network: support ID_NET_MANAGED_BY udev property

If the property is set, networkd manages the interface only when its
value is "io.systemd.Network".

Closes systemd#29768.

man/systemd.network.xml

@@ -77,6 +77,14 @@
     configured. The first (in alphanumeric order) of the network files that matches a given interface
     is applied, all later files are ignored, even if they match as well.</para>
 
+    <para>Note that any network interfaces that have the <varname>ID_NET_MANAGED_BY=</varname> udev property
+    set will never be matched by any .network files – unless the property's value is the string
+    <literal>io.systemd.Network</literal> – even if the [Match] section would otherwise match. This may be
+    used to exclude specific network interfaces from <command>systemd-networkd</command>'s management, while
+    keeping the [Match] section generic. The <varname>ID_NET_MANAGED_BY=</varname> poperty thus declares
+    intended <emphasis>ownership</emphasis> of the device, and permits ensuring that concurrent network
+    management implementations do not compete for management of specific devices.</para>
+
     <para>A network file is said to match a network interface if all matches specified by the [Match]
     section are satisfied. When a network file does not contain valid settings in [Match] section, then
     the file will match all interfaces and <command>systemd-networkd</command> warns about that. Hint:

src/network/networkd-link.c

@@ -1473,6 +1473,7 @@ static int link_check_initialized(Link *link) {
 
 int manager_udev_process_link(Manager *m, sd_device *device, sd_device_action_t action) {
         int r, ifindex;
+        const char *s;
         Link *link;
 
         assert(m);
@@ -1507,6 +1508,15 @@ int manager_udev_process_link(Manager *m, sd_device *device, sd_device_action_t
                 return 0;
         }
 
+        r = sd_device_get_property_value(device, "ID_NET_MANAGED_BY", &s);
+        if (r < 0 && r != -ENOENT)
+                log_device_debug_errno(device, r, "Failed to get ID_NET_MANAGED_BY udev property, ignoring: %m");
+        if (r >= 0 && !streq(s, "io.systemd.Network")) {
+                log_device_debug(device, "Interface is requested to be managed by '%s', not managing the interface.", s);
+                link_set_state(link, LINK_STATE_UNMANAGED);
+                return 0;
+        }
+
         r = link_initialized(link, device);
         if (r < 0)
                 link_enter_failed(link);