shared: rework _NM_GET_PRIVATE() to use _Generic()

_NM_GET_PRIVATE() used typeof() to propagate constness of the @self
pointer. However, that means, it could only be used with a self pointer
of the exact type. That means, you explicitly had to cast from (GObject *)
or from (void *).
The requirement is cumbersome, and often led us to either create @self
pointer we didn't need:

    NMDeviceVlan *self = NM_DEVICE_VLAN (device);
    NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE (self);

or casting:

    NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE ((NMDevice *) device);

In both cases we forcefully cast the source variable, loosing help from
the compiler to detect a bug.

For "nm-linux-platform.c", instead we commonly have a pointer of type
NMPlatform. Hence, we always forcefully cast the type via _NM_GET_PRIVATE_VOID().

Rework the macro to use _Generic(). If compiler supports _Generic(), then we
will get all compile time checks as desired. If the compiler doesn't support
_Generic(), it will still work. You don't get the compile-time checking of course,
but you'd notice that something is wrong once you build with a suitable
compiler.

3 files changed, 57 insertions, 47 deletions

diff --git a/shared/nm-utils/nm-macros-internal.h b/shared/nm-utils/nm-macros-internal.h
index d9dab83d73..68b69bf403 100644
--- a/shared/nm-utils/nm-macros-internal.h
+++ b/shared/nm-utils/nm-macros-internal.h
@@ -270,26 +270,65 @@ NM_G_ERROR_MSG (GError *error)
 #endif
 
 #if _NM_CC_SUPPORT_GENERIC
-#define _NM_CONSTCAST(type, obj) \
-	(_Generic ((obj), \
-	           void *           : ((type *) (obj)), \
-	           void *const      : ((type *) (obj)), \
+#define _NM_CONSTCAST_FULL(type, obj_expr, obj) \
+	(_Generic ((obj_expr), \
 	           const void *     : ((const type *) (obj)), \
-	           const void *const: ((const type *) (obj)), \
-	           const type *     : (obj), \
-	           const type *const: (obj), \
-	           type *           : (obj), \
-	           type *const      : (obj)))
+	           void *           : ((      type *) (obj)), \
+	           const type *     : ((const type *) (obj)), \
+	           type *           : ((      type *) (obj))))
+#define _NM_CONSTCAST2_FULL(type, obj_expr, obj, alias_type2) \
+	(_Generic ((obj_expr), \
+	           const void        *: ((const type *) (obj)), \
+	                 void        *: ((      type *) (obj)), \
+	           const alias_type2 *: ((const type *) (obj)), \
+	                 alias_type2 *: ((      type *) (obj)), \
+	           const type        *: ((const type *) (obj)), \
+	                 type        *: ((      type *) (obj))))
+#define _NM_CONSTCAST3_FULL(type, obj_expr, obj, alias_type2, alias_type3) \
+	(_Generic ((obj_expr), \
+	           const void        *: ((const type *) (obj)), \
+	                 void        *: ((      type *) (obj)), \
+	           const alias_type2 *: ((const type *) (obj)), \
+	                 alias_type2 *: ((      type *) (obj)), \
+	           const alias_type3 *: ((const type *) (obj)), \
+	                 alias_type3 *: ((      type *) (obj)), \
+	           const type        *: ((const type *) (obj)), \
+	                 type        *: ((      type *) (obj))))
 #else
 /* _NM_CONSTCAST() is there to preserve constness of a pointer.
  * It uses C11's _Generic(). If that is not supported, we fall back
  * to casting away constness. So, with _Generic, we get some additional
  * static type checking by preserving constness, without, we cast it
  * to a non-const pointer. */
-#define _NM_CONSTCAST(type, obj) \
+#define _NM_CONSTCAST_FULL(type, obj_expr, obj) \
+	((type *) (obj))
+#define _NM_CONSTCAST2_FULL(type, obj_expr, obj, alias_type2) \
+	((type *) (obj))
+#define _NM_CONSTCAST3_FULL(type, obj_expr, obj, alias_type2, alias_type2) \
 	((type *) (obj))
 #endif
 
+#define _NM_CONSTCAST(type, obj) \
+	_NM_CONSTCAST_FULL(type, (obj), (obj))
+
+#define _NM_CONSTCAST2(type, obj, alias_type2) \
+	_NM_CONSTCAST2_FULL(type, (obj), (obj), alias_type2)
+
+#define _NM_GOBJECT_CAST(type, obj, is_check) \
+	({ \
+		const void *_obj = (obj); \
+		\
+		nm_assert (is_check (_obj)); \
+		_NM_CONSTCAST2_FULL (type, (obj), _obj, GObject); \
+	})
+#define _NM_GOBJECT_CAST2(type, obj, is_check, alias_type2) \
+	({ \
+		const void *_obj = (obj); \
+		\
+		nm_assert (is_check (_obj)); \
+		_NM_CONSTCAST3_FULL (type, (obj), _obj, GObject, alias_type2); \
+	})
+
 #if _NM_CC_SUPPORT_GENERIC
 /* returns @value, if the type of @value matches @type.
  * This requires support for C11 _Generic(). If no support is
@@ -581,39 +620,10 @@ _notify (obj_type *obj, _PropertyEnums prop) \
 
 /*****************************************************************************/
 
-/* these are implemented as a macro, because they accept self
- * as both (type*) and (const type*), and return a const
- * private pointer accordingly. */
-#define __NM_GET_PRIVATE(self, type, is_check, addrop) \
-	({ \
-		/* preserve the const-ness of self. Unfortunately, that
-		 * way, @self cannot be a void pointer */ \
-		typeof (self) _self = (self); \
-		\
-		/* Get compiler error if variable is of wrong type */ \
-		_nm_unused const type *const _self2 = (_self); \
-		\
-		nm_assert (is_check (_self)); \
-		( addrop ( _NM_CONSTCAST (type, _self)->_priv) ); \
-	})
-
-#define _NM_GET_PRIVATE(self, type, is_check)     __NM_GET_PRIVATE(self, type, is_check, &)
-#define _NM_GET_PRIVATE_PTR(self, type, is_check) __NM_GET_PRIVATE(self, type, is_check,  )
-
-#define __NM_GET_PRIVATE_VOID(self, type, is_check, result_cmd) \
-	({ \
-		/* (self) can be any non-const pointer. It will be cast to "type *".
-		 * We don't explicitly cast but assign first to (void *) which
-		 * will fail if @self is pointing to const. */ \
-		void *const _self1 = (self); \
-		type *const _self = _self1; \
-		\
-		nm_assert (is_check (_self)); \
-		( result_cmd ); \
-	})
-
-#define _NM_GET_PRIVATE_VOID(self, type, is_check)     __NM_GET_PRIVATE_VOID(self, type, is_check, &_self->_priv)
-#define _NM_GET_PRIVATE_PTR_VOID(self, type, is_check) __NM_GET_PRIVATE_VOID(self, type, is_check,  _self->_priv)
+#define _NM_GET_PRIVATE(     self, type, is_check)               (&(_NM_GOBJECT_CAST  (type, (self), is_check             )->_priv))
+#define _NM_GET_PRIVATE_PTR( self, type, is_check)               ( (_NM_GOBJECT_CAST  (type, (self), is_check             )->_priv))
+#define _NM_GET_PRIVATE2(    self, type, is_check, alias_type2)  (&(_NM_GOBJECT_CAST2 (type, (self), is_check, alias_type2)->_priv))
+#define _NM_GET_PRIVATE2_PTR(self, type, is_check, alias_type2)  ( (_NM_GOBJECT_CAST2 (type, (self), is_check, alias_type2)->_priv))
 
 /*****************************************************************************/
 
diff --git a/src/devices/nm-device-vlan.c b/src/devices/nm-device-vlan.c
index e30dae7449..1253a66418 100644
--- a/src/devices/nm-device-vlan.c
+++ b/src/devices/nm-device-vlan.c
@@ -66,7 +66,7 @@ struct _NMDeviceVlanClass {
 
 G_DEFINE_TYPE (NMDeviceVlan, nm_device_vlan, NM_TYPE_DEVICE)
 
-#define NM_DEVICE_VLAN_GET_PRIVATE(self) _NM_GET_PRIVATE (self, NMDeviceVlan, NM_IS_DEVICE_VLAN)
+#define NM_DEVICE_VLAN_GET_PRIVATE(self) _NM_GET_PRIVATE2 (self, NMDeviceVlan, NM_IS_DEVICE_VLAN, NMDevice)
 
 /*****************************************************************************/
 
@@ -418,7 +418,7 @@ complete_connection (NMDevice *device,
 static void
 update_connection (NMDevice *device, NMConnection *connection)
 {
-	NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE ((NMDeviceVlan *) device);
+	NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE (device);
 	NMSettingVlan *s_vlan = nm_connection_get_setting_vlan (connection);
 	int ifindex = nm_device_get_ifindex (device);
 	const char *setting_parent, *new_parent;
@@ -558,7 +558,7 @@ static void
 get_property (GObject *object, guint prop_id,
               GValue *value, GParamSpec *pspec)
 {
-	NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE ((NMDeviceVlan *) object);
+	NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE (object);
 
 	switch (prop_id) {
 	case PROP_VLAN_ID:
diff --git a/src/platform/nm-linux-platform.c b/src/platform/nm-linux-platform.c
index c4c93ed3f1..9ce520bdcc 100644
--- a/src/platform/nm-linux-platform.c
+++ b/src/platform/nm-linux-platform.c
@@ -2867,7 +2867,7 @@ struct _NMLinuxPlatformClass {
 
 G_DEFINE_TYPE (NMLinuxPlatform, nm_linux_platform, NM_TYPE_PLATFORM)
 
-#define NM_LINUX_PLATFORM_GET_PRIVATE(self) _NM_GET_PRIVATE_VOID(self, NMLinuxPlatform, NM_IS_LINUX_PLATFORM)
+#define NM_LINUX_PLATFORM_GET_PRIVATE(self) _NM_GET_PRIVATE2(self, NMLinuxPlatform, NM_IS_LINUX_PLATFORM, NMPlatform)
 
 NMPlatform *
 nm_linux_platform_new (gboolean log_with_ptr, gboolean netns_support)