connectivity: avoid D-Bus activating systemd-resolved when we know it's not used

Every (failed) attempt to D-Bus activate a service results in log-messages
from dbus-daemon. It must be avoided to spam the logs that way.

Let connectivity check not only ask whether systemd-resolved is enabled
(and NetworkManager would like to push information there), but also
whether it looks like the service is actually available. That is,
either it has a name-owner or it's not blocked from starting.

The previous workaround was to configure main.systemd-resolved=no
in NetworkManager.conf. But that requires explict configuration.

4 files changed, 39 insertions, 3 deletions

diff --git a/src/dns/nm-dns-manager.c b/src/dns/nm-dns-manager.c
index c7c561c459..27c3e7109c 100644
--- a/src/dns/nm-dns-manager.c
+++ b/src/dns/nm-dns-manager.c
@@ -347,13 +347,20 @@ gboolean
 nm_dns_manager_has_systemd_resolved (NMDnsManager *self)
 {
 	NMDnsManagerPrivate *priv;
+	NMDnsSystemdResolved *plugin = NULL;
 
 	g_return_val_if_fail (NM_IS_DNS_MANAGER (self), FALSE);
 
 	priv = NM_DNS_MANAGER_GET_PRIVATE (self);
 
-	return    priv->sd_resolve_plugin
-	       || NM_IS_DNS_SYSTEMD_RESOLVED (priv->plugin);
+	if (priv->sd_resolve_plugin) {
+		nm_assert (!NM_IS_DNS_SYSTEMD_RESOLVED (priv->plugin));
+		plugin = NM_DNS_SYSTEMD_RESOLVED (priv->sd_resolve_plugin);
+	} else if (NM_IS_DNS_SYSTEMD_RESOLVED (priv->plugin))
+		plugin = NM_DNS_SYSTEMD_RESOLVED (priv->plugin);
+
+	return    plugin
+	       && nm_dns_systemd_resolved_is_running (plugin);
 }
 
 /*****************************************************************************/
diff --git a/src/dns/nm-dns-systemd-resolved.c b/src/dns/nm-dns-systemd-resolved.c
index 140776a598..a1cf4f89df 100644
--- a/src/dns/nm-dns-systemd-resolved.c
+++ b/src/dns/nm-dns-systemd-resolved.c
@@ -476,6 +476,22 @@ resolved_proxy_created (GObject *source, GAsyncResult *r, gpointer user_data)
 
 /*****************************************************************************/
 
+gboolean
+nm_dns_systemd_resolved_is_running (NMDnsSystemdResolved *self)
+{
+	NMDnsSystemdResolvedPrivate *priv;
+
+	g_return_val_if_fail (NM_IS_DNS_SYSTEMD_RESOLVED (self), FALSE);
+
+	priv = NM_DNS_SYSTEMD_RESOLVED_GET_PRIVATE (self);
+
+	return    priv->resolve
+	       && (   priv->dbus_has_owner
+	           || !priv->try_start_blocked);
+}
+
+/*****************************************************************************/
+
 static void
 nm_dns_systemd_resolved_init (NMDnsSystemdResolved *self)
 {
diff --git a/src/dns/nm-dns-systemd-resolved.h b/src/dns/nm-dns-systemd-resolved.h
index 800a60c154..b79ff5e4ec 100644
--- a/src/dns/nm-dns-systemd-resolved.h
+++ b/src/dns/nm-dns-systemd-resolved.h
@@ -36,4 +36,6 @@ GType nm_dns_systemd_resolved_get_type (void);
 
 NMDnsPlugin *nm_dns_systemd_resolved_new (void);
 
+gboolean nm_dns_systemd_resolved_is_running (NMDnsSystemdResolved *self);
+
 #endif /* __NETWORKMANAGER_DNS_SYSTEMD_RESOLVED_H__ */
diff --git a/src/nm-connectivity.c b/src/nm-connectivity.c
index b72413d2f3..2aab23862b 100644
--- a/src/nm-connectivity.c
+++ b/src/nm-connectivity.c
@@ -840,7 +840,18 @@ nm_connectivity_check_start (NMConnectivity *self,
 		 *
 		 * Yes, this makes NMConnectivity singleton dependent on NMDnsManager singleton.
 		 * Well, not really: it makes connectivity-check-start dependent on NMDnsManager
-		 * which merely means, not to start a connectivity check, late during shutdown. */
+		 * which merely means, not to start a connectivity check, late during shutdown.
+		 *
+		 * NMDnsSystemdResolved tries to D-Bus activate systemd-resolved only once,
+		 * to not spam syslog with failures messages from dbus-daemon.
+		 * Note that unless NMDnsSystemdResolved tried and failed to start systemd-resolved,
+		 * it guesses that systemd-resolved is activatable and returns %TRUE here. That
+		 * means, while NMDnsSystemdResolved would not try to D-Bus activate systemd-resolved
+		 * more than once, NMConnectivity might -- until NMDnsSystemdResolved tried itself
+		 * and noticed that systemd-resolved is not available.
+		 * This is relatively cumbersome to avoid, because we would have to go through
+		 * NMDnsSystemdResolved trying to asynchronously start the service, to ensure there
+		 * is only one attempt to start the service. */
 		has_systemd_resolved = nm_dns_manager_has_systemd_resolved (nm_dns_manager_get ());
 
 		if (has_systemd_resolved) {