这个page的前40个字节就是XLogLongPageHeaderData。写入完之后整个的XLOG_SEGMENT的结构如下图所示：

据xlog中的xl_rmid调用资源管理器中不同资源的rm_redo回放函数进行回放。

/* symbol name, textual name, redo, desc, identify, startup, cleanup */
PG_RMGR(RM_XLOG_ID, "XLOG", xlog_redo, xlog_desc, xlog_identify, NULL, NULL, NULL)
PG_RMGR(RM_XACT_ID, "Transaction", xact_redo, xact_desc, xact_identify, NULL, NULL, NULL)
PG_RMGR(RM_SMGR_ID, "Storage", smgr_redo, smgr_desc, smgr_identify, NULL, NULL, NULL)
PG_RMGR(RM_CLOG_ID, "CLOG", clog_redo, clog_desc, clog_identify, NULL, NULL, NULL)
PG_RMGR(RM_DBASE_ID, "Database", dbase_redo, dbase_desc, dbase_identify, NULL, NULL, NULL)
PG_RMGR(RM_TBLSPC_ID, "Tablespace", tblspc_redo, tblspc_desc, tblspc_identify, NULL, NULL, NULL)
PG_RMGR(RM_MULTIXACT_ID, "MultiXact", multixact_redo, multixact_desc, multixact_identify, NULL, NULL, NULL)
PG_RMGR(RM_RELMAP_ID, "RelMap", relmap_redo, relmap_desc, relmap_identify, NULL, NULL, NULL)
PG_RMGR(RM_STANDBY_ID, "Standby", standby_redo, standby_desc, standby_identify, NULL, NULL, NULL)
PG_RMGR(RM_HEAP2_ID, "Heap2", heap2_redo, heap2_desc, heap2_identify, NULL, NULL, heap_mask)
PG_RMGR(RM_HEAP_ID, "Heap", heap_redo, heap_desc, heap_identify, NULL, NULL, heap_mask)
PG_RMGR(RM_BTREE_ID, "Btree", btree_redo, btree_desc, btree_identify, NULL, NULL, btree_mask)
PG_RMGR(RM_HASH_ID, "Hash", hash_redo, hash_desc, hash_identify, NULL, NULL, hash_mask)
PG_RMGR(RM_GIN_ID, "Gin", gin_redo, gin_desc, gin_identify, gin_xlog_startup, gin_xlog_cleanup, gin_mask)
PG_RMGR(RM_GIST_ID, "Gist", gist_redo, gist_desc, gist_identify, gist_xlog_startup, gist_xlog_cleanup, gist_mask)
PG_RMGR(RM_SEQ_ID, "Sequence", seq_redo, seq_desc, seq_identify, NULL, NULL, seq_mask)
PG_RMGR(RM_SPGIST_ID, "SPGist", spg_redo, spg_desc, spg_identify, spg_xlog_startup, spg_xlog_cleanup, spg_mask)
PG_RMGR(RM_BRIN_ID, "BRIN", brin_redo, brin_desc, brin_identify, NULL, NULL, brin_mask)
PG_RMGR(RM_COMMIT_TS_ID, "CommitTs", commit_ts_redo, commit_ts_desc, commit_ts_identify, NULL, NULL, NULL)
PG_RMGR(RM_REPLORIGIN_ID, "ReplicationOrigin", replorigin_redo, replorigin_desc, replorigin_identify, NULL, NULL, NULL)
PG_RMGR(RM_GENERIC_ID, "Generic", generic_redo, generic_desc, generic_identify, NULL, NULL, generic_mask)
PG_RMGR(RM_LOGICALMSG_ID, "LogicalMessage", logicalmsg_redo, logicalmsg_desc, logicalmsg_identify, NULL, NULL, NULL)

在 PostgreSQL 内核中，rmgrlist.h 文件定义了各种类型的 rmgr（record manager），每个 rmgr 对应一个不同的数据库操作。这些 rmgr 在事务日志（XLog）中起着重要作用，用于记录不同类型的数据库操作，以便在之后的恢复过程中重做（redo）或撤销（undo）这些操作。

1. RM_XLOG_ID: 这是 XLog 自身的 rmgr，负责记录 XLog 中的操作，用于数据库的恢复和故障转移。
2. RM_XACT_ID: 这个 rmgr 负责记录事务提交和回滚的操作，用于数据库的一致性和事务管理。
3. RM_SMGR_ID: 这个 rmgr 负责记录存储管理器（SMGR）相关的操作，用于数据库的物理存储管理。
4. RM_CLOG_ID: 这个 rmgr 负责记录提交日志（CLOG）相关的操作，用于事务的可见性和并发控制。
5. RM_DBASE_ID: 这个 rmgr 负责记录数据库的创建和删除操作。
6. RM_TBLSPC_ID: 这个 rmgr 负责记录表空间的创建和删除操作。
7. RM_MULTIXACT_ID: 这个 rmgr 负责记录多版本并发控制（MVCC）相关的操作。
8. RM_RELMAP_ID: 这个 rmgr 负责记录关系映射（RelMap）相关的操作。
9. RM_STANDBY_ID: 这个 rmgr 负责记录热备相关的操作。
10. RM_HEAP2_ID: 这个 rmgr 也负责记录堆表（Heap）的修改操作。
11. RM_HEAP_ID: 这个 rmgr 负责记录堆表（Heap）的修改操作，包括插入、更新和删除等。
12. RM_BTREE_ID: 这个 rmgr 负责记录 B 树索引的修改操作。
13. RM_HASH_ID: 这个 rmgr 负责记录哈希索引的修改操作。
14. RM_GIN_ID: 这个 rmgr 负责记录 GIN 索引的修改操作。
15. RM_GIST_ID: 这个 rmgr 负责记录 GIST 索引的修改操作。
16. RM_SEQ_ID: 这个 rmgr 负责记录序列（Sequence）的修改操作。
17. RM_SPGIST_ID: 这个 rmgr 负责记录 SP-GiST 索引的修改操作。
18. RM_BRIN_ID: 这个 rmgr 负责记录 BRIN 索引的修改操作。
19. RM_COMMIT_TS_ID: 这个 rmgr 负责记录提交时间戳的修改操作。
20. RM_REPLORIGIN_ID: 这个 rmgr 负责记录复制源的修改操作。
21. RM_GENERIC_ID: 这个 rmgr 用于实验性目的，可用于自定义类型的操作。
22. RM_LOGICALMSG_ID: 这个 rmgr 用于逻辑复制相关的操作。

-----------------LONGHEADER------------------------
xlp_magic: D101
xlp_info: 7
xlp_tli: 1
xlp_pageaddr: 77000000
xlp_rem_len: 1257
xlp_sysid: 64B6A0F56CFBE0AA
xlp_seg_size: 1000000
xlp_seg_size in MB: 16.0
xlp_xlog_blcksz: 2000
xlp_xlog_blcksz in KB: 8.0
---------------RECORD-----------------------
xl_tot_len: F5
xl_xid: A048
xl_prev: 76FFF238
xl_info: 0
xl_rmid: A
xl_crc: AA2568A1
---------------INFO-----------------------
当前info:  0
current opera --> insert
---------------XLogRecordBlockHeader-----------------------
id: 00
fork_flags: 20
data_length: 00C4
ref.spcNode: 0000067F
ref.dbNode: 0000401C
ref.relNode: 000040FE
blockNumber: 0000151D
---------------XLogRecordDataHeaderShort-----------------------
id: 255
data_length: 3
---------------xl_heap_header-----------------------
t_infomask2: 4
t_infomask: 2050
t_hoff: 24

import ctypes
import sys
class XlogPageHeaderDate(ctypes.Structure):
    _fields_ = [
        ('xlp_magic',ctypes.c_uint16),
        ('xlp_info',ctypes.c_uint16),
        ('xlp_tli',ctypes.c_uint32),
        ('xlp_pageaddr',ctypes.c_uint64),
        ('xlp_rem_len',ctypes.c_uint32),
        ('xlp_al',ctypes.c_uint32) #用于对齐的4个byte
    ]
class XLogLongPageHeaderData(ctypes.Structure):
    _fields_ = [
        ('std', XlogPageHeaderDate),
        ('xlp_sysid', ctypes.c_uint64),
        ('xlp_seg_size', ctypes.c_uint32),
        ('xlp_xlog_blcksz', ctypes.c_uint32)
    ]

class XLogRecord(ctypes.Structure):
    _fields_ = [
        ('xl_tot_len', ctypes.c_uint32),
        ('xl_xid', ctypes.c_uint32),
        ('xl_prev', ctypes.c_uint64),
        ('xl_info', ctypes.c_uint8),
        ('xl_rmid', ctypes.c_uint8),
        ('_padding', ctypes.c_uint16),  # 2 bytes of padding, initialize to zero
        ('xl_crc', ctypes.c_uint32),
    ]

class RelFileNode(ctypes.Structure):  
    _fields_ = [  
        ('spcNode', ctypes.c_uint32),  
        ('dbNode', ctypes.c_uint32),  
        ('relNode', ctypes.c_uint32),  
    ]

class XLogRecordBlockHeader(ctypes.Structure):  
    _fields_ = [  
        ('id', ctypes.c_uint8),  
        ('fork_flags', ctypes.c_uint8),  
        ('data_length', ctypes.c_uint16), 
        ('ref',  RelFileNode),
        ('block_num', ctypes.c_int32),
    ]

class XLogRecordDataHeaderShort(ctypes.Structure):
    _fields_ = [
        ('id', ctypes.c_uint8),
        ('data_length', ctypes.c_uint8),
    ]

class XLHeapHeader(ctypes.Structure):
    _fields_ = [
        ('t_infomask2', ctypes.c_uint16),
        ('t_infomask', ctypes.c_uint16),
        ('t_hoff',ctypes.c_ubyte)
        # ('t_hoff', ctypes.c_uint8),
    ]

typedef struct XLogPageHeaderData
{
	uint16		xlp_magic;		/* magic value for correctness checks */
	uint16		xlp_info;		/* flag bits, see below */
	TimeLineID	xlp_tli;		/* TimeLineID of first record on page */--32
	XLogRecPtr	xlp_pageaddr;	/* XLOG address of this page */--64
	uint32		xlp_rem_len;	/* total len of remaining data for record */
} XLogPageHeaderData;

typedef struct XLogLongPageHeaderData
{
    XLogPageHeaderData std;     /* standard header fields  */
    uint64      xlp_sysid;      /* system identifier from pg_control */
    uint32      xlp_seg_size;   /* just as a cross-check */
    uint32      xlp_xlog_blcksz;    /* just as a cross-check */
} XLogLongPageHeaderData;

#define SizeOfXLogShortPHD	MAXALIGN(sizeof(XLogPageHeaderData))

#define MAXALIGN(LEN)			TYPEALIGN(MAXIMUM_ALIGNOF, (LEN))

/* ----------------
 * Alignment macros: align a length or address appropriately for a given type.
 * The fooALIGN() macros round up to a multiple of the required alignment,
 * while the fooALIGN_DOWN() macros round down.  The latter are more useful
 * for problems like "how many X-sized structures will fit in a page?".
 *
 * NOTE: TYPEALIGN[_DOWN] will not work if ALIGNVAL is not a power of 2.
 * That case seems extremely unlikely to be needed in practice, however.
 *
 * NOTE: MAXIMUM_ALIGNOF, and hence MAXALIGN(), intentionally exclude any
 * larger-than-8-byte types the compiler might have.
 * ----------------
 */

#define TYPEALIGN(ALIGNVAL,LEN)  \
	(((uintptr_t) (LEN) + ((ALIGNVAL) - 1)) & ~((uintptr_t) ((ALIGNVAL) - 1)))

static void
ReserveXLogInsertLocation(int size, XLogRecPtr *StartPos, XLogRecPtr *EndPos,
						  XLogRecPtr *PrevPtr)
{
	XLogCtlInsert *Insert = &XLogCtl->Insert;
	uint64		startbytepos;
	uint64		endbytepos;
	uint64		prevbytepos;

	size = MAXALIGN(size);
  
  ...
    
	SpinLockAcquire(&Insert->insertpos_lck);

	startbytepos = Insert->CurrBytePos;
	endbytepos = startbytepos + size;
	prevbytepos = Insert->PrevBytePos;
	Insert->CurrBytePos = endbytepos;
	Insert->PrevBytePos = startbytepos;

	SpinLockRelease(&Insert->insertpos_lck);

	*StartPos = XLogBytePosToRecPtr(startbytepos);
	*EndPos = XLogBytePosToEndRecPtr(endbytepos);
	*PrevPtr = XLogBytePosToRecPtr(prevbytepos);

  ...
}

#define UsableBytesInPage (XLOG_BLCKSZ - SizeOfXLogShortPHD)
#define UsableBytesInSegment ((XLOG_SEG_SIZE / XLOG_BLCKSZ) * UsableBytesInPage - (SizeOfXLogLongPHD - SizeOfXLogShortPHD))

// UsableBytesInSegment = (XLOG_SEG_SIZE / XLOG_BLCKSZ) * UsableBytesInPage - (SizeOfXLogLongPHD - SizeOfXLogShortPHD)
// UsableBytesInSegment = (16 * 1024 * 1024 / 8192) * (8192 - SizeOfXLogShortPHD) - (XlogHeaderDiff)
// UsableBytesInSegment = 页面数量 * 每个页面除了HEADER之外的空间 - 第一个页面的HEADER多出来的一部分
// UsableBytesInSegment = 一个SEG内能用来保存XLOG的空间总大小
// UsableBytesInSegment = 16728048
// 如果没有HEADER，UsableBytesInSegment = 16 * 1024 * 1024 = 16777216

uint8				info;
	
	info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
	info &= XLOG_HEAP_OPMASK;
	
	if (XLOG_HEAP_INSERT == info)
	{
		插入
	}
	else if(XLOG_HEAP_DELETE == info)
	{
		删除
	}
	else if (XLOG_HEAP_UPDATE == info)
	{
		更新
	}
	else if (XLOG_HEAP_HOT_UPDATE == info)
	{
		更新
	}

def judge_info(info_num):
    info = info_num & ~XLR_INFO_MASK
    info &= XLOG_HEAP_OPMASK

    if XLOG_HEAP_INSERT == info:
        print('current opera --> insert')
    elif XLOG_HEAP_DELETE == info:
        print('current opera --> delete')
    elif XLOG_HEAP_UPDATE == info or XLOG_HEAP_HOT_UPDATE == info:
        print('current opera --> update')

# rmid定义判断
RM_XLOG_ID = 0x00
RM_XACT_ID = 0x01
RM_SMGR_ID = 0x02
RM_CLOG_ID = 0x03
RM_DBASE_ID = 0x04
RM_TBLSPC_ID = 0x05
RM_MULTIXACT_ID = 0x06
RM_RELMAP_ID = 0x07
RM_STANDBY_ID = 0x08
RM_HEAP2_ID = 0x09
RM_HEAP_ID = 0x0A
RM_BTREE_ID = 0x0B
RM_HASH_ID = 0x0C
RM_GIN_ID = 0x0D
RM_GIST_ID = 0x0E
RM_SEQ_ID = 0x0F
RM_SPGIST_ID = 0x10
RM_BRIN_ID = 0x11
RM_COMMIT_TS_ID = 0x12
RM_REPLORIGIN_ID = 0x13
RM_GENERIC_ID = 0x14
RM_LOGICALMSG_ID = 0x15

def read_wal(path):
    with open(path,mode='rb') as f:
        buf = f.read(XLOG_BLCKSZ)
        # print(buf)
        hdr = buf[:XLOG_HDRLEN]
        # hdr = buf
        lhdr = buf[:XLOG_LONGHDRLEN]
        rem = buf[XLOG_LONGHDRLEN:XLOG_REM+XLOG_LONGHDRLEN] # 看看是不是page的第一个！！！
        record = buf[XLOG_REM+XLOG_LONGHDRLEN : XLOG_REM+XLOG_LONGHDRLEN + XLOG_RECORD]
        rbh = buf[XLOG_REM+XLOG_LONGHDRLEN + XLOG_RECORD : XLOG_REM+XLOG_LONGHDRLEN + XLOG_RECORD + XLOGRECORDBLOCKHEADER]
        rdhs = buf[XLOG_REM+XLOG_LONGHDRLEN + XLOG_RECORD + XLOGRECORDBLOCKHEADER 
                  : XLOG_REM+XLOG_LONGHDRLEN + XLOG_RECORD + XLOGRECORDBLOCKHEADER + XLOGRECORDDATAHEADERSHORT]
        xhh = buf[XLOG_REM+XLOG_LONGHDRLEN + XLOG_RECORD + XLOGRECORDBLOCKHEADER + XLOGRECORDDATAHEADERSHORT 
                  : XLOG_REM+XLOG_LONGHDRLEN + XLOG_RECORD + XLOGRECORDBLOCKHEADER + XLOGRECORDDATAHEADERSHORT + XLOGHEAPHEADER +1]