sd-network: add _get_network_file api

src/libsystemd/sd-network/sd-network.c

@@ -120,6 +120,30 @@ _public_ int sd_network_link_get_setup_state(int ifindex, char **state) {
         return 0;
 }
 
+_public_ int sd_network_link_get_network_file(int ifindex, char **filename) {
+        _cleanup_free_ char *s = NULL, *p = NULL;
+        int r;
+
+        assert_return(ifindex > 0, -EINVAL);
+        assert_return(filename, -EINVAL);
+
+        if (asprintf(&p, "/run/systemd/netif/links/%d", ifindex) < 0)
+                return -ENOMEM;
+
+        r = parse_env_file(p, NEWLINE, "NETWORK_FILE", &s, NULL);
+        if (r == -ENOENT)
+                return -ENODATA;
+        if (r < 0)
+                return r;
+        if (isempty(s))
+                return -ENODATA;
+
+        *filename = s;
+        s = NULL;
+
+        return 0;
+}
+
 _public_ int sd_network_link_get_operational_state(int ifindex, char **state) {
         _cleanup_free_ char *s = NULL, *p = NULL;
         int r;

src/network/networkd-link.c

@@ -1785,6 +1785,8 @@ int link_save(Link *link) {
                 char **address, **domain;
                 bool space;
 
+                fprintf(f, "NETWORK_FILE=%s\n", link->network->filename);
+
                 fputs("DNS=", f);
                 space = false;
                 STRV_FOREACH(address, link->network->dns) {

src/systemd/sd-network.h

@@ -93,6 +93,9 @@ int sd_network_link_get_setup_state(int ifindex, char **state);
  */
 int sd_network_link_get_operational_state(int ifindex, char **state);
 
+/* Get path to .network file applied to link */
+int sd_network_link_get_network_file(int ifindex, char **filename);
+
 /* Get DNS entries for a given link. These are string representations of
  * IP addresses */
 int sd_network_link_get_dns(int ifindex, char ***addr);