Sync-diff-inspector's GC keeper with specific snapshot does not work with TiDB v6.1 or below

[kennytm]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

Use TiDB as a data-source of sync-diff-inspector.
Set snapshot config to some fixed value.
Inspect the Service GC Safepoint via pd-ctl service-gc-safepoint

2. What did you expect to see?

The service-gc-safepoint list contains Sync_diff_xxxxxxxxx.

3. What did you see instead?

The safepoint is not registered.

The sync-diff log suggested the safepoint is 0.

[2024/12/19 16:32:30.080 +08:00] [INFO] [pd.go:228] ["generate dumpling gc safePoint id"] [id=Sync_diff_1734597150080542000]
[2024/12/19 16:32:30.080 +08:00] [DEBUG] [pd.go:230] ["update PD safePoint limit with ttl"] [safePoint=0] [updateInterval=2m30s]

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

v5.4.1, v6.1.7

5. which tool are you using?

sync-diff-inspector

6. what versionof tool are you using (pump -V or tidb-lightning -V or syncer -V)?

v6.5.3

[kennytm]

tidb-tools/sync_diff_inspector/diff.go

Lines 327 to 351 in 2109c5e

	if pdCli != nil {
	// Get latest snapshot
	latestSnap, err := utils.GetSnapshot(ctx, db)
	if err != nil {
	log.Info("failed to get snapshot, user should guarantee the GC stopped during diff progress.")
	return
	}
	if len(latestSnap) == 1 {
	if len(snap) == 0 {
	snap = latestSnap[0]
	}
	// compare the snapshot and choose the small one to lock
	if strings.Compare(latestSnap[0], snap) < 0 {
	snap = latestSnap[0]
	}
	}
	err = utils.StartGCSavepointUpdateService(ctx, pdCli, db, snap)
	if err != nil {
	log.Info("failed to keep snapshot, user should guarantee the GC stopped during diff progress.")
	} else {
	log.Info("start update service to keep GC stopped automatically")
	}
	}

Sync-diff-inspector determines the safepointTS using the following method:

1. Use SHOW MASTER STATUS to get the cluster's current TSO.
2. Compare with the given data-sources.*.snapshot, and use the earlier one (minimum).
3. Set as the safe point TS.

Unfortunately, on TiDB v6.1 and before, there is a bug in SHOW MASTER STATUS that when @@tidb_snapshot is defined it will always return 0:

$ # v6.1.7
$ mysql -u root -h 127.1 -P 4000 -e 'set tidb_snapshot = now(); show master status'             
+-------------+----------+--------------+------------------+-------------------+
| File        | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+-------------+----------+--------------+------------------+-------------------+
| tidb-binlog |        0 |              |                  |                   |
+-------------+----------+--------------+------------------+-------------------+

$ # v6.5.3
$ mysql -u root -h 127.1 -P 4000 -e 'set tidb_snapshot = now(); show master status;'
+-------------+--------------------+--------------+------------------+-------------------+
| File        | Position           | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+-------------+--------------------+--------------+------------------+-------------------+
| tidb-binlog | 454714341457920000 |              |                  |                   |
+-------------+--------------------+--------------+------------------+-------------------+

Since 0 is smaller than any given value, sync-diff-inspector will pick this as the safepoint and issued an invalid request to PD which is ignored.

We are not going to figure out how to fix TiDB. But sync-diff-inspector should prepare for a zero reply from SHOW MASTER STATUS and treat this as invalid.