Summary

DropCollectionCoordinator::removeTagsMetadataFromConfig has the same namespace mismatch bug as SERVER-127450, but is triggered by a plain drop — no resharding required.

Root Cause

In src/mongo/db/global_catalog/ddl/drop_collection_coordinator.cpp:412, the coordinator calls sharding_ddl_util::removeTagsMetadataFromConfig(opCtx, nss(), session). For a viewful timeseries collection, nss() resolves to the bucket namespace (e.g. db.system.buckets.coll) because _translateTimeseriesNss sets bucketNss on the coordinator metadata. Zone tags written via updateZoneKeyRange / sh.addTagRange() are always stored under the user-facing namespace (e.g. db.coll). The drop therefore only removes tags for the bucket namespace, leaving user-facing tags orphaned in config.tags.

Impact

Steps to reproduce:

1. Call updateZoneKeyRange on testDb.testColl (a viewful timeseries collection) → tag stored under testDb.testColl
2. Drop testDb.testColl → removeTagsMetadataFromConfig is called with the bucket nss → tag under testDb.testColl is not removed
3. Recreate testDb.testColl as a non-timeseries collection → the balancer looks up zones by exact namespace, finds the orphaned tag, and incorrectly applies it to the new collection

This reproduces without resharding and without a version upgrade, broadening the surface area relative to SERVER-127450.

Pre-existing orphaned config.tags entries — created either by this drop bug or by the resharding bug (SERVER-127450) — are also dormant until a v9.0 FCV upgrade, at which point viewless timeseries means config.tags is queried by user-facing namespace and the orphans become live. Since CAR already performs namespace migrations in src/mongo/db/global_catalog/ddl/timeseries_upgrade_downgrade_coordinator.cpp (see updateZonesInTransaction), the scrubbing logic to clean up pre-existing orphans is a natural fit there and is tracked in SERVER-129106.

Related

• SERVER-127450 — resharding variant of the same namespace mismatch (CS-owned fix)
• PR #54371 — discussion thread where both bugs were identified