Interrupt USB polling thread before threading shutdown #480
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While performing `USBContext.handleEvents()` the poller thread is uninterruptible other than through libusb1 API. Since the Python runtime joins all non-daemon threads on threading shutdown it is necessary to interrupt the thread by calling the corresponding libusb1 function. A seemingly obvious solution is to make the thread a daemon thread so that it is not joined on threading shutdown. Unfortunately this only makes the problem worse. All daemon threads are instantly killed on shutdown, leaving a lock within libusb1 permanently taken, and causing a hang later when python-libusb1 objects are garbage collected by the main thread. Fixes GlasgowEmbedded#413.
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WIP: requires libusb1 version bump after vpelletier/python-libusb1#90 is merged.
While performing
USBContext.handleEvents()the poller thread is uninterruptible other than through libusb1 API. Since the Python runtime joins all non-daemon threads on threading shutdown it is necessary to interrupt the thread by calling the corresponding libusb1 function.A seemingly obvious solution is to make the thread a daemon thread so that it is not joined on threading shutdown. Unfortunately this only makes the problem worse. All daemon threads are instantly killed, leaving a lock within libusb1 permanently taken, and causing a hang later when python-libusb1 objects are garbage collected.
Fixes #413.