PG守护进程（Postmaster）——ApplyLauncherRegister

后台二等公民进程logical replication launcher注册（Register the apply launcher. Since it registers a background worker, it needs to be called before InitializeMaxBackends(), and it’s probably a good idea to call it before any modules had chance to take the background worker slots.）。
如果max_logical_replication_workers为零，就不需要注册logical replication launcher后台进程。
logical replication launcher也是使用后台二等公民进程框架。

void ApplyLauncherRegister(void){
  BackgroundWorker bgw;
  if (max_logical_replication_workers == 0) return;

  memset(&bgw, 0, sizeof(bgw));
  bgw.bgw_flags = BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION;
  bgw.bgw_start_time = BgWorkerStart_RecoveryFinished;
  snprintf(bgw.bgw_library_name, BGW_MAXLEN, "postgres");
  snprintf(bgw.bgw_function_name, BGW_MAXLEN, "ApplyLauncherMain");
  snprintf(bgw.bgw_name, BGW_MAXLEN, "logical replication launcher");
  snprintf(bgw.bgw_type, BGW_MAXLEN, "logical replication launcher");
  bgw.bgw_restart_time = 5;
  bgw.bgw_notify_pid = 0;
  bgw.bgw_main_arg = (Datum) 0;

  RegisterBackgroundWorker(&bgw);
}

logical replication launcher运行的函数ApplyLauncherMain