[Bug 1890572] Re: USB backend never ends if the printer is not connected

[Alejandro]

Hi Sebastian,

the patch set the job state to backend error.

I don't think openprinting is the right error here. The problem is in
the backend (the USB backend) when it's not possible to communicate with
a USB printer. This usually means that the printer is not connected.

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Title:
  USB backend never ends if the printer is not connected

Status in cups package in Ubuntu:
  New

Bug description:
  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://Printer/Model?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 manually stopping work are not options
  for us. We may have jobs that take hours to complete.

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