The documentation correctly states that a foreground index build is a blocking operation. However, it does not explain what this means.

When a foreground index build is occurring on a particular collection in a particular database, it holds a write lock for that database for the entire duration of the index build, and does not release it until the index build is complete. This means that while the index build is ongoing:

• You cannot access (read, insert, update, delete) any documents in any collection in the same database as the index build
• You cannot perform any operation that requires a read or write lock on all databases ( "listDatabases" comes to mind, but there are probably others ).

If and when collection-level locking gets implemented, this will change.