device: reset MTU when VLAN's parent device changes MTU

Kernel does not allow setting the MTU of a VLAN larger
then the MTU of the underlying device. Hence, we might
initially fail to set a large MTU of the VLAN, but we
have to retry when the MTU of the parent changes.

https://bugzilla.redhat.com/show_bug.cgi?id=1414901

src/devices/nm-device-private.h

@@ -118,6 +118,8 @@ gint64 nm_device_get_configured_mtu_from_connection_default (NMDevice *self,
 
 guint32 nm_device_get_configured_mtu_for_wired (NMDevice *self, gboolean *out_is_user_config);
 
+void nm_device_commit_mtu (NMDevice *self);
+
 /*****************************************************************************/
 
 #define NM_DEVICE_CLASS_DECLARE_TYPES(klass, conn_type, ...) \

src/devices/nm-device-vlan.c

@@ -51,6 +51,7 @@ NM_GOBJECT_PROPERTIES_DEFINE (NMDeviceVlan,
 typedef struct {
 	gulong parent_state_id;
 	gulong parent_hwaddr_id;
+	gulong parent_mtu_id;
 	guint vlan_id;
 } NMDeviceVlanPrivate;
 
@@ -85,6 +86,17 @@ parent_state_changed (NMDevice *parent,
 	nm_device_set_unmanaged_by_flags (NM_DEVICE (self), NM_UNMANAGED_PARENT, !nm_device_get_managed (parent, FALSE), reason);
 }
 
+static void
+parent_mtu_maybe_changed (NMDevice *parent,
+                          GParamSpec *pspec,
+                          gpointer user_data)
+{
+	/* the MTU of a VLAN device is limited by the parent's MTU.
+	 *
+	 * When the parent's MTU changes, try to re-set the MTU. */
+	nm_device_commit_mtu (user_data);
+}
+
 static void
 parent_hwaddr_maybe_changed (NMDevice *parent,
                              GParamSpec *pspec,
@@ -143,6 +155,7 @@ parent_changed_notify (NMDevice *device,
 	 *  parent_changed_notify(). */
 	nm_clear_g_signal_handler (old_parent, &priv->parent_state_id);
 	nm_clear_g_signal_handler (old_parent, &priv->parent_hwaddr_id);
+	nm_clear_g_signal_handler (old_parent, &priv->parent_mtu_id);
 
 	if (new_parent) {
 		priv->parent_state_id = g_signal_connect (new_parent,
@@ -154,6 +167,10 @@ parent_changed_notify (NMDevice *device,
 		                                           G_CALLBACK (parent_hwaddr_maybe_changed), device);
 		parent_hwaddr_maybe_changed (new_parent, NULL, self);
 
+		priv->parent_mtu_id = g_signal_connect (new_parent, "notify::" NM_DEVICE_MTU,
+		                                        G_CALLBACK (parent_mtu_maybe_changed), device);
+		parent_mtu_maybe_changed (new_parent, NULL, self);
+
 		/* Set parent-dependent unmanaged flag */
 		nm_device_set_unmanaged_by_flags (device,
 		                                  NM_UNMANAGED_PARENT,
@@ -482,8 +499,10 @@ act_stage1_prepare (NMDevice *device, NMDeviceStateReason *out_failure_reason)
 
 	/* Change MAC address to parent's one if needed */
 	parent_device = nm_device_parent_get_device (device);
-	if (parent_device)
+	if (parent_device) {
 		parent_hwaddr_maybe_changed (parent_device, NULL, device);
+		parent_mtu_maybe_changed (parent_device, NULL, device);
+	}
 
 	s_vlan = (NMSettingVlan *) nm_device_get_applied_setting (device, NM_TYPE_SETTING_VLAN);
 	if (s_vlan) {

src/devices/nm-device.c

@@ -7231,8 +7231,7 @@ _commit_mtu (NMDevice *self, const NMIP4Config *config)
 		return;
 
 	if (nm_device_sys_iface_state_is_external_or_assume (self)) {
-		/* for assumed connections we don't tamper with the MTU. This is
+		/* for assumed connections we don't tamper with the MTU. */
-		 * a bug and supposed to be fixed by the unmanaged/assumed rework. */
 		return;
 	}
 
@@ -7354,6 +7353,22 @@ _commit_mtu (NMDevice *self, const NMIP4Config *config)
 #undef _IP6_MTU_SYS
 }
 
+void
+nm_device_commit_mtu (NMDevice *self)
+{
+	NMDeviceState state;
+
+	g_return_if_fail (NM_IS_DEVICE (self));
+
+	state = nm_device_get_state (self);
+	if (   state >= NM_DEVICE_STATE_CONFIG
+	    && state < NM_DEVICE_STATE_DEACTIVATING) {
+		_LOGT (LOGD_DEVICE, "mtu: commit-mtu...");
+		_commit_mtu (self, NM_DEVICE_GET_PRIVATE (self)->ip4_config);
+	} else
+		_LOGT (LOGD_DEVICE, "mtu: commit-mtu... skip due to state %s", nm_device_state_to_str (state));
+}
+
 static void
 ndisc_config_changed (NMNDisc *ndisc, const NMNDiscData *rdata, guint changed_int, NMDevice *self)
 {