log_level seems to be unused in this final version, can it be removed?

style(9) dictates explicit comparison against NULL here

Is there some implication here for no_discovery? Should this be reachable at all if it's set?

The no_discovery in upstream libusb is utilized when init the context. It will not start any hotplug scan thread if this option is true on the upstream libusb. That means this option is useless after libusb_init is called.

However, our implementation on the hotplug scan thread starts when the hotplug register is called instead of the libusb_init. I think the time point of no_discovery on upstream libusb is tighter because after init it will not work. So our implementation to do check on hotplug handler won't break any usecase of this option as it is expected to be set before init the context and actually hotplug register should be called after init.

It is a miss. Sorry, I will update a newer version later.

This probably calls for a comment explaining why the value is reused (was already true before your changes)

The upstream code change in here: https://github.com/libusb/libusb/commit/33d9949921423d0911527e04af8372d320f2489b.

They first use LIBUSB_OPTION_NO_DEVICE_DISCOVERY to replace LIBUSB_OPTION_WEAK_AUTHORITY. Then for the compatibility, they define LIBUSB_OPTION_WEAK_AUTHORITY as a macro again. I would send a different patch about this.