There are two parts to this job. The immediate task is to reduce contention on the fil_system::mutex during the extension of datafiles. This WL for now focuses on reducing the contention on fil_system::mutex. With this fix multiple files can be extended at the same time and also other threads won't trying to do a normal read/write won't block during the course of extension. This should resolve the issue of stalls reported in BUG#56433. A future task will be to extend datafiles in the background, described in WL#5993.