PG守护进程（Postmaster）——后台一等公民进程StartupDataBase

后台一等公民进程StartupDataBase调用函数链，通过fork子进程AuxiliaryProcessMain来执行StartupProcessMain函数。

StartupDataBase() --> StartChildProcess(StartupProcess) --fork--> 子进程 AuxiliaryProcessMain --> StartupProcessMain函数
                                                              --> 父进程 return

StartChildProcess

StartChildProcess函数执行流程如下，子进程执行如下函数InitPostmasterChild、ClosePostmasterPorts、MemoryContextSwitchTo、MemoryContextDelete、AuxiliaryProcessMain，最后执行ExitPostmaster(0)。

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 */  
    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. */
    if (type == StartupProcess) ExitPostmaster(1);
    return 0;
  }
  /* in parent, successful fork */
  return pid;
}

InitPostmasterChild

InitPostmasterChild函数将IsUnderPostmaster设置为true，也就是postmaster的子进程。重置proc_exit proc_exit，初始化进程本地latch支持。调用PostmasterDeathSignalInit注册postmaster death信号处理函数。

信号	信号处理函数	功能
POSTMASTER_DEATH_SIGNAL	postmaster_death_handler	子进程处理postmaster death信号（部分平台支持）

void InitPostmasterChild(void) {
  IsUnderPostmaster = true;  /* we are a postmaster subprocess now */
  InitProcessGlobals();  
  on_exit_reset(); /* We don't want the postmaster's proc_exit() handlers */

  /* Initialize process-local latch support */
  InitializeLatchSupport();
  MyLatch = &LocalLatchData;
  InitLatch(MyLatch);

  /* If possible, make this process a group leader, so that the postmaster
   * can signal any child processes too. Not all processes will have
   * children, but for consistency we make all postmaster child processes do
   * this. */
#ifdef HAVE_SETSID
  if (setsid() < 0) elog(FATAL, "setsid() failed: %m");
#endif

  /* Request a signal if the postmaster dies, if possible. */
  PostmasterDeathSignalInit();
}

ClosePostmasterPorts

AuxiliaryProcessMain

处理argv命令行参数

AuxiliaryProcessMain(int argc, char *argv[]) {
  char     *progname = argv[0];
  int      flag; char     *userDoption = NULL;
  ...
  /* Ignore the initial --boot argument, if present */
  if (argc > 1 && strcmp(argv[1], "--boot") == 0) {
    argv++; argc--;
  }  
  /* If no -x argument, we are a CheckerProcess */
  MyAuxProcType = CheckerProcess;
  while ((flag = getopt(argc, argv, "B:c:d:D:Fkr:x:X:-:")) != -1) {
    switch (flag) {
      case 'B': SetConfigOption("shared_buffers", optarg, PGC_POSTMASTER, PGC_S_ARGV); break;
      case 'D': userDoption = pstrdup(optarg); break;
      case 'd': { /* Turn on debugging for the bootstrap process. */      
          char     *debugstr;
          debugstr = psprintf("debug%s", optarg);
          SetConfigOption("log_min_messages", debugstr, PGC_POSTMASTER, PGC_S_ARGV);
          SetConfigOption("client_min_messages", debugstr, PGC_POSTMASTER, PGC_S_ARGV);
          pfree(debugstr);
        }
        break;
      case 'F': SetConfigOption("fsync", "false", PGC_POSTMASTER, PGC_S_ARGV); break;
      case 'k': bootstrap_data_checksum_version = PG_DATA_CHECKSUM_VERSION; break;
      case 'r': strlcpy(OutputFileName, optarg, MAXPGPATH); break;
      case 'x': MyAuxProcType = atoi(optarg); break;
      case 'X':{
          int      WalSegSz = strtoul(optarg, NULL, 0);
          if (!IsValidWalSegSize(WalSegSz)) ereport(ERROR,(errcode(ERRCODE_INVALID_PARAMETER_VALUE), errmsg("-X requires a power of two value between 1 MB and 1 GB")));
          SetConfigOption("wal_segment_size", optarg, PGC_INTERNAL, PGC_S_OVERRIDE);
        }
        break;
      case 'c':
      case '-': {
          char     *name, *value;
          ParseLongOption(optarg, &name, &value);
          if (!value) {
            if (flag == '-') ereport(ERROR,(errcode(ERRCODE_SYNTAX_ERROR),errmsg("--%s requires a value",optarg)));
            else ereport(ERROR,(errcode(ERRCODE_SYNTAX_ERROR),errmsg("-c %s requires a value",optarg)));
          }
          SetConfigOption(name, value, PGC_POSTMASTER, PGC_S_ARGV); free(name);
          if (value) free(value);
          break;
        }
      default:
        write_stderr("Try \"%s --help\" for more information.\n", progname); proc_exit(1); break;
    }
  }
  if (argc != optind) {
    write_stderr("%s: invalid command-line arguments\n", progname); proc_exit(1);
  }

通过ps显示自己的进程情况

/* Identify myself via ps */
  if (IsUnderPostmaster){
    const char *statmsg;
    switch (MyAuxProcType){
      case StartupProcess:statmsg = pgstat_get_backend_desc(B_STARTUP);break;
      case BgWriterProcess:statmsg = pgstat_get_backend_desc(B_BG_WRITER);break;
      case CheckpointerProcess:statmsg = pgstat_get_backend_desc(B_CHECKPOINTER);break;
      case WalWriterProcess:statmsg = pgstat_get_backend_desc(B_WAL_WRITER);break;
      case WalReceiverProcess:statmsg = pgstat_get_backend_desc(B_WAL_RECEIVER);break;
      default:statmsg = "??? process";break;
    }
    init_ps_display(statmsg, "", "", "");
  }

子进程环境初始化

BaseInit函数在standalone或under postmaster进行初期初始化后端backend，且是在InitPostgres之前执行。主要调用如下函数：InitCommunication、DebugFileOpen、InitFileAccess、InitSync、smgrinit、InitBufferPoolAccess。

SetProcessingMode(BootstrapProcessing);
    IgnoreSystemIndexes = true;
    ...
    BaseInit();

在auxiliary process中不能像后端postgres一样直接执行InitPostgres进行初始化，需要执行另外的函数进行初始化。

/* When we are an auxiliary process, we aren't going to do the full
   * InitPostgres pushups, but there are a couple of things that need to get
   * lit up even in an auxiliary process. */
  if (IsUnderPostmaster) {   
    /* Assign the ProcSignalSlot for an auxiliary process.  Since it
     * doesn't have a BackendId, the slot is statically allocated based on
     * the auxiliary process type (MyAuxProcType).  Backends use slots
     * indexed in the range from 1 to MaxBackends (inclusive), so we use
     * MaxBackends + AuxProcType + 1 as the index of the slot for an
     * auxiliary process.
     * This will need rethinking if we ever want more than one of a
     * particular auxiliary process type. */
    ProcSignalInit(MaxBackends + MyAuxProcType + 1);  
    
    InitBufferPoolBackend(); /* finish setting up bufmgr.c */
    /* Auxiliary processes don't run transactions, but they may need a
     * resource owner anyway to manage buffer pins acquired outside
     * transactions (and, perhaps, other things in future). */
    CreateAuxProcessResourceOwner();
    
    pgstat_initialize(); /* Initialize backend status information */
    pgstat_bestart();  
    before_shmem_exit(ShutdownAuxiliaryProcess, 0); /* register a before-shutdown callback for LWLock cleanup */
  }

ProcSignalInit函数在procsignal数组中注册当前进程。传入参数可以是后台进行的BackendId，或者是MaxBackends + aux process type。函数执行流程参考PostgreSQL数据库PMsignal——后端进程\Postmaster信号通信中的进程间信号通信。

BootstrapProcess

/* XLOG operations */
  SetProcessingMode(NormalProcessing);
  switch (MyAuxProcType){
    case StartupProcess:  
      /* don't set signals, startup process has its own agenda */
      StartupProcessMain();
      proc_exit(1);    /* should never return */

StartupProcessMain函数

StartupProcessMain函数主要工作是注册信号处理函数、PG_SETMASK(&UnBlockSig)和运行StartupXLOG函数。

信号	信号处理函数	功能
SIGHUP	StartupProcSigHupHandler	重载配置文件
SIGINT	SIG_IGN	忽略查询取消
SIGTERM	StartupProcShutdownHandler	request shutdown
SIGQUIT	startupproc_quickdie	hard crash time
SIGALRM	handle_sig_alarm	timeout
SIGPIPE	SIG_IGN
SIGUSR1	StartupProcSigUsr1Handler
SIGUSR2	StartupProcTriggerHandler
SIGCHLD	SIG_DFL
STANDBY_DEADLOCK_TIMEOUT	StandbyDeadLockHandler
STANDBY_TIMEOUT	StandbyTimeoutHandler
STANDBY_LOCK_TIMEOUT	StandbyLockTimeoutHandler

StartupXLOG
proc_exit(0)