Add convenience function to find and open a device by USB VID+PID

Lots of libusb apps I write are simple test apps not intended to be real
apps. Having a function available to quickly locate my device will be
handy in such situations.

examples/dpfp.c

@@ -81,36 +81,8 @@ static struct libusb_bulk_transfer irqtrf = {
 
 static int find_dpfp_device(void)
 {
-	struct libusb_device *dev;
+	devh = libusb_open_device_with_vid_pid(0x05ba, 0x000a);
-	struct libusb_device *found = NULL;
+	return devh ? 0 : -EIO;
-	struct libusb_device **devs;
-	size_t i = 0;
-	int r;
-
-	r = libusb_get_device_list(&devs);
-	if (r < 0)
-		return r;
-
-	while ((dev = devs[i++]) != NULL) {
-		struct libusb_dev_descriptor *desc = libusb_device_get_descriptor(dev);
-		if (desc->idVendor == 0x05ba && desc->idProduct == 0x000a) {
-			found = dev;
-			break;
-		}
-	}
-
-	if (!found) {
-		r = -ENODEV;
-		goto out;
-	}
-
-	devh = libusb_open(dev);
-	if (!devh)
-		r = -EIO;
-
-out:
-	libusb_free_device_list(devs, 1);
-	return r;
 }
 
 static int print_f0_data(void)

libusb/core.c

@@ -385,6 +385,37 @@ API_EXPORTED struct libusb_dev_handle *libusb_open(struct libusb_device *dev)
 	return devh;
 }
 
+/* convenience function for finding a device with a particular vendor/product
+ * combination. has limitations and is hence not intended for use in "real
+ * applications": if multiple devices have the same VID+PID it'll only
+ * give you the first one, etc. */
+API_EXPORTED struct libusb_dev_handle *libusb_open_device_with_vid_pid(
+	uint16_t vendor_id, uint16_t product_id)
+{
+	struct libusb_device **devs;
+	struct libusb_device *found = NULL;
+	struct libusb_device *dev;
+	struct libusb_dev_handle *devh;
+	size_t i = 0;
+
+	if (libusb_get_device_list(&devs) < 0)
+		return NULL;
+
+	while ((dev = devs[i++]) != NULL) {
+		struct libusb_dev_descriptor *desc = libusb_device_get_descriptor(dev);
+		if (desc->idVendor == vendor_id && desc->idProduct == product_id) {
+			found = dev;
+			break;
+		}
+	}
+
+	if (found)
+		devh = libusb_open(found);
+
+	libusb_free_device_list(devs, 1);
+	return devh;
+}
+
 static void do_close(struct libusb_dev_handle *devh)
 {
 	usbi_remove_pollfd(devh->fd);

libusb/libusb.h

@@ -231,10 +231,13 @@ void libusb_device_unref(libusb_device *dev);
 
 libusb_dev_handle *libusb_open(libusb_device *dev);
 void libusb_close(libusb_dev_handle *devh);
-struct libusb_device *libusb_devh_get_device(libusb_dev_handle *devh);
+libusb_device *libusb_devh_get_device(libusb_dev_handle *devh);
 int libusb_claim_interface(libusb_dev_handle *dev, int iface);
 int libusb_release_interface(libusb_dev_handle *dev, int iface);
 
+libusb_dev_handle *libusb_open_device_with_vid_pid(uint16_t vendor_id,
+	uint16_t product_id);
+
 /* async I/O */
 
 libusb_urb_handle *libusb_async_control_transfer(libusb_dev_handle *devh,