Changes 4.6.21:
* Fix a bug where mutex contention in database environments configured for
  hybrid mutex support could result in performance degradation.
* Fix a bug where closing a database handle after aborting a transaction
  which included a failed open of that database handle could result in
  application failure.
* Fix multiple MVCC bugs including a race which could result in incorrect
  data being returned to the application.
* Fix a bug where a database store into a Hash database could self-deadlock
  in a database environment configured for the Berkeley DB Concurrent Data
  Store product and with a free-threaded DB_ENV or DB handle.
* Fix an installation bug where Berkeley DB's PHP header file was not installed
  in the correct place.

(adam)