Prevent Lost Updates In Concurrent Category Bitmap Writes

[shaleenji]

Description

Category add/remove uses read-modify-write across separate transactions. Concurrent writes to the same category key can overwrite each other.

Steps to Reproduce

1. Run two concurrent writes adding different IDs to the same category field/value.
2. Make both writes overlap between bitmap read and bitmap write.
3. Query that category value.

Expected Behavior

The bitmap should contain both IDs after both successful writes.

Actual Behavior

One writer can overwrite the other writer's bitmap update.