High level overview of libuv looping.

libuv-overview.c

int uv_run(uv_loop_t* loop, uv_run_mode mode) {
  int timeout;
  int r;
  int ran_pending;

  r = uv__loop_alive(loop);
  if (!r)
    uv__update_time(loop);

  while (r != 0 && loop->stop_flag == 0) {
    uv__update_time(loop);
    uv__run_timers(loop);
    ran_pending = uv__run_pending(loop);
    uv__run_idle(loop);
    uv__run_prepare(loop);

    timeout = 0;
    if ((mode == UV_RUN_ONCE && !ran_pending) || mode == UV_RUN_DEFAULT)
      timeout = uv_backend_timeout(loop);

    uv__io_poll(loop, timeout);
    uv__run_check(loop);
    uv__run_closing_handles(loop);

    if (mode == UV_RUN_ONCE) {
      /* UV_RUN_ONCE implies forward progress: at least one callback must have
       * been invoked when it returns. uv__io_poll() can return without doing
       * I/O (meaning: no callbacks) when its timeout expires - which means we
       * have pending timers that satisfy the forward progress constraint.
       *
       * UV_RUN_NOWAIT makes no guarantees about progress so it's omitted from
       * the check.
       */
      uv__update_time(loop);
      uv__run_timers(loop);
    }

    r = uv__loop_alive(loop);
    if (mode == UV_RUN_ONCE || mode == UV_RUN_NOWAIT)
      break;
  }

  /* The if statement lets gcc compile it to a conditional store. Avoids
   * dirtying a cache line.
   */
  if (loop->stop_flag != 0)
    loop->stop_flag = 0;

  return r;
}

static int uv__run_pending(uv_loop_t* loop) {
  QUEUE* q;
  QUEUE pq;
  uv__io_t* w;

  if (QUEUE_EMPTY(&loop->pending_queue))
    return 0;

  QUEUE_MOVE(&loop->pending_queue, &pq);

  while (!QUEUE_EMPTY(&pq)) {
    q = QUEUE_HEAD(&pq);
    QUEUE_REMOVE(q);
    QUEUE_INIT(q);
    w = QUEUE_DATA(q, uv__io_t, pending_queue);
    w->cb(loop, w, POLLOUT);
  }

  return 1;
}

void uv__run_timers(uv_loop_t* loop) {
  struct heap_node* heap_node;
  uv_timer_t* handle;

  for (;;) {
    heap_node = heap_min(timer_heap(loop));
    if (heap_node == NULL)
      break;

    handle = container_of(heap_node, uv_timer_t, heap_node);
    if (handle->timeout > loop->time)
      break;

    uv_timer_stop(handle);
    uv_timer_again(handle);
    handle->timer_cb(handle);
  }
}

void uv__io_poll(uv_loop_t* loop, int timeout) {
  sigset_t* pset;
  sigset_t set;
  uint64_t time_base;
  uint64_t time_diff;
  QUEUE* q;
  uv__io_t* w;
  size_t i;
  unsigned int nevents;
  int nfds;
  int have_signals;
  struct pollfd* pe;
  int fd;

  if (loop->nfds == 0) {
    assert(QUEUE_EMPTY(&loop->watcher_queue));
    return;
  }

  /* Take queued watchers and add their fds to our poll fds array.  */
  while (!QUEUE_EMPTY(&loop->watcher_queue)) {
    q = QUEUE_HEAD(&loop->watcher_queue);
    QUEUE_REMOVE(q);
    QUEUE_INIT(q);

    w = QUEUE_DATA(q, uv__io_t, watcher_queue);
    assert(w->pevents != 0);
    assert(w->fd >= 0);
    assert(w->fd < (int) loop->nwatchers);

    uv__pollfds_add(loop, w);

    w->events = w->pevents;
  }

  /* Prepare a set of signals to block around poll(), if any.  */
  pset = NULL;
  if (loop->flags & UV_LOOP_BLOCK_SIGPROF) {
    pset = &set;
    sigemptyset(pset);
    sigaddset(pset, SIGPROF);
  }

  assert(timeout >= -1);
  time_base = loop->time;

  /* Loop calls to poll() and processing of results.  If we get some
   * results from poll() but they turn out not to be interesting to
   * our caller then we need to loop around and poll() again.
   */
  for (;;) {
    if (pset != NULL)
      if (pthread_sigmask(SIG_BLOCK, pset, NULL))
        abort();
    nfds = poll(loop->poll_fds, (nfds_t)loop->poll_fds_used, timeout);
    if (pset != NULL)
      if (pthread_sigmask(SIG_UNBLOCK, pset, NULL))
        abort();

    /* Update loop->time unconditionally. It's tempting to skip the update when
     * timeout == 0 (i.e. non-blocking poll) but there is no guarantee that the
     * operating system didn't reschedule our process while in the syscall.
     */
    SAVE_ERRNO(uv__update_time(loop));

    if (nfds == 0) {
      assert(timeout != -1);
      return;
    }

    if (nfds == -1) {
      if (errno != EINTR)
        abort();

      if (timeout == -1)
        continue;

      if (timeout == 0)
        return;

      /* Interrupted by a signal. Update timeout and poll again. */
      goto update_timeout;
    }

    /* Tell uv__platform_invalidate_fd not to manipulate our array
     * while we are iterating over it.
     */
    loop->poll_fds_iterating = 1;

    /* Initialize a count of events that we care about.  */
    nevents = 0;
    have_signals = 0;

    /* Loop over the entire poll fds array looking for returned events.  */
    for (i = 0; i < loop->poll_fds_used; i++) {
      pe = loop->poll_fds + i;
      fd = pe->fd;

      /* Skip invalidated events, see uv__platform_invalidate_fd.  */
      if (fd == -1)
        continue;

      assert(fd >= 0);
      assert((unsigned) fd < loop->nwatchers);

      w = loop->watchers[fd];

      if (w == NULL) {
        /* File descriptor that we've stopped watching, ignore.  */
        uv__platform_invalidate_fd(loop, fd);
        continue;
      }

      /* Filter out events that user has not requested us to watch
       * (e.g. POLLNVAL).
       */
      pe->revents &= w->pevents | POLLERR | POLLHUP;

      if (pe->revents != 0) {
        /* Run signal watchers last.  */
        if (w == &loop->signal_io_watcher) {
          have_signals = 1;
        } else {
          w->cb(loop, w, pe->revents);
        }

        nevents++;
      }
    }

    if (have_signals != 0)
      loop->signal_io_watcher.cb(loop, &loop->signal_io_watcher, POLLIN);

    loop->poll_fds_iterating = 0;

    /* Purge invalidated fds from our poll fds array.  */
    uv__pollfds_del(loop, -1);

    if (have_signals != 0)
      return;  /* Event loop should cycle now so don't poll again. */

    if (nevents != 0)
      return;

    if (timeout == 0)
      return;

    if (timeout == -1)
      continue;

update_timeout:
    assert(timeout > 0);

    time_diff = loop->time - time_base;
    if (time_diff >= (uint64_t) timeout)
      return;

    timeout -= time_diff;
  }
}

static void uv__run_closing_handles(uv_loop_t* loop) {
  uv_handle_t* p;
  uv_handle_t* q;

  p = loop->closing_handles;
  loop->closing_handles = NULL;

  while (p) {
    q = p->next_closing;
    uv__finish_close(p);
    p = q;
  }
}