etcd(ticdc): update etcd cluster member correctly (#12646)

[ti-chi-bot]

This is an automated cherry-pick of #12646

What problem does this PR solve?

Issue Number: close #12368

What is changed and how it works?

When updating etcd client URLs, remove the client that is not a member of the etcd cluster.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

tiup playground $VERSION --db 1 --kv 1 --pd 3 --ticdc 1 --tiflash 0 --without-monitor
tiup cdc:$VERSION cli changefeed create --sink-uri 'blackhole://'
tiup playground scale-out --pd 1
tiup playground display
tiup playground scale-in --pid 153389 # pick any one from the display

Print only once

[2026/05/21 10:03:39.588 +00:00] [INFO] [client.go:447] ["update endpoints"] [numChange=3->4] [lastEndpoints="[http://127.0.0.1:2382,http://127.0.0.1:2379,http://127.0.0.1:2384]"] [endpoints="[http://127.0.0.1:44765,http://127.0.0.1:2382,http://127.0.0.1:2384,http://127.0.0.1:2379]"]

Questions

Will it cause performance regression or break compatibility?

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

Release note

Fixed the bug of using an incorrect etcd client when scaling PD nodes

[ti-chi-bot]

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Adding the do-not-merge/cherry-pick-not-approved label.

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[gemini-code-assist]

Code Review

This pull request updates the etcd client by making the health check function private, removing the AutoSyncInterval configuration, and refining the healthyChecker to properly close and remove stale or offline clients. Feedback identifies a risk where an empty endpoint list could lead to the deletion of all healthy clients, suggesting an early return to preserve state. Additionally, it was noted that the timeout logic should use an else-if structure to prevent performing operations on clients that have already been closed and deleted.

[gemini-code-assist]

If eps is empty (which occurs if syncUrls fails to list members), this function will proceed to iterate over all existing clients in the checker and delete them because they won't be present in the empty updateEps map. This can lead to a complete loss of the healthy checker state due to transient etcd errors. Since an etcd cluster used by TiCDC should always have members, it is safer to return early if no endpoints are provided to preserve the current state.

	if len(eps) == 0 {
		return
	}
	updateEps := make(map[string]struct{}, len(eps))

[gemini-code-assist]

The second if condition for etcdServerDisconnectedTimeout (1m) will always be true if the first if for etcdServerOfflineTimeout (30m) is met. This results in calling SetEndpoints on a client that has already been closed and deleted from the checker. Using an else if ensures that these operations are mutually exclusive and avoids unnecessary operations on a closed client.

			if time.Since(lastHealthy) > etcdServerOfflineTimeout {
				log.Info("some etcd server maybe offline", zap.String("endpoint", ep))
				client.Close()
				checker.Delete(ep)
			} else if time.Since(lastHealthy) > etcdServerDisconnectedTimeout {
				// try to reset client endpoint to trigger reconnect
				client.Client.SetEndpoints([]string{}...)
				client.Client.SetEndpoints(ep)
			}