Make dhcp-all-interfaces block all interfaces

Previously dhcp-all-interfaces was only blocking the first network
interface to come up. We add an instance stanza to make it run one
instance of generate-interfaces-file per interface. We then use flock to
serialize runs of generate-interfaces-file.

Fixes bug #1233577

Change-Id: Ib16bed6b37ce0789e315ef57e05ac561470a6f2a

elements/dhcp-all-interfaces/install.d/dhcp-all-interfaces.conf

@@ -3,7 +3,8 @@
 
 description "DHCP any connected, but unconfigured network interfaces"
 
-start on starting cloud-init-nonet
+start on starting network-interface
+instance $INTERFACE
 
 task
 

elements/dhcp-all-interfaces/install.d/generate-interfaces-file.sh

@@ -6,6 +6,12 @@
 
 INTERFACES_FILE="/etc/network/interfaces"
 
+# Serialize runs so that we don't miss hot-add interfaces
+FLOCK=${1:-}
+if [ -z "$FLOCK" ] ; then
+    exec flock -x $INTERFACES_FILE $0 flocked
+fi
+
 function get_if_link() {
   cat /sys/class/net/${1}/carrier
 }