Side note:

> One concern I do have - while writing the patch, I noticed the existing submit method (specifically the adjust_thread_count function) isn't thread safe.

True.  The executor is obviously thread-safe internally (as it handles multiple worker threads).  But the user should not /call/ it from multiple threads.

(most primitives exposed by the Python stdlib are not thread-safe, except for the simplest ones such as lists, dicts etc.)