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libuv源码阅读(22)--spawn

先看下用例源码:

#include <stdio.h>
#include <inttypes.h>

#include <uv.h>

uv_loop_t *loop;
uv_process_t child_req;
uv_process_options_t options;

void on_exit(uv_process_t *req, int64_t exit_status, int term_signal) {
    fprintf(stderr, "Process exited with status %" PRId64 ", signal %d\n", exit_status, term_signal);
    uv_close((uv_handle_t*) req, NULL);
}

int main() {
    loop = uv_default_loop();

    char* args[3];
    args[0] = "mkdir";
    args[1] = "test-dir";
    args[2] = NULL;

    options.exit_cb = on_exit;
    options.file = "mkdir";
    options.args = args;

    int r;
    // fork 出子进程
    if ((r = uv_spawn(loop, &child_req, &options))) {
        fprintf(stderr, "%s\n", uv_strerror(r));
        return 1;
    } else {
        fprintf(stderr, "Launched process with ID %d\n", child_req.pid);
    }
    // 父进程等待结束
    return uv_run(loop, UV_RUN_DEFAULT);
}

// 进程类型的handler
struct uv_process_s {
  UV_HANDLE_FIELDS
  uv_exit_cb exit_cb;
  int pid;
  UV_PROCESS_PRIVATE_FIELDS
};

#define UV_PROCESS_PRIVATE_FIELDS                                             \
  void* queue[2];                                                             \
  int status;                                                                 \

// 配置参数
typedef struct uv_process_options_s {
  uv_exit_cb exit_cb; /* Called after the process exits. */
  const char* file;   /* Path to program to execute. */
  /*
   * Command line arguments. args[0] should be the path to the program. On
   * Windows this uses CreateProcess which concatenates the arguments into a
   * string this can cause some strange errors. See the note at
   * windows_verbatim_arguments.
   */
  char** args;
  /*
   * This will be set as the environ variable in the subprocess. If this is
   * NULL then the parents environ will be used.
   */
  char** env;
  /*
   * If non-null this represents a directory the subprocess should execute
   * in. Stands for current working directory.
   */
  const char* cwd;
  /*
   * Various flags that control how uv_spawn() behaves. See the definition of
   * `enum uv_process_flags` below.
   */
  unsigned int flags;
  /*
   * The `stdio` field points to an array of uv_stdio_container_t structs that
   * describe the file descriptors that will be made available to the child
   * process. The convention is that stdio[0] points to stdin, fd 1 is used for
   * stdout, and fd 2 is stderr.
   *
   * Note that on windows file descriptors greater than 2 are available to the
   * child process only if the child processes uses the MSVCRT runtime.
   */
  int stdio_count;
  uv_stdio_container_t* stdio;
  /*
   * Libuv can change the child process' user/group id. This happens only when
   * the appropriate bits are set in the flags fields. This is not supported on
   * windows; uv_spawn() will fail and set the error to UV_ENOTSUP.
   */
  uv_uid_t uid;
  uv_gid_t gid;
} uv_process_options_t;

// 子进程继承的fd 可用于子进程向传递数据
typedef struct uv_stdio_container_s {
  uv_stdio_flags flags;

  union {
    uv_stream_t* stream;
    int fd;
  } data;
} uv_stdio_container_t;

然后看下主要的函数:

int uv_spawn(uv_loop_t* loop,
             uv_process_t* process,
             const uv_process_options_t* options) {
#if defined(__APPLE__) && (TARGET_OS_TV || TARGET_OS_WATCH)
  /* fork is marked __WATCHOS_PROHIBITED __TVOS_PROHIBITED. */
  return UV_ENOSYS;
#else
  int signal_pipe[2] = { -1, -1 };
  int pipes_storage[8][2];
  int (*pipes)[2];
  int stdio_count;
  ssize_t r;
  pid_t pid;
  int err;
  int exec_errorno;
  int i;
  int status;

  assert(options->file != NULL);
  assert(!(options->flags & ~(UV_PROCESS_DETACHED |
                              UV_PROCESS_SETGID |
                              UV_PROCESS_SETUID |
                              UV_PROCESS_WINDOWS_HIDE |
                              UV_PROCESS_WINDOWS_HIDE_CONSOLE |
                              UV_PROCESS_WINDOWS_HIDE_GUI |
                              UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS)));

  // 初始化handler
  uv__handle_init(loop, (uv_handle_t*)process, UV_PROCESS);
  QUEUE_INIT(&process->queue);

  stdio_count = options->stdio_count;
  if (stdio_count < 3)
    stdio_count = 3;

  err = UV_ENOMEM;
  pipes = pipes_storage;
  if (stdio_count > (int) ARRAY_SIZE(pipes_storage))
    pipes = uv__malloc(stdio_count * sizeof(*pipes));

  if (pipes == NULL)
    goto error;
  
  // 传递给子进程用于通信的fd
  for (i = 0; i < stdio_count; i++) {
    pipes[i][0] = -1;
    pipes[i][1] = -1;
  }

  // 根据flag决定如何传递fd 目前是传默认值-1 
  for (i = 0; i < options->stdio_count; i++) {
    err = uv__process_init_stdio(options->stdio + i, pipes[i]);
    if (err)
      goto error;
  }

  /* This pipe is used by the parent to wait until
   * the child has called `execve()`. We need this
   * to avoid the following race condition:
   *
   *    if ((pid = fork()) > 0) {
   *      kill(pid, SIGTERM);
   *    }
   *    else if (pid == 0) {
   *      execve("/bin/cat", argp, envp);
   *    }
   *
   * The parent sends a signal immediately after forking.
   * Since the child may not have called `execve()` yet,
   * there is no telling what process receives the signal,
   * our fork or /bin/cat.
   *
   * To avoid ambiguity, we create a pipe with both ends
   * marked close-on-exec. Then, after the call to `fork()`,
   * the parent polls the read end until it EOFs or errors with EPIPE.
   */
  // 捕获子进程的信号管道
  err = uv__make_pipe(signal_pipe, 0);
  if (err)
    goto error;

  // 监听信号fd
  uv_signal_start(&loop->child_watcher, uv__chld, SIGCHLD);

  /* Acquire write lock to prevent opening new fds in worker threads */
  uv_rwlock_wrlock(&loop->cloexec_lock);
  pid = fork();

  if (pid == -1) {
    err = UV__ERR(errno);
    uv_rwlock_wrunlock(&loop->cloexec_lock);
    uv__close(signal_pipe[0]);
    uv__close(signal_pipe[1]);
    goto error;
  }

  if (pid == 0) {
    // 子进程执行逻辑
    uv__process_child_init(options, stdio_count, pipes, signal_pipe[1]);
    abort();
  }

  /* Release lock in parent process */
  uv_rwlock_wrunlock(&loop->cloexec_lock);
  uv__close(signal_pipe[1]);

  process->status = 0;
  exec_errorno = 0;
  // 等待子进程完成 替换操作
  do
    r = read(signal_pipe[0], &exec_errorno, sizeof(exec_errorno));
  while (r == -1 && errno == EINTR);

  if (r == 0)
    ; /* okay, EOF */
  else if (r == sizeof(exec_errorno)) {
    do
      err = waitpid(pid, &status, 0); /* okay, read errorno */
    while (err == -1 && errno == EINTR);
    assert(err == pid);
  } else if (r == -1 && errno == EPIPE) {
    do
      err = waitpid(pid, &status, 0); /* okay, got EPIPE */
    while (err == -1 && errno == EINTR);
    assert(err == pid);
  } else
    abort();

  uv__close_nocheckstdio(signal_pipe[0]);

  for (i = 0; i < options->stdio_count; i++) {
    // 建立stream流 用于存储子进程输出的信息
    err = uv__process_open_stream(options->stdio + i, pipes[i]);
    if (err == 0)
      continue;
    // 不需要传递fd的时候对应上面的关闭操作
    while (i--)
      uv__process_close_stream(options->stdio + i);

    goto error;
  }

  /* Only activate this handle if exec() happened successfully */
  if (exec_errorno == 0) {
    // 子进程替换成功 父进程激活process handler
    QUEUE_INSERT_TAIL(&loop->process_handles, &process->queue);
    uv__handle_start(process);
  }

  process->pid = pid;
  process->exit_cb = options->exit_cb;

  if (pipes != pipes_storage)
    uv__free(pipes);

  return exec_errorno;

error:
  if (pipes != NULL) {
    for (i = 0; i < stdio_count; i++) {
      if (i < options->stdio_count)
        if (options->stdio[i].flags & (UV_INHERIT_FD | UV_INHERIT_STREAM))
          continue;
      if (pipes[i][0] != -1)
        uv__close_nocheckstdio(pipes[i][0]);
      if (pipes[i][1] != -1)
        uv__close_nocheckstdio(pipes[i][1]);
    }

    if (pipes != pipes_storage)
      uv__free(pipes);
  }

  return err;
#endif
}

// 创建fd
static int uv__process_init_stdio(uv_stdio_container_t* container, int fds[2]) {
  int mask;
  int fd;

  mask = UV_IGNORE | UV_CREATE_PIPE | UV_INHERIT_FD | UV_INHERIT_STREAM;

  switch (container->flags & mask) {
  // 这里是执行这个逻辑
  case UV_IGNORE:
    return 0;

  case UV_CREATE_PIPE:
    assert(container->data.stream != NULL);
    if (container->data.stream->type != UV_NAMED_PIPE)
      return UV_EINVAL;
    else
      return uv_socketpair(SOCK_STREAM, 0, fds, 0, 0);

  case UV_INHERIT_FD:
  case UV_INHERIT_STREAM:
    if (container->flags & UV_INHERIT_FD)
      fd = container->data.fd;
    else
      fd = uv__stream_fd(container->data.stream);

    if (fd == -1)
      return UV_EINVAL;

    fds[1] = fd;
    return 0;

  default:
    assert(0 && "Unexpected flags");
    return UV_EINVAL;
  }
}

// 父进程捕获子进程退出后的信号 代表子进程任务执行结束了
static void uv__chld(uv_signal_t* handle, int signum) {
  uv_process_t* process;
  uv_loop_t* loop;
  int exit_status;
  int term_signal;
  int status;
  pid_t pid;
  QUEUE pending;
  QUEUE* q;
  QUEUE* h;

  assert(signum == SIGCHLD);

  QUEUE_INIT(&pending);
  loop = handle->loop;

  h = &loop->process_handles;
  q = QUEUE_HEAD(h);
  while (q != h) {
    process = QUEUE_DATA(q, uv_process_t, queue);
    q = QUEUE_NEXT(q);

    // 只捕获结束的子进程
    do
      pid = waitpid(process->pid, &status, WNOHANG);
    while (pid == -1 && errno == EINTR);

    if (pid == 0)
      continue;

    if (pid == -1) {
      if (errno != ECHILD)
        abort();
      continue;
    }

    process->status = status;
    QUEUE_REMOVE(&process->queue);
    // 一次性放入pending中
    QUEUE_INSERT_TAIL(&pending, &process->queue);
  }

  h = &pending;
  q = QUEUE_HEAD(h);
  while (q != h) {
    process = QUEUE_DATA(q, uv_process_t, queue);
    q = QUEUE_NEXT(q);

    QUEUE_REMOVE(&process->queue);
    QUEUE_INIT(&process->queue);
    uv__handle_stop(process);

    if (process->exit_cb == NULL)
      continue;

    exit_status = 0;
    if (WIFEXITED(process->status))
      exit_status = WEXITSTATUS(process->status);

    term_signal = 0;
    if (WIFSIGNALED(process->status))
      term_signal = WTERMSIG(process->status);
    
    // 执行用户设置 进程结束后的cb
    process->exit_cb(process, exit_status, term_signal);
  }
  assert(QUEUE_EMPTY(&pending));
}

看下子进程接下来:

#if !(defined(__APPLE__) && (TARGET_OS_TV || TARGET_OS_WATCH))
/* execvp is marked __WATCHOS_PROHIBITED __TVOS_PROHIBITED, so must be
 * avoided. Since this isn't called on those targets, the function
 * doesn't even need to be defined for them.
 */
static void uv__process_child_init(const uv_process_options_t* options,
                                   int stdio_count,
                                   int (*pipes)[2],
                                   int error_fd) {
  sigset_t set;
  int close_fd;
  int use_fd;
  int err;
  int fd;
  int n;

  if (options->flags & UV_PROCESS_DETACHED)
    setsid();

  /* First duplicate low numbered fds, since it's not safe to duplicate them,
   * they could get replaced. Example: swapping stdout and stderr; without
   * this fd 2 (stderr) would be duplicated into fd 1, thus making both
   * stdout and stderr go to the same fd, which was not the intention. */
  for (fd = 0; fd < stdio_count; fd++) {
    use_fd = pipes[fd][1];
    if (use_fd < 0 || use_fd >= fd)
      continue;
    pipes[fd][1] = fcntl(use_fd, F_DUPFD, stdio_count);
    if (pipes[fd][1] == -1) {
      uv__write_int(error_fd, UV__ERR(errno));
      _exit(127);
    }
  }

  for (fd = 0; fd < stdio_count; fd++) {
    close_fd = pipes[fd][0];
    use_fd = pipes[fd][1];

    if (use_fd < 0) {
      if (fd >= 3)
        continue;
      else {
        /* redirect stdin, stdout and stderr to /dev/null even if UV_IGNORE is
         * set
         */
        // 当前我们传入的是 3个-1 都会走下面的逻辑
        use_fd = open("/dev/null", fd == 0 ? O_RDONLY : O_RDWR);
        close_fd = use_fd;

        if (use_fd < 0) {
          uv__write_int(error_fd, UV__ERR(errno));
          _exit(127);
        }
      }
    }

    if (fd == use_fd)
      uv__cloexec_fcntl(use_fd, 0);
    else
      // 把 0 1 2 重定向到 /dev/null
      fd = dup2(use_fd, fd);

    if (fd == -1) {
      uv__write_int(error_fd, UV__ERR(errno));
      _exit(127);
    }

    if (fd <= 2)
      uv__nonblock_fcntl(fd, 0);
    
    // 关闭不需要的fd了
    if (close_fd >= stdio_count)
      uv__close(close_fd);
  }

  for (fd = 0; fd < stdio_count; fd++) {
    use_fd = pipes[fd][1];

    if (use_fd >= stdio_count)
      uv__close(use_fd);
  }

  if (options->cwd != NULL && chdir(options->cwd)) {
    uv__write_int(error_fd, UV__ERR(errno));
    _exit(127);
  }

  if (options->flags & (UV_PROCESS_SETUID | UV_PROCESS_SETGID)) {
    /* When dropping privileges from root, the `setgroups` call will
     * remove any extraneous groups. If we don't call this, then
     * even though our uid has dropped, we may still have groups
     * that enable us to do super-user things. This will fail if we
     * aren't root, so don't bother checking the return value, this
     * is just done as an optimistic privilege dropping function.
     */
    SAVE_ERRNO(setgroups(0, NULL));
  }

  if ((options->flags & UV_PROCESS_SETGID) && setgid(options->gid)) {
    uv__write_int(error_fd, UV__ERR(errno));
    _exit(127);
  }

  if ((options->flags & UV_PROCESS_SETUID) && setuid(options->uid)) {
    uv__write_int(error_fd, UV__ERR(errno));
    _exit(127);
  }

  if (options->env != NULL) {
    environ = options->env;
  }

  /* Reset signal disposition.  Use a hard-coded limit because NSIG
   * is not fixed on Linux: it's either 32, 34 or 64, depending on
   * whether RT signals are enabled.  We are not allowed to touch
   * RT signal handlers, glibc uses them internally.
   */
  for (n = 1; n < 32; n += 1) {
    if (n == SIGKILL || n == SIGSTOP)
      continue;  /* Can't be changed. */

#if defined(__HAIKU__)
    if (n == SIGKILLTHR)
      continue;  /* Can't be changed. */
#endif

    if (SIG_ERR != signal(n, SIG_DFL))
      continue;

    uv__write_int(error_fd, UV__ERR(errno));
    _exit(127);
  }

  /* Reset signal mask. */
  sigemptyset(&set);
  err = pthread_sigmask(SIG_SETMASK, &set, NULL);

  if (err != 0) {
    uv__write_int(error_fd, UV__ERR(err));
    _exit(127);
  }
  
  // 子进程执行目标程序
  execvp(options->file, options->args);
  // 如果失败的话通知父进程
  uv__write_int(error_fd, UV__ERR(errno));
  _exit(127);
}
#endif

总结:父进程fork出子进程去执行指定的文件或者应用,双方根据参数设置的共享fd来通信,这个示例比较简单,父进程只需要等待结束捕获信号就可以了。

detach用例则是设置子进程分离属性,让子进程脱离父进程独立运行,同时解除引用,让父进程不会永远运行下去。

#include <stdio.h>

#include <uv.h>

uv_loop_t *loop;
uv_process_t child_req;
uv_process_options_t options;

int main() {
    loop = uv_default_loop();

    char* args[3];
    args[0] = "sleep";
    args[1] = "100";
    args[2] = NULL;

    options.exit_cb = NULL;
    options.file = "sleep";
    options.args = args;
    options.flags = UV_PROCESS_DETACHED;

    int r;
    if ((r = uv_spawn(loop, &child_req, &options))) {
        fprintf(stderr, "%s\n", uv_strerror(r));
        return 1;
    }
    fprintf(stderr, "Launched sleep with PID %d\n", child_req.pid);
    uv_unref((uv_handle_t*) &child_req);

    return uv_run(loop, UV_RUN_DEFAULT);
}

而pro_streams示例则是双方共享fd 2,让子进程的错误输出重定向到父进程的fd 2:

#include <stdio.h>
#include <string.h>
#include <inttypes.h>

#include <uv.h>

uv_loop_t *loop;
uv_process_t child_req;
uv_process_options_t options;

void on_exit(uv_process_t *req, int64_t exit_status, int term_signal) {
    fprintf(stderr, "Process exited with status %" PRId64 ", signal %d\n", exit_status, term_signal);
    uv_close((uv_handle_t*) req, NULL);
}

int main() {
    loop = uv_default_loop();

    size_t size = 500;
    char path[size];
    uv_exepath(path, &size);
    strcpy(path + (strlen(path) - strlen("proc-streams")), "test");

    char* args[2];
    args[0] = path;
    args[1] = NULL;

    /* ... */

    options.stdio_count = 3;
    uv_stdio_container_t child_stdio[3];
    child_stdio[0].flags = UV_IGNORE;
    child_stdio[1].flags = UV_IGNORE;
    // 这里设置继承fd
    child_stdio[2].flags = UV_INHERIT_FD;
    child_stdio[2].data.fd = 2;
    options.stdio = child_stdio;

    options.exit_cb = on_exit;
    options.file = args[0];
    options.args = args;

    int r;
    if ((r = uv_spawn(loop, &child_req, &options))) {
        fprintf(stderr, "%s\n", uv_strerror(r));
        return 1;
    }

    return uv_run(loop, UV_RUN_DEFAULT);
}

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