hotplug_exit: remove parents recursively

Remove all parents with a single child remaining in parent tree.
This ensures that no parents of the direct parent of the device being considered
are left in the list, when appearing before their child(ren) in the processing order
of the context device list cleaning.

libusb/hotplug.c

@@ -161,6 +161,27 @@ void usbi_hotplug_init(struct libusb_context *ctx)
 	usbi_atomic_store(&ctx->hotplug_ready, 1);
 }
 
+static void usbi_recursively_remove_parents(struct libusb_device *dev, struct libusb_device *next_dev)
+{
+	if (dev && dev->parent_dev) {
+		if (usbi_atomic_load(&dev->parent_dev->refcnt) == 1) {
+			/* the parent was processed before this device in the list
+			 * and therefore has its ref count already decrement for its own ref.
+			 * The only remaining counted ref come from its remaining single child.
+			 * It will thus be released when its child will be released.
+			 * We remove it from the list. This is safe as parent_dev can not be
+			 * equal to next_dev given we know at this point that it was
+			 * previously seen in the list. */
+			assert (dev->parent_dev != next_dev);
+			if(dev->parent_dev->list.next && dev->parent_dev->list.prev) {
+				list_del(&dev->parent_dev->list);
+			}
+		}
+
+		usbi_recursively_remove_parents(dev->parent_dev, next_dev);
+	}
+}
+
 void usbi_hotplug_exit(struct libusb_context *ctx)
 {
 	struct usbi_hotplug_callback *hotplug_cb, *next_cb;
@@ -193,21 +214,17 @@ void usbi_hotplug_exit(struct libusb_context *ctx)
 		free(msg);
 	}
 
-	/* free all discovered devices. due to parent references loop until no devices are freed. */
+	/* free all discovered devices */
 	for_each_device_safe(ctx, dev, next_dev) {
 		/* remove the device from the usb_devs list only if there are no
 		 * references held, otherwise leave it on the list so that a
 		 * warning message will be shown */
 		if (usbi_atomic_load(&dev->refcnt) == 1) {
 			list_del(&dev->list);
 		}
-		if (dev->parent_dev && usbi_atomic_load(&dev->parent_dev->refcnt) == 1) {
-			/* the parent was before this device in the list and will be released.
-			   remove it from the list. this is safe as parent_dev can not be
-			   equal to next_dev. */
-			assert (dev->parent_dev != next_dev);
-			list_del(&dev->parent_dev->list);
-		}
+
+		usbi_recursively_remove_parents(dev, next_dev);
+
 		libusb_unref_device(dev);
 	}