Mark watchdog thread daemonic #16
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A python application will only quit when all non-daemonic threads have stopped. The watchdog thread is non-daemonic and so oresat_c3 can only quit once the watchdog thread has stopped. The watchdog thread will only quit if `event` is set, and `event` only gets set if oresat_c3 successfully makes it to the end of `main()`. If oresat_c3 doesn't make it successfully to the end of `main()` (say an exception occurs during service initialization) then the watchdog thread will never stop, even though the main thread has, keeping the application running in a sort of unresponsive zombie mode. It is in fact so unresponsive that it ignores SIGINT (ctrl-c) so the only way to get it to stop is a SIGKILL, which is uncouth. Daemonic threads on the other hand will stop abruptly when the last non-daemon thread stops. This can be bad if resources in the thread need finalization but the only resource in the watchdog thread is a UDP socket, which the kernel will helpfully close for us. Setting the daemonic flag on the watchdog thread allows oresat_c3 to stop gracefully in exceptional circumstances. An alternative to daemonic threads would be to wrap everything after starting the thread in a try-finally block but I felt that would be a bit more intrusive.
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A python application will only quit when all non-daemonic threads have stopped. The watchdog thread is non-daemonic and so oresat_c3 can only quit once the watchdog thread has stopped. The watchdog thread will only quit if
eventis set, andeventonly gets set if oresat_c3 successfully makes it to the end ofmain().If oresat_c3 doesn't make it successfully to the end of
main()(say an exception occurs during service initialization) then the watchdog thread will never stop, even though the main thread has, keeping the application running in a sort of unresponsive zombie mode. It is in fact so unresponsive that it ignores SIGINT (ctrl-c) so the only way to get it to stop is a SIGKILL, which is uncouth.Daemonic threads on the other hand will stop abruptly when the last non-daemon thread stops. This can be bad if resources in the thread need finalization but the only resource in the watchdog thread is a UDP socket, which the kernel will helpfully close for us. Setting the daemonic flag on the watchdog thread allows oresat_c3 to stop gracefully in exceptional circumstances.
An alternative to daemonic threads would be to wrap everything after starting the thread in a try-finally block but I felt that would be a bit more intrusive.