release-8.5 CI: same_upstream_downstream fails because TiDB 8.5.x lacks mysql.tidb.cluster_id

[wlwilliamx]

What did you do?

• Ran TiCDC release-8.5 MySQL light integration tests (group G06), which includes same_upstream_downstream.
• CI example: https://prow.tidb.net/jenkins/blue/organizations/jenkins/pingcap%2Fticdc%2Fpull_cdc_mysql_integration_light/detail/pull_cdc_mysql_integration_light/687/pipeline/451/

What did you expect to see?

Creating a changefeed with the same TiDB cluster as both upstream and downstream should be rejected with CDC:ErrSameUpstreamDownstream.

What did you see instead?

The changefeed creation succeeds and the test fails with:
Expected create to fail with ErrSameUpstreamDownstream, got: Create changefeed successfully!

Root cause:

• TiCDC determines downstream identity by querying mysql.tidb for cluster_id.
• TiDB 8.5.x (release-8.5 line) does not write cluster_id into mysql.tidb because bootstrap: add cluster_id to the mysql.tidb table tidb#59511 was not cherry-picked to release-8.5.
• As a result, TiCDC cannot detect "same upstream downstream" and the safeguard is skipped.

Versions of the cluster

Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

CI log shows Release Version v8.5.6 (release-8.5 line); `mysql.tidb` does not contain `cluster_id`.

Upstream TiKV version (execute tikv-server --version):

(unknown / CI env)

TiCDC version (execute cdc version):

TiCDC release-8.5 (MySQL light IT includes `same_upstream_downstream`)