libudev: always return NULL in _unref() APIs

Returning anything else but NULL would suggest the caller's reference
might still be valid, but it isn't, because the caller just invoked
_unref() after all.

This turns the return value into a typesafe shortcut that allows
unreffing and resetting a reference in one line. In contrast to
solutions for this which take a pointer to a pointer to accomplish the
same this solution is just syntactic sugar the developer can make use of
but doesn't have to, and this is particularly useful when immediately
unreffing objects returned by function calls.

src/libudev/libudev-device.c

@@ -1090,7 +1090,7 @@ _public_ struct udev_device *udev_device_ref(struct udev_device *udev_device)
  * Drop a reference of a udev device. If the refcount reaches zero,
  * the resources of the device will be released.
  *
- * Returns: the passed udev device if it has still an active reference, or #NULL otherwise.
+ * Returns: #NULL
  **/
 _public_ struct udev_device *udev_device_unref(struct udev_device *udev_device)
 {
@@ -1098,7 +1098,7 @@ _public_ struct udev_device *udev_device_unref(struct udev_device *udev_device)
                 return NULL;
         udev_device->refcount--;
         if (udev_device->refcount > 0)
-                return udev_device;
+                return NULL;
         if (udev_device->parent_device != NULL)
                 udev_device_unref(udev_device->parent_device);
         free(udev_device->syspath);

src/libudev/libudev-enumerate.c

@@ -122,7 +122,7 @@ _public_ struct udev_enumerate *udev_enumerate_ref(struct udev_enumerate *udev_e
  * Drop a reference of an enumeration context. If the refcount reaches zero,
  * all resources of the enumeration context will be released.
  *
- * Returns: the passed enumeration context if it has still an active reference, or #NULL otherwise.
+ * Returns: #NULL
  **/
 _public_ struct udev_enumerate *udev_enumerate_unref(struct udev_enumerate *udev_enumerate)
 {
@@ -132,7 +132,7 @@ _public_ struct udev_enumerate *udev_enumerate_unref(struct udev_enumerate *udev
                 return NULL;
         udev_enumerate->refcount--;
         if (udev_enumerate->refcount > 0)
-                return udev_enumerate;
+                return NULL;
         udev_list_cleanup(&udev_enumerate->sysattr_match_list);
         udev_list_cleanup(&udev_enumerate->sysattr_nomatch_list);
         udev_list_cleanup(&udev_enumerate->subsystem_match_list);

src/libudev/libudev-hwdb.c

@@ -334,14 +334,14 @@ _public_ struct udev_hwdb *udev_hwdb_ref(struct udev_hwdb *hwdb) {
  * Drop a reference of a hwdb context. If the refcount reaches zero,
  * all resources of the hwdb context will be released.
  *
- * Returns: the passed hwdb context if it has still an active reference, or #NULL otherwise.
+ * Returns: #NULL
  **/
 _public_ struct udev_hwdb *udev_hwdb_unref(struct udev_hwdb *hwdb) {
         if (!hwdb)
                 return NULL;
         hwdb->refcount--;
         if (hwdb->refcount > 0)
-                return hwdb;
+                return NULL;
         if (hwdb->map)
                 munmap((void *)hwdb->map, hwdb->st.st_size);
         if (hwdb->f)

src/libudev/libudev-monitor.c

@@ -408,7 +408,7 @@ _public_ struct udev_monitor *udev_monitor_ref(struct udev_monitor *udev_monitor
  * the bound socket will be closed, and the resources of the monitor
  * will be released.
  *
- * Returns: the passed udev monitor if it has still an active reference, or #NULL otherwise.
+ * Returns: #NULL
  **/
 _public_ struct udev_monitor *udev_monitor_unref(struct udev_monitor *udev_monitor)
 {
@@ -416,7 +416,7 @@ _public_ struct udev_monitor *udev_monitor_unref(struct udev_monitor *udev_monit
                 return NULL;
         udev_monitor->refcount--;
         if (udev_monitor->refcount > 0)
-                return udev_monitor;
+                return NULL;
         if (udev_monitor->sock >= 0)
                 close(udev_monitor->sock);
         udev_list_cleanup(&udev_monitor->filter_subsystem_list);

src/libudev/libudev-queue.c

@@ -101,7 +101,7 @@ _public_ struct udev_queue *udev_queue_ref(struct udev_queue *udev_queue)
  * Drop a reference of a udev queue context. If the refcount reaches zero,
  * the resources of the queue context will be released.
  *
- * Returns: the passed queue context if it has still an active reference, or #NULL otherwise.
+ * Returns: #NULL
  **/
 _public_ struct udev_queue *udev_queue_unref(struct udev_queue *udev_queue)
 {
@@ -109,7 +109,7 @@ _public_ struct udev_queue *udev_queue_unref(struct udev_queue *udev_queue)
                 return NULL;
         udev_queue->refcount--;
         if (udev_queue->refcount > 0)
-                return udev_queue;
+                return NULL;
         udev_list_cleanup(&udev_queue->queue_list);
         free(udev_queue);
         return NULL;