's nothing to do. This can occur if a descriptor has
      // multiple active interests, since only one interest needs to be
      // satisfied in order for us to process an epoll event.
      continue;
    }

    if (interest->params().one_shot) {
      // This is a one-shot event watch which is about to be triggered. We
      // deactivate the interest and update epoll immediately. The event handler
      // may reactivate it.
      interest->set_active(false);
      UpdateEpollEvent(entry);
    }

    if (!interest->was_controller_destroyed()) {
      HandleEvent(entry.fd, can_read, can_write, interest->controller());
    }
  }

  for (const auto& interest : interests) {
    interest->StopWatchingForControllerDestruction();
  }
}

void MessagePumpEpoll::HandleEvent(int fd,
                                   bool can_read,
                                   bool can_write,
                                   FdWatchController* controller) {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
  // Make the MessagePumpDelegate aware of this other form of "DoWork". Skip if
  // HandleNotification() is called outside of Run() (e.g. in unit tests).
  Delegate::ScopedDoWorkItem scoped_do_work_item;
  if (run_state_) {
    scoped_do_work_item = run_state_->delegate->BeginWorkItem();
  }

  // Trace events must begin after the above BeginWorkItem() so that the
  // ensuing "ThreadController active" outscopes all the events under it.
  TRACE_EVENT("toplevel", "EpollEvent", "controller_created_from",
              controller->created_from_location(), "fd", fd, "can_read",
              can_read, "can_write", can_write, "context",
              static_cast<void*>(controller));
  TRACE_HEAP_PROFILER_API_SCOPED_TASK_EXECUTION heap_profiler_scope(
      controller->created_from_location().file_name());
  if (can_read && can_write) {
    bool controller_was_destroyed = false;
    bool* previous_was_destroyed_flag =
        std::exchange(controller->was_destroyed_, &controller_was_destroyed);

    controller->OnFdWritable();
    if (!controller_was_destroyed) {
      controller->OnFdReadable();
    }
    if (!controller_was_destroyed) {
      controller->was_destroyed_ = previous_was_destroyed_flag;
    } else if (previous_was_destroyed_flag) {
      *previous_was_destroyed_flag = true;
    }
  } else if (can_write) {
    controller->OnFdWritable();
  } else if (can_read) {
    controller->OnFdReadable();
  }
}

void MessagePumpEpoll::HandleWakeUp() {
  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
  uint64_t value;
  ssize_t n = HANDLE_EINTR(read(wake_event_.get(), &value, sizeof(value)));
  DPCHECK(n == sizeof(value));
}

MessagePumpEpoll::EpollEventEntry::EpollEventEntry(int fd) : fd(fd) {}

MessagePumpEpoll::EpollEventEntry::~EpollEventEntry() {
  if (active_event) {
    DCHECK_EQ(this, active_event->data.ptr);
    active_event->data.ptr = nullptr;
  }
}

uint32_t MessagePumpEpoll::EpollEventEntry::ComputeActiveEvents() {
  uint32_t events = 0;
  bool one_shot = true;
  for (const auto& interest : interests) {
    if (!interest->active()) {
      continue;
    }
    const InterestParams& params = interest->params();
    events |= (params.read ? EPOLLIN : 0) | (params.write ? EPOLLOUT : 0);
    one_shot &= params.one_shot;
  }
  if (events != 0 && one_shot) {
    return events | EPOLLONESHOT;
  }
  return events;
}

}  // namespace base