virt-builder stucks in endless loop due to /sbin/dhclient-script failure

[Kamil Madac]

virt-builder cannot build VM image as it uses locally installed /sbin/dhclient-script. dhclient-script tries to find out whether / is RW and reads /etc/fstab. Issue is that it does not exists in supermin machine which virt-builder uses and it end up in an endless loop of messages:

...
/sbin/dhclient-script: 31: /sbin/dhclient-script: cannot open /etc/fstab: No such file
RTNETLINK answers: File exists
/sbin/dhclient-script: 31: /sbin/dhclient-script: cannot open /etc/fstab: No such file
RTNETLINK answers: File exists
/sbin/dhclient-script: 31: /sbin/dhclient-script: cannot open /etc/fstab: No such file
RTNETLINK answers: File exists
/sbin/dhclient-script: 31: /sbin/dhclient-script: cannot open /etc/fstab: No such file
RTNETLINK answers: File exists
/sbin/dhclient-script: 31: /sbin/dhclient-script: cannot open /etc/fstab: No such file
^C
WARNING: CTRL+C pressed!!!

How to reproduce the bug (on 16.04):
~# apt install libguestfs-tools
~# virt-builder debian-8 --root-password password:xxx --hostname testvm --format qcow2 -v

Proposed solution:
Do not use /etc/fstab for finding out the state of / in dhclient-script, but use /proc/mounts instead of it. It makes more sense as it shows real state of filesystems and it exists on supermin machines, so virt-builder runs flawlessly.

Patch:
--- /sbin/dhclient-script.orig 2016-12-12 14:48:18.062438451 +0100
+++ /sbin/dhclient-script 2016-12-12 14:53:24.897500263 +0100
@@ -28,7 +28,7 @@
 wait_for_rw() {
     local file=$1
     # Find out whether we are going to mount / rw
- exec 9>&0 </etc/fstab
+ exec 9>&0 </proc/mounts
     rootmode=rw
     while read dev mnt type opts dump pass junk; do
         [ "$mnt" != / ] && continue

After you apply the patch delete virt-builder cache by deleting of /var/tmp/.guestfs-* directories and run virt-builder again.

Description: Ubuntu 16.04.1 LTS
Release: 16.04
libguestfs-tools 1:1.32.2-4ubuntu2
isc-dhcp-client 4.3.3-5ubuntu12.3