[Static DNS] Optimize DNS configuration update during interface-confi…

…g service restart (sonic-net#19583)

#### Why I did it
The `resolvconfig` service updates the DNS configuration in the host OS and each running Docker container when a configuration change is detected. The DNS configuration update during the shutdown of the management interface is redundant, as the management interface is going down temporarily and, when it is back online, the DNS configuration will remain the same. The update of the DNS configuration adds a couple of seconds (depending on the CPU) to the interface-config service restart time when the management interface uses a dynamic IP address. This can affect the fast boot. To optimize the flow and execute the service restart faster, the update should be skipped.

#### How I did it
Do not run DNS configuration update during the shutdown of the management interface.

#### How to verify it
Measure the execution time of the `service interfaces-config restart` command on the device with the static IP address configuration on the management interface and compare it to the execution time of the same command with the dynamic IP address. The difference should be insignificant.

files/image_config/interfaces/interfaces-config.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+resolvconf_updates=true
 
 function wait_networking_service_done() {
     local -i _WDOG_CNT="1"
@@ -23,6 +24,24 @@ function wait_networking_service_done() {
     systemctl kill networking 2>&1
 }
 
+function resolvconf_updates_disable() {
+    resolvconf --updates-are-enabled
+    if [[ $? -ne 0 ]]; then
+        resolvconf_updates=false
+    fi
+    resolvconf --disable-updates
+}
+
+function resolvconf_updates_restore() {
+    if [[ $resolvconf_updates == true ]]; then
+        resolvconf --enable-updates
+    fi
+}
+
+# Do not run DNS configuration update during the shutdowning of the management interface. 
+# This operation is redundant as there will be an update after the start of the interface.
+resolvconf_updates_disable
+
 if [[ $(ifquery --running eth0) ]]; then
     wait_networking_service_done
     ifdown --force eth0
@@ -61,6 +80,8 @@ for intf_pid in $(ls -1 /var/run/dhclient*.Ethernet*.pid 2> /dev/null); do
 done
 
 /usr/bin/resolv-config.sh cleanup
+# Restore DNS configuration update to the previous state.
+resolvconf_updates_restore
 
 # Read sysctl conf files again
 sysctl -p /etc/sysctl.d/90-dhcp6-systcl.conf

files/image_config/resolv-config/update-containers

@@ -1,6 +1,11 @@
 #!/bin/bash
 
-for container in $(docker ps -a --format=" {{ .ID }}"); do
+networking_status=$(systemctl is-active networking.service 2>/dev/null)
+if [[ $networking_status != "active" ]]; then
+    exit 0
+fi
+
+for container in $(docker ps -q); do
     docker cp -L /etc/resolv.conf ${container}:/_resolv.conf
     docker exec -t ${container} bash -c "cat /_resolv.conf > /etc/resolv.conf"
     docker exec -t ${container} bash -c "rm /_resolv.conf"