On at least freebsd-13 trying to cancel a transfer whilst the libusb thread
is not running results in the transfers not completing cancellation.
The next time they're attempted to be re-added the libusb code thinks
they're still active, and returns BUSY on the buffers.
This causes gqrx to error out when one makes DSP changes or stops/starts it.
You have to restart gqrx to fix it.
After digging into it a bit, the libusb code expects that you're actively
running the main loop in order to have some deferred actions run in the
context of said main loop thread. This includes processing cancelled
transfers - the callbacks have to be run (if they exist) before the
buffers are properly cancelled and have their tracking metadata (a couple of
private pointers and state) removed from inside of libusb.
This patch does the following:
* separate out adding and cancelling transfers from the libusb worker thread
create/destroy path
* create the libusb worker thread when opening the device
* destroy the libusb worker thread when closing the device
* only add and cancel transfers when starting and stopping tx/rx
* handle cancelled transfers gracefully in the USB callback
Whilst here, also make the libusb device memory zeroed by using
calloc instead of malloc.
This fixes all of the weird libusb related buffer management problems
on FreeBSD.
0961a76f26