InitPostmasterDeathWatchHandle函数初始化仅仅一个句柄用于监控postmaster death。在postmaster中调用，这样子进程在父进程挂掉之后才能监控到（subsequently monitor）。创建postmaster_alive_fds管道，postmaster拥有管道的写端，子进程拥有管道的读端。子进程可以将读端的文件描述符传递到select中在postmaster die后唤醒，或者通过read函数返回0来检查postmaster death。子进程必须在forking之后立刻关闭该管道的写端（只有在所有子进程关闭了写端之后，EOF在read端才会有信号，EOF won’t be signaled in the read end until all processes have closed the write fd）

static void InitPostmasterDeathWatchHandle(void){
  /* Create a pipe. Postmaster holds the write end of the pipe open
   * (POSTMASTER_FD_OWN), and children hold the read end. Children can pass
   * the read file descriptor to select() to wake up in case postmaster
   * dies, or check for postmaster death with a (read() == 0). Children must
   * close the write end as soon as possible after forking, because EOF
   * won't be signaled in the read end until all processes have closed the
   * write fd. That is taken care of in ClosePostmasterPorts(). */
  Assert(MyProcPid == PostmasterPid);
  if (pipe(postmaster_alive_fds) < 0)
    ereport(FATAL,
        (errcode_for_file_access(),
         errmsg_internal("could not create pipe to monitor postmaster death: %m")));

  /*
   * Set O_NONBLOCK to allow testing for the fd's presence with a read()
   * call.
   */
  if (fcntl(postmaster_alive_fds[POSTMASTER_FD_WATCH], F_SETFL, O_NONBLOCK) == -1)
    ereport(FATAL,
        (errcode_for_socket_access(),
         errmsg_internal("could not set postmaster death monitoring pipe to nonblocking mode: %m")));

PG守护进程（Postmaster）——InitPostmasterDeathWatchHandle_postgresql