PostgreSQL数据库WAL——XLogBeginInsert初始化

XLogBeginInsert初始化主要工作：通过设置begininsert_called标志防止递归调用日志生成函数；通过XLogInsertAllowed函数和一些Assert做代码检查工作。
XLogBeginInsert函数必须在XLogRegister*和XLogInsert之前调用

/* Begin constructing a WAL record. This must be called before the XLogRegister* functions and XLogInsert(). */
void XLogBeginInsert(void) {
  Assert(max_registered_block_id == 0);
  Assert(mainrdata_last == (XLogRecData *) &mainrdata_head);
  Assert(mainrdata_len == 0);

  /* cross-check on whether we should be here or not */
  if (!XLogInsertAllowed()) elog(ERROR, "cannot make new WAL entries during recovery");

  if (begininsert_called) elog(ERROR, "XLogBeginInsert was already called");
  begininsert_called = true;
}

在编译PostgreSQL时，打开WAL_DEBUG编译开关，开发者可借助wal_debug参数来显示每个写操作所产生的事务日志。