For test #5 above, the unit test "hooks" malloc - using dlsym to force a NULL return on demand to simulate a "NO MEM" condition.

The BUSY test currently fails because we're are blocked by D54218. Currently, opening the same device twice (via a different backend or dev-list) isn't an error. I'm proposing we proceed as-is. If/when D54218 is accepted, this code need not change.

D54296: Bug 291774: expand range of errors reported by libusb_open(3)
Unexpected one-line fix to backend.

DIOCGETRULENV takes the write lock as well but I believe this is only
required when clearing rule counters. (It might not be required even
then, on platforms where counter increment is done atomically.) Acquire
the read lock if that is not the case.

what's next?