network: enable KeepConfiguration= when running on network filesystem

Also, set KeepConfiguration=dhcp-on-stop by default when running in
initrd.

Fixes #21967.

src/network/networkd-manager.c

@@ -398,6 +398,30 @@ static int signal_restart_callback(sd_event_source *s, const struct signalfd_sig
         return sd_event_exit(sd_event_source_get_event(s), 0);
 }
 
+static int manager_set_keep_configuration(Manager *m) {
+        int r;
+
+        assert(m);
+
+        if (in_initrd()) {
+                log_debug("Running in initrd, keep DHCPv4 addresses on stopping networkd by default.");
+                m->keep_configuration = KEEP_CONFIGURATION_DHCP_ON_STOP;
+                return 0;
+        }
+
+        r = path_is_network_fs("/");
+        if (r < 0)
+                return log_error_errno(r, "Failed to detect if root is network filesystem: %m");
+        if (r == 0) {
+                m->keep_configuration = _KEEP_CONFIGURATION_INVALID;
+                return 0;
+        }
+
+        log_debug("Running on network filesystem, enabling KeepConfiguration= by default.");
+        m->keep_configuration = KEEP_CONFIGURATION_YES;
+        return 0;
+}
+
 int manager_setup(Manager *m) {
         int r;
 
@@ -453,6 +477,10 @@ int manager_setup(Manager *m) {
         if (r < 0)
                 return r;
 
+        r = manager_set_keep_configuration(m);
+        if (r < 0)
+                return r;
+
         m->state_file = strdup("/run/systemd/netif/state");
         if (!m->state_file)
                 return -ENOMEM;
@@ -468,6 +496,7 @@ int manager_new(Manager **ret, bool test_mode) {
                 return -ENOMEM;
 
         *m = (Manager) {
+                .keep_configuration = _KEEP_CONFIGURATION_INVALID,
                 .test_mode = test_mode,
                 .speed_meter_interval_usec = SPEED_METER_DEFAULT_TIME_INTERVAL,
                 .online_state = _LINK_ONLINE_STATE_INVALID,