Advertise _BBPP-ipp-usb._tcp for each handled device (see #28)

    For each device that ipp-usb handles, a service with the
    name in the form _BBPP-ipp-usb._tcp, where BB and PP are
    USB bus/port numbers in hex, is advertised

    The purpose of this change is to let legacy drivers to easily
    detect that device is handled by the ipp-usb, to avoid possible
    conflicts

    See discussion at #28

device.go

@@ -10,6 +10,7 @@ package main
 
 import (
 	"context"
+	"fmt"
 	"net"
 	"net/http"
 	"time"
@@ -132,6 +133,18 @@ func NewDevice(desc UsbDeviceDesc) (*Device, error) {
 	// Advertise Web service. Assume it always exists
 	dnssdServices.Add(DNSSdSvcInfo{Type: "_http._tcp", Port: dev.State.HTTPPort})
 
+	// Advertise "_BBPP-ipp-usb._tcp" service, where BB and PP are bus and
+	// port numbers in hex, so legacy drivers can easily check if device
+	// is handled by the ipp-usb
+	//
+	// See the following for details:
+	//     https://github.com/OpenPrinting/ipp-usb/issues/28
+	dnssdServices.Add(DNSSdSvcInfo{
+		Type: fmt.Sprintf("_%.2x%.2x-ipp-usb._tcp",
+			desc.Bus,
+			info.PortNum),
+	})
+
 	// Enable handling incoming requests
 	dev.UsbTransport.SetDeadline(time.Time{})
 	dev.HTTPProxy.Enable()

usbcommon.go

@@ -176,6 +176,7 @@ type UsbDeviceInfo struct {
 	SerialNumber string // Device serial number
 	Manufacturer string // Manufacturer name
 	ProductName  string // Product name
+	PortNum      int    // USB port number
 
 	// Precomputed fields
 	MfgAndProduct string // Product with Manufacturer prefix, if needed

usbio_libusb.go

@@ -497,6 +497,8 @@ func (devhandle *UsbDevHandle) UsbDeviceInfo() (UsbDeviceInfo, error) {
 		}
 	}
 
+	info.PortNum = int(C.libusb_get_port_number(dev))
+
 	info.FixUp()
 
 	return info, nil