network: restart dnsmasq after adding/removing txt and srv records

Although dns host records are stored in a separate configuration file
that is reread by dnsmasq when it receives a SIGHUP, the txt and srv
records are directly in the dnsmasq .conf file which can't be reread
after initial dnsmasq startup. This means that if an srv or txt record
is modified in a network config, libvirt needs to restart the dnsmasq
process rather than just sending a SIGHUP.

This was pointed out in a question in
https://bugzilla.redhat.com/show_bug.cgi?id=988718 , but no separate
BZ was filed.

src/network/bridge_driver.c

@@ -3404,9 +3404,14 @@ networkUpdate(virNetworkPtr net,
         if (section == VIR_NETWORK_SECTION_BRIDGE ||
             section == VIR_NETWORK_SECTION_DOMAIN ||
             section == VIR_NETWORK_SECTION_IP ||
-            section == VIR_NETWORK_SECTION_IP_DHCP_RANGE) {
-            /* these sections all change things on the dnsmasq commandline,
-             * so we need to kill and restart dnsmasq.
+            section == VIR_NETWORK_SECTION_IP_DHCP_RANGE ||
+            section == VIR_NETWORK_SECTION_DNS_TXT ||
+            section == VIR_NETWORK_SECTION_DNS_SRV) {
+            /* these sections all change things on the dnsmasq
+             * commandline (i.e. in the .conf file), so we need to
+             * kill and restart dnsmasq, because dnsmasq sets its uid
+             * to "nobody" after it starts, and is unable to re-read
+             * the conf file (owned by root, mode 600)
              */
             if (networkRestartDhcpDaemon(driver, network) < 0)
                 goto cleanup;
@@ -3434,12 +3439,10 @@ networkUpdate(virNetworkPtr net,
                 goto cleanup;
             }
 
-        } else if (section == VIR_NETWORK_SECTION_DNS_HOST ||
-                   section == VIR_NETWORK_SECTION_DNS_TXT ||
-                   section == VIR_NETWORK_SECTION_DNS_SRV) {
-            /* these sections only change things in config files, so we
-             * can just update the config files and send SIGHUP to
-             * dnsmasq.
+        } else if (section == VIR_NETWORK_SECTION_DNS_HOST) {
+            /* this section only changes data in an external file
+             * (not the .conf file) so we can just update the config
+             * files and send SIGHUP to dnsmasq.
              */
             if (networkRefreshDhcpDaemon(driver, network) < 0)
                 goto cleanup;