以heap_insert为例，简述WAL的插入过程。
在构建WAL日志记录的过程中主要涉及2个数据变量：static XLogRecData rdatas数组和static registered_buffer registered_buffers数组。这两个数组分别用来保存WAL数据和管理rdatas链表。
主要涉及3个重要的函数：XLogRegisterData、XLogRegisterBuffer和XLogRegisterBufData。这三个函数的作用分别是将WAL记录的特殊结构体数据注册到WAL，比如heap_insert中的xl_heap_insert结构体；将涉及到的buf注册到wal记录，比如heap_insert中page页赋予regbuf->page；将元组内容注册到WAL记录，比如insert语句的元组数据等。
下面首先介绍相关数据结构。
1、数据结构
HeapTupleData
typedef struct HeapTupleData { uint32 t_len; /* length of *t_data */ ItemPointerData t_self; /* SelfItemPointer */ Oid t_tableOid; /* table the tuple came from */ HeapTupleHeader t_data; /* -> tuple header and data */ } HeapTupleData;
xl_heap_header
`/*

• We don’t store the whole fixed part (HeapTupleHeaderData) of an inserted
• or updated tuple in WAL; we can save a few bytes by reconstructing the
• fields that are available elsewhere in the WAL record, or perhaps just
• plain needn’t be reconstructed. These are the fields we must store.
• NOTE: t_hoff could be recomputed, but we may as well store it because
• it will come for free due to alignment considerations.
  /
  typedef struct xl_heap_header
  {
  uint16 t_infomask2;
  uint16 t_infomask;
  uint8 t_hoff;
  } xl_heap_header; xl_heap_insert
  / This is what we need to know about insert /
  typedef struct xl_heap_insert
  {
  OffsetNumber offnum; / inserted tuple’s offset /
  uint8 flags;
  / xl_heap_header & TUPLE DATA in backup block 0 /
  } xl_heap_insert; XLogRecData/
• The functions in xloginsert.c construct a chain of XLogRecData structs
• to represent the final WAL record.
  /
  typedef struct XLogRecData
  {
  struct XLogRecData next; / next struct in chain, or NULL /
  char data; / start of rmgr data to include /
  uint32 len; / length of rmgr data to include /
  } XLogRecData; registered_buffer
  /
• For each block reference registered with XLogRegisterBuffer, we fill in
• a registered buffer struct.
  /
  typedef struct
  {
  bool in_use; / is this slot in use? /
  uint8 flags; / REGBUF flags /
  RelFileNode rnode; / identifies the relation and block /
  ForkNumber forkno;
  BlockNumber block;
  Page page; / page content /
  uint32 rdata_len; / total length of data in rdata chain /
  XLogRecData rdata_head; / head of the chain of data registered with this block /
  XLogRecData rdata_tail; / last entry in the chain, or &rdata_head if empty /
  XLogRecData bkp_rdatas[2]; /* temporary rdatas used to hold references to
• backup block data in XLogRecordAssemble() /
  / buffer to store a compressed version of backup block image */
  char compressed_page[PGLZ_MAX_BLCKSZ];
  } registered_buffer;`
  2、heap_insert涉及WAL的流程