Right now, the listDatabases command checks for locks on each database (https://github.com/mongodb/mongo/blob/d7bd800d3d06004c4ca9114627ee882442e4d5e4/src/mongo/db/commands/list_databases.cpp#L129-L146) before actually checking the filter on the query (https://github.com/mongodb/mongo/blob/d7bd800d3d06004c4ca9114627ee882442e4d5e4/src/mongo/db/commands/list_databases.cpp#L148-L151).

This means that if any user is doing a command that takes a lock on database a, (i.e. building an index), no one can run a listDatabase command, even if they've used a filter that should exclude database a.

cc geert.bosch