text: /* Copyright Joyent, Inc. and other Node contributors. All rights reserved.
   * Permission is hereby granted, free of charge, to any person obtaining a copy
   * of this software and associated documentation files (the "Software"), to
   * deal in the Software without restriction, including without limitation the
   * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
   * sell copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in
   * all copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
   * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
   * IN THE SOFTWARE.
   */
  
  /* This file contains both the uv__async internal infrastructure and the
   * user-facing uv_async_t functions.
   */
  
  #include "uv.h"
  #include "internal.h"
  
  #include <errno.h>
  #include <stdio.h>  /* snprintf() */
  #include <assert.h>
  #include <stdlib.h>
  #include <string.h>
  #include <unistd.h>
  
  static void uv__async_event(uv_loop_t* loop,
                              struct uv__async* w,
                              unsigned int nevents);
  static int uv__async_make_pending(int* pending);
  static int uv__async_eventfd(void);
  
  
  int uv_async_init(uv_loop_t* loop, uv_async_t* handle, uv_async_cb async_cb) {
    int err;
  
    err = uv__async_start(loop, &loop->async_watcher, uv__async_event);
    if (err)
      return err;
  
    uv__handle_init(loop, (uv_handle_t*)handle, UV_ASYNC);
    handle->async_cb = async_cb;
    handle->pending = 0;
  
    QUEUE_INSERT_TAIL(&loop->async_handles, &handle->queue);
    uv__handle_start(handle);
  
    return 0;
  }
  
  
  int uv_async_send(uv_async_t* handle) {
    if (uv__async_make_pending(&handle->pending) == 0)
      uv__async_send(&handle->loop->async_watcher);
  
    return 0;
  }
  
  
  void uv__async_close(uv_async_t* handle) {
    QUEUE_REMOVE(&handle->queue);
    uv__handle_stop(handle);
  }
  
  
  static void uv__async_event(uv_loop_t* loop,
                              struct uv__async* w,
                              unsigned int nevents) {
    QUEUE* q;
    uv_async_t* h;
  
    QUEUE_FOREACH(q, &loop->async_handles) {
      h = QUEUE_DATA(q, uv_async_t, queue);
  
      if (h->pending == 0)
        continue;
      h->pending = 0;
  
      if (h->async_cb == NULL)
        continue;
      h->async_cb(h);
    }
  }
  
  
  static int uv__async_make_pending(int* pending) {
    /* Do a cheap read first. */
    if (ACCESS_ONCE(int, *pending) != 0)
      return 1;
  
    /* Micro-optimization: use atomic memory operations to detect if we've been
     * preempted by another thread and don't have to make an expensive syscall.
     * This speeds up the heavily contended case by about 1-2% and has little
     * if any impact on the non-contended case.
     *
     * Use XCHG instead of the CMPXCHG that __sync_val_compare_and_swap() emits
     * on x86, it's about 4x faster. It probably makes zero difference in the
     * grand scheme of things but I'm OCD enough not to let this one pass.
     */
  #if defined(__i386__) || defined(__x86_64__)
    {
      unsigned int val = 1;
      __asm__ __volatile__ ("xchgl %0, %1"
                           : "+r" (val)
                           : "m"  (*pending));
      return val != 0;
    }
  #elif defined(__GNUC__) && (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ > 0)
    return __sync_val_compare_and_swap(pending, 0, 1) != 0;
  #else
    ACCESS_ONCE(int, *pending) = 1;
    return 0;
  #endif
  }
  
  
  static void uv__async_io(uv_loop_t* loop, uv__io_t* w, unsigned int events) {
    struct uv__async* wa;
    char buf[1024];
    unsigned n;
    ssize_t r;
  
    n = 0;
    for (;;) {
      r = read(w->fd, buf, sizeof(buf));
  
      if (r > 0)
        n += r;
  
      if (r == sizeof(buf))
        continue;
  
      if (r != -1)
        break;
  
      if (errno == EAGAIN || errno == EWOULDBLOCK)
        break;
  
      if (errno == EINTR)
        continue;
  
      abort();
    }
  
    wa = container_of(w, struct uv__async, io_watcher);
  
  #if defined(__linux__)
    if (wa->wfd == -1) {
      uint64_t val;
      assert(n == sizeof(val));
      memcpy(&val, buf, sizeof(val));  /* Avoid alignment issues. */
      wa->cb(loop, wa, val);
      return;
    }
  #endif
  
    wa->cb(loop, wa, n);
  }
  
  
  void uv__async_send(struct uv__async* wa) {
    const void* buf;
    ssize_t len;
    int fd;
    int r;
  
    buf = "";
    len = 1;
    fd = wa->wfd;
  
  #if defined(__linux__)
    if (fd == -1) {
      static const uint64_t val = 1;
      buf = &val;
      len = sizeof(val);
      fd = wa->io_watcher.fd;  /* eventfd */
    }
  #endif
  
    do
      r = write(fd, buf, len);
    while (r == -1 && errno == EINTR);
  
    if (r == len)
      return;
  
    if (r == -1)
      if (errno == EAGAIN || errno == EWOULDBLOCK)
        return;
  
    abort();
  }
  
  
  void uv__async_init(struct uv__async* wa) {
    wa->io_watcher.fd = -1;
    wa->wfd = -1;
  }
  
  
  int uv__async_start(uv_loop_t* loop, struct uv__async* wa, uv__async_cb cb) {
    int pipefd[2];
    int err;
  
    if (wa->io_watcher.fd != -1)
      return 0;
  
    err = uv__async_eventfd();
    if (err >= 0) {
      pipefd[0] = err;
      pipefd[1] = -1;
    }
    else if (err == -ENOSYS) {
      err = uv__make_pipe(pipefd, UV__F_NONBLOCK);
  #if defined(__linux__)
      /* Save a file descriptor by opening one of the pipe descriptors as
       * read/write through the procfs.  That file descriptor can then
       * function as both ends of the pipe.
       */
      if (err == 0) {
        char buf[32];
        int fd;
  
        snprintf(buf, sizeof(buf), "/proc/self/fd/%d", pipefd[0]);
        fd = uv__open_cloexec(buf, O_RDWR);
        if (fd >= 0) {
          uv__close(pipefd[0]);
          uv__close(pipefd[1]);
          pipefd[0] = fd;
          pipefd[1] = fd;
        }
      }
  #endif
    }
  
    if (err < 0)
      return err;
  
    uv__io_init(&wa->io_watcher, uv__async_io, pipefd[0]);
    uv__io_start(loop, &wa->io_watcher, UV__POLLIN);
    wa->wfd = pipefd[1];
    wa->cb = cb;
  
    return 0;
  }
  
  
  void uv__async_stop(uv_loop_t* loop, struct uv__async* wa) {
    if (wa->io_watcher.fd == -1)
      return;
  
    if (wa->wfd != -1) {
      if (wa->wfd != wa->io_watcher.fd)
        uv__close(wa->wfd);
      wa->wfd = -1;
    }
  
    uv__io_stop(loop, &wa->io_watcher, UV__POLLIN);
    uv__close(wa->io_watcher.fd);
    wa->io_watcher.fd = -1;
  }
  
  
  static int uv__async_eventfd() {
  #if defined(__linux__)
    static int no_eventfd2;
    static int no_eventfd;
    int fd;
  
    if (no_eventfd2)
      goto skip_eventfd2;
  
    fd = uv__eventfd2(0, UV__EFD_CLOEXEC | UV__EFD_NONBLOCK);
    if (fd != -1)
      return fd;
  
    if (errno != ENOSYS)
      return -errno;
  
    no_eventfd2 = 1;
  
  skip_eventfd2:
  
    if (no_eventfd)
      goto skip_eventfd;
  
    fd = uv__eventfd(0);
    if (fd != -1) {
      uv__cloexec(fd, 1);
      uv__nonblock(fd, 1);
      return fd;
    }
  
    if (errno != ENOSYS)
      return -errno;
  
    no_eventfd = 1;
  
  skip_eventfd:
  
  #endif
  
    return -ENOSYS;
  }
  