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Calling libusb_cancel_transfer only starts the cancellation of a transfer. The process is not complete until the transfer callback has been called with status LIBUSB_TRANSFER_CANCELLED. If hackrf_start_rx() is called soon after hackrf_stop_rx(), prepare_transfers() may be called before the previous cancellations are completed, resulting in a LIBUSB_ERROR_BUSY when a transfer is reused with libusb_submit_transfer(). To prevent this happening, we keep track of which transfers have finished (either by completion, or cancellation), and make cancel_transfers() wait until all transfers are finished. This is implemented using a pthread condition variable which is signalled from the transfer thread.