PG守护进程（Postmaster）——后台一等公民进程

AuxiliaryProcessMain函数是后台一等公民进程的入口，我们来看一看哪里调用了该入口函数：

2. src/backend/main/main.c main函数，如下图所示，argv第二个参数为–boot



4. src/backend/postmaster/postmaster.c SubPostmasterMain函数 argv第二个参数为–forkboot



6. src/backend/postmaster/postmaster.c StartChildProcess函数，我们以该函数为主要讲解点。主要看这个分支流程，上面两个分支流程不重要。



7. 调用链：StartChildProcess --> AuxiliaryProcessMain

StartChildProcess函数

StartChildProcess函数为postmaster启动一个an auxiliary process。为AuxiliaryProcessMain拼接输入参数数组argv：“postgres -x%d”。fork一个子进程，调用函数AuxiliaryProcessMain(ac, av)。

static pid_t StartChildProcess(AuxProcType type) {
  pid_t    pid;
  char     *av[10]; int      ac = 0;
  char    typebuf[32];

  /* Set up command-line arguments for subprocess */
  av[ac++] = "postgres";
  snprintf(typebuf, sizeof(typebuf), "-x%d", type);
  av[ac++] = typebuf;
  av[ac] = NULL;
  Assert(ac < lengthof(av));

  pid = fork_process();
  if (pid == 0) {        /* child 子进程 */
    InitPostmasterChild();    
    ClosePostmasterPorts(false); /* Close the postmaster's sockets */
    /* Release postmaster's working memory context */
    MemoryContextSwitchTo(TopMemoryContext);
    MemoryContextDelete(PostmasterContext);
    PostmasterContext = NULL;
    AuxiliaryProcessMain(ac, av);
    ExitPostmaster(0);
  }

  if (pid < 0) {
    /* in parent, fork failed */
    int      save_errno = errno; errno = save_errno;
    switch (type) {
      case StartupProcess:
        ereport(LOG,(errmsg("could not fork startup process: %m"))); break;
      case BgWriterProcess:
        ereport(LOG, (errmsg("could not fork background writer process: %m"))); break;
      case CheckpointerProcess:
        ereport(LOG, (errmsg("could not fork checkpointer process: %m"))); break;
      case WalWriterProcess:
        ereport(LOG, (errmsg("could not fork WAL writer process: %m"))); break;
      case WalReceiverProcess:
        ereport(LOG, (errmsg("could not fork WAL receiver process: %m"))); break;
      default:
        ereport(LOG, (errmsg("could not fork process: %m")));
        break;
    }
    /* fork failure is fatal during startup, but there's no need to choke immediately if starting other child types fails. 如果是StartupProcess子进程fork失败，fatal级别的错误，需要调用ExitPostmaster(1) */
    if (type == StartupProcess) ExitPostmaster(1);
    return 0;
  }

  /*
   * in parent, successful fork
   */
  return pid;
}

AuxProcType的类型如下，下面就是后台一等公民进程，和PG的执行逻辑紧密耦合。

后台一等公民进程

第一组一等公民：StartupProcess、BgWriterProcess、CheckpointerProcess、WalWriterProcess、WalReceiverProcess。调用流程如下：

• PostmasterMain --> StartupDataBase
• pmdie/reaper/sigusr1_handler -> PostmasterStateMachine --> StartupDataBase
• ServerLoop(postmaster.c)/reaper/sigusr1_handler --> StartBackgroundWriter
• ServerLoop(postmaster.c)/reaper/PostmasterStateMachine/sigusr1_handler --> StartCheckpointer
• ServerLoop(postmaster.c)/reaper --> StartWalWriter
• ServerLoop(postmaster.c)/sigusr1_handler --> MaybeStartWalReceiver --> StartWalReceiver

第二组一等公民：CheckerProcess、BootstrapProcess

• AuxiliaryProcessMain -> CheckerProcess
• AuxiliaryProcessMain -> BootstrapProcess