PG守护进程（Postmaster）——读取控制文件LocalProcessControlFile

为错误检查和配置信息读取控制文件。我们已经检查了控制文件的CRC，需要的CRC run-time test支持设施已经加载。postmaster在启动过程中会进行测试CRC，子进程会继承该函数指针，不需要重复进行测试。

LocalProcessControlFile(false)

LocalProcessControlFile函数会读取控制文件，设置respective GUC。该函数是在启动过程中调用，包括在crash recovery cycle（除了bootstrap模式，因为这个模式下还没有生产控制文件）。这时还没有建立共享内存，所以需要将控制文件的内容放置本地被称中，并在XLOGShmemInit函数中拷贝到共享内存中。

void LocalProcessControlFile(bool reset) {
  Assert(reset || ControlFile == NULL);
  ControlFile = palloc(sizeof(ControlFileData));
  ReadControlFile();
}

执行过程

1. 打开控制文件，读取控制文件

static void ReadControlFile(void) {
  pg_crc32c  crc;
  int      fd;
  static char wal_segsz_str[20];
  int      r;

  /* Read data... */
  fd = BasicOpenFile(XLOG_CONTROL_FILE, O_RDWR | PG_BINARY); // 打开文件
  if (fd < 0) ereport(PANIC,(errcode_for_file_access(),errmsg("could not open file \"%s\": %m", XLOG_CONTROL_FILE)));

  pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_READ); // 将WAIT_EVENT_CONTROL_FILE_READ事件设置到MyProc->wait_event_info中
  r = read(fd, ControlFile, sizeof(ControlFileData));
  if (r != sizeof(ControlFileData)) {
    if (r < 0) ereport(PANIC,(errcode_for_file_access(), errmsg("could not read file \"%s\": %m",XLOG_CONTROL_FILE)));
    else ereport(PANIC,(errcode(ERRCODE_DATA_CORRUPTED),errmsg("could not read file \"%s\": read %d of %zu", XLOG_CONTROL_FILE, r, sizeof(ControlFileData))));
  }
  pgstat_report_wait_end();
  close(fd);

2. 检查控制文件：版本，CRC，兼容性检查
3. 设置respective GUC：wal_segment_size、data_checksums

= ControlFile->xlog_seg_size;
  if (!IsValidWalSegSize(wal_segment_size)) ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),errmsg_plural("WAL segment size must be a power of two between 1 MB and 1 GB, but the control file specifies %d byte","WAL segment size must be a power of two between 1 MB and 1 GB, but the control file specifies %d bytes",wal_segment_size,wal_segment_size)));

  snprintf(wal_segsz_str, sizeof(wal_segsz_str), "%d", wal_segment_size);
  SetConfigOption("wal_segment_size", wal_segsz_str, PGC_INTERNAL,PGC_S_OVERRIDE);

  /* check and update variables dependent on wal_segment_size */
  if (ConvertToXSegs(min_wal_size_mb, wal_segment_size) < 2)
    ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),errmsg("\"min_wal_size\" must be at least twice \"wal_segment_size\"")));
  if (ConvertToXSegs(max_wal_size_mb, wal_segment_size) < 2)
    ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),errmsg("\"max_wal_size\" must be at least twice \"wal_segment_size\"")));


  UsableBytesInSegment =(wal_segment_size / XLOG_BLCKSZ * UsableBytesInPage) - (SizeOfXLogLongPHD - SizeOfXLogShortPHD);

  CalculateCheckpointSegments();

  /* Make the initdb settings visible as GUC variables, too */
  SetConfigOption("data_checksums", DataChecksumsEnabled() ? "yes" : "no", PGC_INTERNAL, PGC_S_OVERRIDE);