dnsmasq: properly handling respawning of dnsmask

Otherwise, when killing dnsmasq it does not get respawned:

    dnsmasq[0x560dd7e43cf0]: dnsmasq exited normally
    dns-mgr: plugin dnsmasq child quit unexpectedly
    dns-mgr: update-dns: updating resolv.conf
    dns-mgr: config:      100 best    v4 enp0s25
    dns-mgr: config:      100 best    v6 enp0s25
    dns-mgr: config:      100 default v6 lo
    dns-mgr: config:      100 default v4 lo
    dns-mgr: update-dns: updating plugin dnsmasq
    dnsmasq[0x560dd7e43cf0]: adding nameserver '192.168.0.2@enp0s25'
    dnsmasq[0x560dd7e43cf0]: trying to update dnsmasq nameservers
    dns-mgr: update-resolv-conf: write internal file /var/run/NetworkManager/resolv.conf succeeded but don't update /etc/resolv.conf as it points to resolv.conf.nm
    dnsmasq[0x560dd7e43cf0]: dnsmasq disappeared

Previously, we would create priv->dnsmasq proxy only once,
and not respawn the process at all.

https://bugzilla.gnome.org/show_bug.cgi?id=766996

src/dns-manager/nm-dns-dnsmasq.c

@@ -431,11 +431,16 @@ start_dnsmasq (NMDnsDnsmasq *self)
 	NMBusManager *dbus_mgr;
 	GDBusConnection *connection;
 
-
-	if (   priv->running
-	    || priv->dnsmasq
-	    || priv->dnsmasq_cancellable)
+	if (priv->running) {
+		/* the dnsmasq process is running. Nothing to do. */
 		return;
+	}
+
+	if (nm_dns_plugin_child_pid ((NMDnsPlugin *) self) > 0) {
+		/* if we already have a child process spawned, don't do
+		 * it again. */
+		return;
+	}
 
 	dm_binary = nm_utils_find_helper ("dnsmasq", DNSMASQ_PATH, NULL);
 	if (!dm_binary) {
@@ -467,6 +472,13 @@ start_dnsmasq (NMDnsDnsmasq *self)
 	if (!pid)
 		return;
 
+	if (   priv->dnsmasq
+	    || priv->dnsmasq_cancellable) {
+		/* we already have a proxy or are about to create it.
+		 * We are done. */
+		return;
+	}
+
 	dbus_mgr = nm_bus_manager_get ();
 	g_return_if_fail (dbus_mgr);
 
@@ -522,6 +534,7 @@ static void
 child_quit (NMDnsPlugin *plugin, gint status)
 {
 	NMDnsDnsmasq *self = NM_DNS_DNSMASQ (plugin);
+	NMDnsDnsmasqPrivate *priv = NM_DNS_DNSMASQ_GET_PRIVATE (self);
 	gboolean failed = TRUE;
 	int err;
 
@@ -541,6 +554,8 @@ child_quit (NMDnsPlugin *plugin, gint status)
 	else
 		_LOGW ("dnsmasq died from an unknown cause");
 
+	priv->running = FALSE;
+
 	if (failed)
 		g_signal_emit_by_name (self, NM_DNS_PLUGIN_FAILED);
 }

src/dns-manager/nm-dns-plugin.c

@@ -177,6 +177,17 @@ watch_cb (GPid pid, gint status, gpointer user_data)
 	g_signal_emit (self, signals[CHILD_QUIT], 0, status);
 }
 
+GPid
+nm_dns_plugin_child_pid (NMDnsPlugin *self)
+{
+	NMDnsPluginPrivate *priv;
+
+	g_return_val_if_fail (NM_IS_DNS_PLUGIN (self), 0);
+
+	priv = NM_DNS_PLUGIN_GET_PRIVATE (self);
+	return priv->pid;
+}
+
 GPid
 nm_dns_plugin_child_spawn (NMDnsPlugin *self,
                            const char **argv,

src/dns-manager/nm-dns-plugin.h

@@ -103,6 +103,8 @@ GPid nm_dns_plugin_child_spawn (NMDnsPlugin *self,
                                 const char *pidfile,
                                 const char *kill_match);
 
+GPid nm_dns_plugin_child_pid (NMDnsPlugin *self);
+
 gboolean nm_dns_plugin_child_kill (NMDnsPlugin *self);
 
 #endif /* __NETWORKMANAGER_DNS_PLUGIN_H__ */