[swss]: Wait for vlan intf to start ndppd (#10119)

- Use the `wait_for_link.sh` script to delay ndppd start until after the VLAN interface is ready
- Avoids issue where ndppd tries to change interface attributes before the interface is ready

dockers/docker-orchagent/docker-init.j2

@@ -18,9 +18,12 @@ CFGGEN_PARAMS=" \
     -t /usr/share/sonic/templates/ndppd.conf.j2,/etc/ndppd.conf \
     -t /usr/share/sonic/templates/critical_processes.j2,/etc/supervisor/critical_processes \
     -t /usr/share/sonic/templates/supervisord.conf.j2,/etc/supervisor/conf.d/supervisord.conf
+    -t /usr/share/sonic/templates/wait_for_link.sh.j2,/usr/bin/wait_for_link.sh \
 "
 VLAN=$(sonic-cfggen $CFGGEN_PARAMS)
 
+chmod +x /usr/bin/wait_for_link.sh
+
 # Executed platform specific initialization tasks.
 if [ -x /usr/share/sonic/platform/platform-init ]; then
     /usr/share/sonic/platform/platform-init

dockers/docker-orchagent/ndppd.conf

@@ -6,4 +6,15 @@ autorestart=unexpected
 stdout_logfile=syslog
 stderr_logfile=syslog
 dependent_startup=true
-dependent_startup_wait_for=vlanmgrd:running
+dependent_startup_wait_for=wait_for_link:exited
+
+[program:wait_for_link]
+command=/usr/bin/wait_for_link.sh
+priority=7
+autostart=false
+autorestart=false
+startsecs=0
+stdout_logfile=syslog
+stderr_logfile=syslog
+dependent_startup=true
+dependent_startup_wait_for=swssconfig:exited

dockers/docker-orchagent/wait_for_link.sh.j2

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+
+VLAN_TABLE_PREFIX="VLAN_TABLE"
+
+function wait_until_iface_ready
+{
+    TABLE_PREFIX=$1
+    IFACE=$2
+
+    echo "Waiting until interface $IFACE is ready..."
+
+    # Wait for the interface to come up
+    # (i.e., interface is present in STATE_DB and state is "ok")
+    while true; do
+        RESULT=$(sonic-db-cli STATE_DB HGET "${TABLE_PREFIX}|${IFACE}" "state" 2> /dev/null)
+        if [ x"$RESULT" == x"ok" ]; then
+            break
+        fi
+
+        sleep 1
+    done
+
+    echo "Interface ${IFACE} is ready!"
+}
+
+
+# Wait for all interfaces to be up and ready
+{% for (name, prefix) in VLAN_INTERFACE|pfx_filter %}
+wait_until_iface_ready ${VLAN_TABLE_PREFIX} {{ name }}
+{% endfor %}