stream_migrator: skip OnlineDDL stopped streams during SwitchWrites

[yushuqin]

Description

stream_migrator.readSourceStreams blocks SwitchWrites when it finds vreplication streams in state='Stopped' that aren't 'FROZEN'. This is intended to catch orphaned/problematic streams, but it also catches OnlineDDL cutover entries which legitimately remain Stopped (with message "stopped for online DDL cutover") for up to 24h before gcArtifacts() cleans them up.

This causes reshard SwitchWrites to fail if any OnlineDDL ran recently on the source shards.

Fixes #20838

Fix

• Legacy SQL path: added and message != 'stopped for online DDL cutover' to the stopped-streams query
• New gRPC path: filter out WorkflowType_OnlineDDL streams from the stopped check

Checklist

• Tests were added or are not required
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[chatgpt-codex-connector]

Limit the exemption to completed OnlineDDL cutovers

If OnlineDDL setup fails after generateInsertStatement creates its initially Stopped stream but before generateStartStatement runs, this type-only filter treats that orphan as a legitimate cutover artifact even though it does not have the stopped for online DDL cutover message. readSourceStreams then includes the orphan in the full stream set, and the SwitchWrites flow can migrate and eventually restart it on the target shards rather than refusing the reshard as before. Restrict this exemption to OnlineDDL streams carrying the cutover message, matching the legacy path.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Remove cutover artifacts before templatizing streams

When the completed OnlineDDL entry this exemption is meant to permit is present, it is excluded only from the stopped-stream guard; the subsequent unconstrained read still adds it to tabletStreams. OnlineDDL rules match the generated internal shadow-table name and their filter has no in_keyrange, so BuildStreamMigrator passes the entry to templatizeKeyRange, which fails to find that internal table in SourceKeyspaceSchema().Tables and dereferences the nil vtable. Thus the reported recent-OnlineDDL scenario can turn SwitchWrites from a validation error into a panic; exclude these completed cutover entries from the stream set being migrated in both read paths.

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[yushuqin]

Closing — branch was based on slack-22.0 and included unrelated commits. Re-opening with a clean single-commit branch based on main.