Error context and the Errors object seems to be shared between threads, so there is a race condition. We could perhaps use a separate Errors per thread as a quick fix.

A more efficient approach would be to have parse just return a list of error descriptions, and then the main thread would report any errors via the shared Errors instance. This way we wouldn't need to set up error context for each file unless there are errors (which is rare), so the sequential part would do less work, possibly increasing the maximum possible concurrency.

It's okay to do something simple at first. We can always improve it later, but it would be good to fix race conditions before merging.

Also in BuildManager.parse_file, the add_stats call is racy. In this case we could add a stats_enabled check to quickly skip stats reporting most of the time (again reducing sequential part), and then use a lock around stats updates if stats are enabled.