Registered blocks aren't shared between threads

[snej]

If you instantiate multiple CBLDatabases on a single database file, they mostly stay in sync because they operate on the same documents. But map/reduce/validation/filter functions registered with one aren't seen by the other.

This causes problems when you start a CBLListener, because it creates new CBLDatabase instances on its background thread. HTTP requests to the listener that involve views or filters or validation won't work correctly.