Postgresql源码（5）Xlog注册

[1] XLogBeginInsert
----> 注册数据
[2] XLogRegisterData：注册生成日志记录的数据，每调一次使用一个rdatas数组位置
[3] XLogRegisterBuffer：注册数据页面相关信息
[4] XLogRegisterBlock
[4] XLogRegisterBufData
...
[5] XLogSetRecordFlags
----> 组装、写入xlog
[6] XLogInsert
    [6.1] XLogRecordAssemble：组装日志记录
    [6.2] XLogInsertRecord：写入WAL BUFFER
    [6.3] XLogResetInsertion：重置日志写入的各种变量

/*
 * 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 */
【页面的文件虚拟node】
	RelFileNode rnode;			/* identifies the relation and block */
	ForkNumber	forkno;
	BlockNumber block;
【页面内容指针】
	Page		page;			/* page content */
【XLogRegisterBufferData 注册进来的长度】
	uint32		rdata_len;		/* total length of data in rdata chain */
【XLogRegisterBufferData 注册数据到这个链表】
	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;

		XLogBeginInsert();
		XLogRegisterData((char *) &xlrec, SizeOfHeapInsert);

		xlhdr.t_infomask2 = heaptup->t_data->t_infomask2;
		xlhdr.t_infomask = heaptup->t_data->t_infomask;
		xlhdr.t_hoff = heaptup->t_data->t_hoff;

【insert单次使用一个页面，占用registered_buffer的0号位置】
		XLogRegisterBuffer(0, buffer, REGBUF_STANDARD | bufflags);
		
【注册完了写数据，写什么数据？ xl_heap_header{t_infomask2 = ?, t_infomask = ?, t_hoff = 24}】
		XLogRegisterBufData(0, (char *) &xlhdr, SizeOfHeapHeader);
		/* PG73FORMAT: write bitmap [+ padding] [+ oid] + data */

|-------------------------------|       <------ tuple
|           HeapTuple           |
|-------------------------------|       <------ tuple->t_data      header->t_hoff = len(head)
|        HeapTupleHeader        |
|-------------------------------|
|                               |
|                               |
|             DATA              |
|                               |
|                               |
|-------------------------------|

【接着写重要的数据！写tuple的实际数据】
【heaptup->t_data这个是把HeapTuple和HeapTupleHeader都跳过去了，指向tuple实际数据】
【heaptup->t_len - SizeofHeapTupleHeader】
【 = sizeof(HeapTupleHeader + DATA) - sizeof(HeapTupleHeader)】
		XLogRegisterBufData(0,
							(char *) heaptup->t_data + SizeofHeapTupleHeader,
							heaptup->t_len - SizeofHeapTupleHeader);


		/* filtering by origin on a row level is much more efficient */
		XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);
【写入WAL BUFFER】
		recptr = XLogInsert(RM_HEAP_ID, info);

typedef struct
{
	...
	XLogRecData *rdata_head;
	XLogRecData *rdata_tail;
	...
} registered_buffer;

【初始化：】
【链表初始化把tail指向head的地址，第一次往TAIL里面写其实就是写HEAD】
【这样可以免去初始化HEAD的动作】
regbuf->rdata_tail = (XLogRecData *) &regbuf->rdata_head;
regbuf->rdata_len = 0;

typedef struct XLogRecData
{
	struct XLogRecData *next;
	char	   *data;
	uint32		len;
} XLogRecData;

【数组出初始化20个位置】
static XLogRecData *rdatas;
rdatas = MemoryContextAlloc(xloginsert_cxt, sizeof(XLogRecData) * XLR_NORMAL_RDATAS);
max_rdatas = XLR_NORMAL_RDATAS; // 20

【num_rdatas做index依次使用，有严格上限】
	if (num_rdatas >= max_rdatas)
		elog(ERROR, "too much WAL data");
	rdata = &rdatas[num_rdatas++];
【数据存在数组元素里面】
	rdata->data = data;
	rdata->len = len;
【链表首先把第一个元素的next指向下一个】
【然后把之前】
	regbuf->rdata_tail->next = rdata;
	regbuf->rdata_tail = rdata;
	regbuf->rdata_len += len;