Yet another workaround for HP LaserJet Enterprise M406 devices

Some HP LaserJet Enterprise M406 devices may return a current
configuration value that does not match the bConfigurationValue of any
existing configuration.

This can happen if the device is not configured, or if the firmware
returns an incorrect number for the current configuration.
Unfortunately, I don't have more detailed information to investigate,
though such investigation would have only academic value.

As a workaround, we assume that if only one configuration exists, it is
the current configuration, regardless of its bConfigurationValue.

Author: alexpevzner

usbio_libusb.go

@@ -542,10 +542,27 @@ func (devhandle *UsbDevHandle) currentInterfaces() ([]int, error) {
 			return nil, UsbError{"libusb_get_configuration", UsbErrCode(rc)}
 		}
 
+		// If bConfigurationValue matches the wanted configuration
+		// value, we are done
 		if conf.bConfigurationValue == C.uint8_t(config) {
 			break
 		}
 
+		// If we have only one configuration, we are done too.
+		//
+		// This may happen if the device is not configured, or if
+		// the firmware returns an incorrect number for the current
+		// configuration.
+		//
+		// This weird behavior has been seen in the wild for some
+		// HP LaserJet Enterprise M406 devices. So, as a last resort,
+		// assume that if only one configuration exists, it is the
+		// current configuration, regardless of its bConfigurationValue.
+		if cDesc.bNumConfigurations == 1 {
+			break
+		}
+
+		// Throw away mismatching configuration and continue
 		C.libusb_free_config_descriptor(conf)
 		conf = nil
 	}