Couchbase Upgrade: Indexes Stuck in 'Indexing' State During Upgrade

During a Couchbase Server upgrade, N1QL indexes sometimes remain in the indexing state, causing query timeouts. This symptom suggests that the upgrade process may not complete the necessary rebuilds automatically.

Key uncertainties: the upgrade logs frequently omit explicit completion timestamps for index rebuilds; it is not clear if all index types are rebuilt automatically or if some require manual reindexing; cluster health metrics may still report healthy while indexes are not ready.

The goal is to determine the exact behavior of the upgrade process regarding index rebuilds and to identify reliable indicators that an index is fully rebuilt.

What specific log entries or API responses confirm that an index rebuild has finished during or after a Couchbase Server upgrade? Does the upgrade automatically trigger a full rebuild for all N1QL index types, or are manual steps required for certain types? How can we programmatically verify that all indexes report ready before resuming normal traffic?