Comment 0 for bug 1890572

USB backend never ends if the printer is not connected. we have been able to identify a infinity loop in print_device function in usb-libusb.c file:

```
  fprintf(stderr, "DEBUG: Printing on printer with URI: %s\n", uri);
  while ((g.printer = find_device(print_cb, uri)) == NULL)
  {
    _cupsLangPrintFilter(stderr, "INFO",
    _("Waiting for printer to become available."));
    sleep(5);
  }
```

It's also easy to test by invoking the backend by hand:

```
# export DEVICE_URI='usb://Printer/Model?serial=?'
# /usr/lib/cups/backend/usb 0 root title 1 '' data.file

DEBUG: Loading USB quirks from "/usr/share/cups/usb".
DEBUG: Loaded 159 quirks.
DEBUG: Printing on printer with URI: usb://Brother/PJ-773?serial=?
DEBUG: libusb_get_device_list=6
INFO: Waiting for printer to become available.

DEBUG: libusb_get_device_list=6
INFO: Waiting for printer to become available.
DEBUG: libusb_get_device_list=6
INFO: Waiting for printer to become available.
DEBUG: libusb_get_device_list=6
INFO: Waiting for printer to become available.
DEBUG: libusb_get_device_list=6
INFO: Waiting for printer to become available.
...

```

Setting a job timeout policy or stopping the job by hand are not options for us. We can have job that take up to an hour to complete.