Wait for VLAN interface to come up before starting DHCP relay (#399)

dockers/docker-dhcp-relay/Dockerfile

@@ -15,8 +15,9 @@ RUN rm -rf /debs
 
 COPY ["isc-dhcp-relay.j2", "/usr/share/sonic/templates/"]
 COPY ["config.sh", "/usr/bin/"]
+COPY ["start.sh", "/usr/bin/"]
 
-ENTRYPOINT /usr/bin/config.sh       \
-    && service rsyslog start        \
-    && service isc-dhcp-relay start \
+ENTRYPOINT /usr/bin/config.sh \
+    && service rsyslog start  \
+    && /usr/bin/start.sh      \
     && /bin/bash

dockers/docker-dhcp-relay/start.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+VLAN_IFACE_NAME=`sonic-cfggen -m /etc/sonic/minigraph.xml -v "minigraph_vlan_interfaces[0]['name']"`
+
+# Wait for the VLAN to come up (i.e., 'ip link show' returns 0)
+until ip link show $VLAN_IFACE_NAME > /dev/null 2>&1; do
+    sleep 1
+done
+
+# Start the DHCP relay
+service isc-dhcp-relay start
+

files/build_templates/dhcp_relay.service.j2

@@ -1,7 +1,7 @@
 [Unit]
 Description=DHCP relay container
 Requires=docker.service
-After=docker.service
+After=interfaces-config.service
 
 [Service]
 User={{ sonicadmin_user }}