dispatcher: out-of-date/mismatched block action still reports DONE

[3AceShowHand]

What did you do?

1. Ran storage integration heavy CI job:
   https://prow.tidb.net/jenkins/blue/organizations/jenkins/pingcap%2Fticdc%2Fpull_cdc_storage_integration_heavy/detail/pull_cdc_storage_integration_heavy/525/pipeline/449/
2. Observed duplicated/mixed block actions (Write/Pass) for the same dispatcher + commitTs.
3. In BasicDispatcher.HandleDispatcherStatus, when action does not match current pending block event, code still executes Step3 and reports BlockStage_DONE:
   • downstreamadapter/dispatcher/basic_dispatcher.go:829-839
4. Maintainer later receives DONE for missing event and logs:
   • maintainer/barrier.go "event is nil"
   • maintainer/barrier.go "No block event found"

What did you expect to see?

• Outdated or action-mismatched status should be ignored (or handled idempotently) without reporting DONE.
• Maintainer should not receive DONE for already-cleared/non-matching events.
• Barrier state should remain clean under duplicated/late actions.

What did you see instead?

• Dispatcher still reports DONE in the mismatch/outdated path.
• Maintainer logs frequent:
  • event is nil
  • No block event found
• In the same failing run, storage consumer later hit downstream inconsistency symptoms (including missing table errors), with potential timing/state-noise amplification.

Versions of the cluster

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

(unknown from CI artifact; please fill if available)

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

(unknown from CI artifact; please fill if available)

TiCDC version (execute cdc version):

CI run on pingcap/ticdc pull_cdc_storage_integration_heavy #525 (2026-03-03).
Local analysis branch head: da1038b2f

[3AceShowHand]

What I observed from one failed run

1) Same dispatcher + same commitTs receives two different actions (Write then later Pass)

From logs/cdc2.log:

5086 [2026/03/03 13:07:51.887 +08:00] ... [dispatcherStatus="... action:<CommitTs:464654033217912854 > "] ...
5087 [2026/03/03 13:07:51.887 +08:00] ... ["pending event get the action"] ... [innerAction=0] [pendingEventCommitTs=464654033217912854]
...
5102 [2026/03/03 13:07:51.973 +08:00] ... [dispatcherStatus="... action:<action:Pass CommitTs:464654033217912854 > "] ...

Interpretation:

- For commitTs 464654033217912854, the dispatcher first processes Write (innerAction=0), and later receives Pass for the same commitTs.
- This creates a stale/mismatched action scenario.

#### 2) Maintainer already removed the event, then receives another DONE

From logs/cdc1.log:

5846 [2026/03/03 13:07:51.988 +08:00] [INFO]  [barrier.go:437] ["all dispatchers reported event done, remove event"] ... [committs=464654033217912854]
5849 [2026/03/03 13:07:51.990 +08:00] [DEBUG] [barrier.go:290] ["No block event found, ignore the event done message"] ... [commitTs=464654033217912854]
5850 [2026/03/03 13:07:51.990 +08:00] [ERROR] [barrier.go:99]  ["handle block status failed, event is nil"] ... [commitTs=464654033217912854]

Interpretation:

- Event 464654033217912854 was removed at 13:07:51.988.
- A late/duplicate DONE for the same commitTs arrives at 13:07:51.990.
- Barrier logs No block event found and then event is nil.

### Why this can happen (code path)

Dispatcher side (downstreamadapter/dispatcher/basic_dispatcher.go):

- If an action does not match the current pending block event, not all branches return early.
- There is a fallback path (Step3) that still reports BlockStage_DONE for out-of-date/mismatched action context.

Maintainer side (maintainer/barrier.go):

- handleEventDone checks whether the block event still exists.
- If not found, it logs and ignores (No block event found, ignore the event done message).

### Scope / impact

- This is a secondary issue: primarily log noise and status churn from duplicated/late DONE.
- It is distinct from the primary data inconsistency issue.
- The maintainer side appears idempotent for this path, but the noise reduces debuggability and can obscure real failures.