Review findings on #372:
- ImageGenerationHandler keeps the workers a stop signalled that had not
ended; every later shutdown/close joins them again, so a second call no
longer returns True while the first worker still runs.
- ImageMonitor.stop_monitoring keeps an observer or handler it left
running and stops and joins it again on the next call.
- A closed handler takes no awaiting history path, and a re-check that
raced close() is not kept due.
- Tests: second stops of a busy worker and a live observer, the closed
handler's re-check paths, the handle test asserts the worker ended and
stops before it asserts, and a no-new-thread check instead of set
equality.
Vito Sansevero
ade967204f