dm: ignore ghost/trash table DDL without gh-ost comment when online-ddl is enabled

[Takashi-kun]

What problem does this PR solve?

Issue Number: close #12564

When a gh-ost migration leaves behind a ghost table (e.g. _t_gho), an operator may manually rename it for cleanup:

RENAME TABLE _t_gho TO will_be_deleted_t_gho;

Because DM identifies gh-ost tables purely by table-name regex, it treats any GhostTable → RealTable rename as a gh-ost cutover and tries to replay stored ALTER TABLE statements against the destination. Since that table doesn't exist in TiDB, the apply fails and blocks replication.

What is changed and how it works?

gh-ost embeds a /* gh-ost */ comment in all SQL statements it issues — CREATE TABLE, ALTER TABLE, RENAME TABLE, DROP TABLE, etc.:

create /* gh-ost */ table `schema`.`_t_gho` like `schema`.`t`
alter /* gh-ost */ table `schema`.`_t_gho` add column `n` int
rename /* gh-ost */ table `schema`.`t` to `schema`.`_t_del`, `schema`.`_t_gho` to `schema`.`t`

This comment is preserved verbatim in the MySQL binlog QUERY event.

Since ghost/trash tables (_gho, _ghc, _del) never exist in TiDB (DM does not replicate their creation downstream), any DDL on these tables that lacks the /* gh-ost */ comment is a manual operation and must be silently ignored. In processOneDDL, before calling Apply(), we now check the first source table type and the presence of the comment. Without the comment on a ghost/trash table, the event returns nil and replication continues unblocked.

Why not pass the statement through?
The initial approach returned the statement as-is for manual renames. But since ghost tables never exist in TiDB, executing RENAME TABLE _t_gho TO will_be_deleted downstream would also fail. Returning nil (ignore) is the correct behaviour.

Check List

Tests

• Unit test (TestGhostTableDDLWithoutGhostComment)
  • Sub-case A: gh-ost ALTER (with /* gh-ost */ comment) is stored; cutover RENAME (with comment) replays the stored ALTER — existing behaviour preserved
  • Sub-case B: manual RENAME without comment — silently ignored, no DDL emitted, no error

Questions

Will it cause performance regression or break compatibility?

No. The added check only runs when onlineDDL is enabled (online-ddl: true). The regexp match is O(n) on the raw SQL string and is negligible compared to the rest of DDL processing.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

Fixed DM incorrectly replaying stored gh-ost ALTER TABLE DDLs when a ghost/trash table is manually operated on without the gh-ost comment. Such events are now silently ignored since ghost tables never exist in the downstream.

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue where DM would erroneously attempt to replay stored DDLs during manual RENAME TABLE operations involving gh-ost tables. By inspecting the original SQL for the presence of a specific gh-ost comment, the system can now correctly distinguish between automated gh-ost cutovers and operator-initiated cleanups, ensuring that only legitimate gh-ost cutovers trigger DDL replay and allowing manual renames to proceed without interference.

Highlights

• Problem Addressed: DM was incorrectly replaying stored ALTER TABLE DDLs when a gh-ost table was manually renamed to a real table without the /* gh-ost */ comment, leading to replication failures.
• Solution Implemented: Introduced a check within processOneDDL to identify RENAME TABLE statements involving ghost-to-real table renames. If the original SQL for such a rename does not contain the /* gh-ost */ comment, it is now treated as a manual operation and passed through as-is, preventing DM from attempting to replay stored ALTER TABLE statements.
• Comment Detection: A new regular expression, ghostCommentRegexp, was added to detect the presence of the /* gh-ost */ marker in the originSQL of RENAME TABLE statements.
• Test Coverage: A new unit test, TestGhostTableRenameWithoutGhostComment, was added to cover both scenarios: gh-ost cutover renames with the comment (preserving existing behavior) and manual renames without the comment (applying the new logic).

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[gemini-code-assist]

Code Review

This pull request correctly fixes an issue where manual renames of gh-ost ghost tables were incorrectly treated as gh-ost cutovers, leading to DDL replay errors. The approach of checking for the /* gh-ost */ comment in the RENAME TABLE statement is a solid and reliable way to distinguish between automated cutovers and manual operations. The implementation is clean, and the addition of TestGhostTableRenameWithoutGhostComment provides good test coverage for both the new and existing logic paths. I have one suggestion to improve code readability.

[gemini-code-assist]

This if condition is a bit long and combines several checks, which can be hard to parse. To improve readability, consider breaking down the logic into a few named boolean variables that describe each part of the condition.

Suggested change

	if _, ok := ddlInfo.stmtCache.(*ast.RenameTableStmt); ok &&
	len(ddlInfo.sourceTables) == parserpkg.SingleRenameTableNameNum &&
	ddl.onlineDDL.TableType(ddlInfo.sourceTables[0].Name) == onlineddl.GhostTable &&
	ddl.onlineDDL.TableType(ddlInfo.sourceTables[1].Name) == onlineddl.RealTable &&
	!ghostCommentRegexp.MatchString(qec.originSQL) {
	return []string{ddlInfo.originDDL}, nil
	}
	if _, ok := ddlInfo.stmtCache.(*ast.RenameTableStmt); ok {
	isGhostToRealRename := len(ddlInfo.sourceTables) == parserpkg.SingleRenameTableNameNum &&
	ddl.onlineDDL.TableType(ddlInfo.sourceTables[0].Name) == onlineddl.GhostTable &&
	ddl.onlineDDL.TableType(ddlInfo.sourceTables[1].Name) == onlineddl.RealTable
	isManualRename := isGhostToRealRename && !ghostCommentRegexp.MatchString(qec.originSQL)
	if isManualRename {
	return []string{ddlInfo.originDDL}, nil
	}
	}