PostgreSQL源码分析——pg_waldump

PG中有个可以打印WAL日志信息的工具，pg_waldump，对于开发人员来说，掌握该工具是十分有必要的。

用法

具体用法这里不去详解，可见官网pg_waldump文档。

shell 复制代码

postgres@slpc:~$ pg_waldump --help
pg_waldump decodes and displays PostgreSQL write-ahead logs for debugging.

Usage:
  pg_waldump [OPTION]... [STARTSEG [ENDSEG]]

Options:
  -b, --bkp-details      output detailed information about backup blocks
  -e, --end=RECPTR       stop reading at WAL location RECPTR
  -f, --follow           keep retrying after reaching end of WAL
  -n, --limit=N          number of records to display
  -p, --path=PATH        directory in which to find log segment files or a
                         directory with a ./pg_wal that contains such files
                         (default: current directory, ./pg_wal, $PGDATA/pg_wal)
  -q, --quiet            do not print any output, except for errors
  -r, --rmgr=RMGR        only show records generated by resource manager RMGR;
                         use --rmgr=list to list valid resource manager names
  -s, --start=RECPTR     start reading at WAL location RECPTR
  -t, --timeline=TLI     timeline from which to read log records
                         (default: 1 or the value used in STARTSEG)
  -V, --version          output version information, then exit
  -x, --xid=XID          only show records with transaction ID XID
  -z, --stats[=record]   show statistics instead of records
                         (optionally, show per-record statistics)
  -?, --help             show this help, then exit

具体的，我们举一个例子：

sql 复制代码

postgres=# create table t1(a int);
CREATE TABLE

postgres=# select pg_current_wal_lsn();
 pg_current_wal_lsn 
--------------------
 0/1696C78
(1 row)
-- 插入一条数据
postgres=# insert into t1 values(1);
INSERT 0 1
postgres=# select pg_current_wal_insert_lsn();
 pg_current_wal_insert_lsn 
---------------------------
 0/1696D18
(1 row)
postgres=# select txid_current();
 txid_current 
--------------
          735
(1 row)

-- 查看WAL日志
postgres@slpc:~/pgsql$ pg_waldump -p pgdata/pg_wal/ -s 0/1696C78 -e 0/1696D18
-- 插入一条数据，堆表， 事务ID 735   
rmgr: Heap        len (rec/tot):     59/    59, tx:        734, lsn: 0/01696C78, prev 0/01696C40, desc: INSERT+INIT off 1 flags 0x00, blkref #0: rel 1663/13010/16384 blk 0 
rmgr: Transaction len (rec/tot):     34/    34, tx:        734, lsn: 0/01696CB8, prev 0/01696C78, desc: COMMIT 2023-09-23 10:58:39.915785 CST
rmgr: Standby     len (rec/tot):     54/    54, tx:          0, lsn: 0/01696CE0, prev 0/01696CB8, desc: RUNNING_XACTS nextXid 735 latestCompletedXid 733 oldestRunningXid 734; 1 xacts: 734

-- 设置wal_debug = on
postgres=# set wal_debug = on;
SET
postgres=# select pg_current_wal_lsn();
 pg_current_wal_lsn 
--------------------
 0/1696E28
(1 row)

postgres=# insert into t1 values(2);
LOG:  INSERT @ 0/1696EC0:  - Heap/INSERT: off 2 flags 0x00
LOG:  INSERT @ 0/1696EE8:  - Transaction/COMMIT: 2023-09-23 11:27:25.399199+08
LOG:  xlog flush request 0/1696EE8; write 0/1696E28; flush 0/1696E28
INSERT 0 1
postgres=# select pg_current_wal_lsn();
 pg_current_wal_lsn 
--------------------
 0/1696F20
(1 row)

postgres@slpc:~/pgsql$ pg_waldump -p pgdata/pg_wal/ -s 0/1696E28 -e 0/1696F20
rmgr: Heap        len (rec/tot):     54/   150, tx:        736, lsn: 0/01696E28, prev 0/01696DF0, desc: INSERT off 2 flags 0x00, blkref #0: rel 1663/13010/16384 blk 0 FPW
rmgr: Transaction len (rec/tot):     34/    34, tx:        736, lsn: 0/01696EC0, prev 0/01696E28, desc: COMMIT 2023-09-23 11:27:25.399199 CST
rmgr: Standby     len (rec/tot):     50/    50, tx:          0, lsn: 0/01696EE8, prev 0/01696EC0, desc: RUNNING_XACTS nextXid 737 latestCompletedXid 736 oldestRunningXid 737

pg_waldump源码分析

pg_waldump的源码比较简单，在src/bin/pg_waldump中，执行pg_waldump -p pgdata/pg_wal/ -s 0/1696E28 -e 0/1696F20进行调试。

c 复制代码

main(int argc, char **argv)
--> XLogReaderAllocate(WalSegSz, waldir,XL_ROUTINE(.page_read = WALDumpReadPage,.segment_open = WALDumpOpenSegment, .segment_close = WALDumpCloseSegment), &private);
	// 因为我们是读wal日志的一个范围，从起点到终点，所以，这里第1步要做的就是先找到起始的wal日志记录
--> XLogFindNextRecord(xlogreader_state, private.startptr);
	for (;;)
	{
		// 开始读取后续WAL日志
		/* try to read the next record */
		record = XLogReadRecord(xlogreader_state, &errormsg);
		/* perform any per-record work */
		if (!config.quiet)
		{
			if (config.stats == true)
				XLogDumpCountRecord(&config, &stats, xlogreader_state);
			else
				XLogDumpDisplayRecord(&config, xlogreader_state);
		}
				
		/* check whether we printed enough */
		config.already_displayed_records++;
		if (config.stop_after_records > 0 &&
			config.already_displayed_records >= config.stop_after_records)
			break;
	}
--> XLogReaderFree(xlogreader_state);

更详细的代码可以查看：

c 复制代码

int main(int argc, char **argv)
{
	uint32		xlogid;
	uint32		xrecoff;
	XLogReaderState *xlogreader_state;
	XLogDumpPrivate private;
	XLogDumpConfig config;
	XLogDumpStats stats;
	XLogRecord *record;
	XLogRecPtr	first_record;

	private.timeline = 1;
	private.startptr = InvalidXLogRecPtr;     // 起点
	private.endptr = InvalidXLogRecPtr;       // 终点
	private.endptr_reached = false;

	while ((option = getopt_long(argc, argv, "be:fn:p:qr:s:t:x:z", long_options, &optindex)) != -1)
	{
		switch (option)
		{
			case 'e':
				if (sscanf(optarg, "%X/%X", &xlogid, &xrecoff) != 2)
				{
					pg_log_error("could not parse end WAL location \"%s\"",
								 optarg);
					goto bad_argument;
				}
				private.endptr = (uint64) xlogid << 32 | xrecoff;
				break;
			case 's':
				if (sscanf(optarg, "%X/%X", &xlogid, &xrecoff) != 2)
				{
					pg_log_error("could not parse start WAL location \"%s\"",
								 optarg);
					goto bad_argument;
				}
				else
					private.startptr = (uint64) xlogid << 32 | xrecoff;
				break;
        // ...
		}
	}

	// ...

	/* done with argument parsing, do the actual work */

	/* we have everything we need, start reading */
	// 初始化XLogReader，分配读取所需的内存空间
	xlogreader_state = XLogReaderAllocate(WalSegSz, waldir, XL_ROUTINE(.page_read = WALDumpReadPage,
									  .segment_open = WALDumpOpenSegment,
									  .segment_close = WALDumpCloseSegment), &private);

	/* first find a valid recptr to start from */
	// 找到第一个lsn >= startptr的有效日志记录
	first_record = XLogFindNextRecord(xlogreader_state, private.startptr);

	if (first_record == InvalidXLogRecPtr)
		fatal_error("could not find a valid record after %X/%X",
					LSN_FORMAT_ARGS(private.startptr));

	/*
	 * Display a message that we're skipping data if `from` wasn't a pointer
	 * to the start of a record and also wasn't a pointer to the beginning of
	 * a segment (e.g. we were used in file mode).
	 */
	if (first_record != private.startptr &&
		XLogSegmentOffset(private.startptr, WalSegSz) != 0)
		printf(ngettext("first record is after %X/%X, at %X/%X, skipping over %u byte\n",
						"first record is after %X/%X, at %X/%X, skipping over %u bytes\n",
						(first_record - private.startptr)),
			   LSN_FORMAT_ARGS(private.startptr),
			   LSN_FORMAT_ARGS(first_record),
			   (uint32) (first_record - private.startptr));

	for (;;)
	{
		/* try to read the next record */
		record = XLogReadRecord(xlogreader_state, &errormsg);
		if (!record)
		{
			if (!config.follow || private.endptr_reached)
				break;
			else
			{
				pg_usleep(1000000L);	/* 1 second */
				continue;
			}
		}

		/* apply all specified filters */
		if (config.filter_by_rmgr != -1 &&
			config.filter_by_rmgr != record->xl_rmid)
			continue;

		if (config.filter_by_xid_enabled &&
			config.filter_by_xid != record->xl_xid)
			continue;

		/* perform any per-record work */
		if (!config.quiet)
		{
			if (config.stats == true)
				XLogDumpCountRecord(&config, &stats, xlogreader_state);
			else
				XLogDumpDisplayRecord(&config, xlogreader_state);
		}

		/* check whether we printed enough */
		config.already_displayed_records++;
		if (config.stop_after_records > 0 &&
			config.already_displayed_records >= config.stop_after_records)
			break;
	}

	if (config.stats == true && !config.quiet)
		XLogDumpDisplayStats(&config, &stats);

	if (errormsg)
		fatal_error("error in WAL record at %X/%X: %s",
					LSN_FORMAT_ARGS(xlogreader_state->ReadRecPtr),
					errormsg);

	XLogReaderFree(xlogreader_state);

	return EXIT_SUCCESS;

bad_argument:
	fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
	return EXIT_FAILURE;
}

参考文档：
pg_waldump