Fix LXC cache resolvers more appropriately

The implementation of I3769053956b958a0ac7cab499ea90af2a40de850
was somewhat brutal in the approach based on the assumption that
systemd networking is implemented in the base cache. This may be
true for Rocky, but is not true for any of the older branches.

This patch returns the previous method, but fixes it in two ways:

1. It uses the -f file test operator, instead of the deprecated
   -a operator.
2. It also tests for the presence of a symbolic link with -L.

The downloaded image has resolvconf installed, so there is a symlink
for /etc/resolv.conf which points to /run/resolvconf/resolv.conf
in the cache. Given that in a chroot /run does not exist, the file
test operator fails. This is why we're adding the symlink check.

Instead of then removing resolv.conf at the end, we return it back
so that resolveconf works as expected. This is important for anyone
implementing their resolvers through configuration of network
interfaces.

This is only implemented for Queens and older because for master
we've moved to using a different image source, and using
systemd-resolved.

Closes-Bug: #1768592
Change-Id: I11d069e2d0cb3becad067689fa8f1e4c9391f368

vars/redhat-7.yml

@@ -58,7 +58,9 @@ lxc_cache_map:
   cache_prep_commands: |
     {{ lxc_cache_prep_pre_commands }}
     mkdir -p /etc/ansible/facts.d/
-    rm /etc/resolv.conf || true
+    if [ -f /etc/resolv.conf ] || [ -L /etc/resolv.conf ]; then
+      mv /etc/resolv.conf /etc/resolv.conf.org
+    fi
     {% for resolver in lxc_cache_prep_dns %}
     echo "nameserver {{ resolver }}" >> /etc/resolv.conf
     {% endfor %}
@@ -82,7 +84,11 @@ lxc_cache_map:
     mkdir -p /var/backup
     echo -e '{{ lxc_container_default_interfaces }}' | tee /etc/sysconfig/network-scripts/ifcfg-eth0
     chage -I -1 -d -1 -m 0 -M 99999 -E -1 root
-    echo "nameserver {{ lxc_net_address }}" > /etc/resolv.conf
+    if [ -f /etc/resolv.conf.org ] || [ -L /etc/resolv.conf.org ]; then
+      mv /etc/resolv.conf.org /etc/resolv.conf
+    else
+      rm -f /etc/resolv.conf
+    fi
     {{ lxc_cache_prep_post_commands }}
 
 _lxc_cache_distro_packages:

vars/suse-42.yml

@@ -50,7 +50,9 @@ lxc_cache_map:
   cache_prep_commands: |
     {{ lxc_cache_prep_pre_commands }}
     mkdir -p /etc/ansible/facts.d/
-    rm /etc/resolv.conf || true
+    if [ -f /etc/resolv.conf ] || [ -L /etc/resolv.conf ]; then
+      mv /etc/resolv.conf /etc/resolv.conf.org
+    fi
     {% for resolver in lxc_cache_prep_dns %}
     echo "nameserver {{ resolver }}" >> /etc/resolv.conf
     {% endfor %}
@@ -68,7 +70,11 @@ lxc_cache_map:
     mkdir -p /var/backup
     echo -e '{{ lxc_container_default_interfaces }}' | tee /etc/sysconfig/network/ifcfg-eth0
     chage -I -1 -d -1 -m 0 -M 99999 -E -1 root
-    echo "nameserver {{ lxc_net_address }}" > /etc/resolv.conf
+    if [ -f /etc/resolv.conf.org ] || [ -L /etc/resolv.conf.org ]; then
+      mv /etc/resolv.conf.org /etc/resolv.conf
+    else
+      rm -f /etc/resolv.conf
+    fi
     # NOTE(hwoarang): Enable sshd which has been explicitely disabled in
     # https://github.com/lxc/lxc-ci/commit/8dc7105399350a59698538a12b6d5a1a880ef2ba
     systemctl -q unmask sshd

vars/ubuntu-16.04.yml

@@ -64,7 +64,9 @@ lxc_cache_map:
   cache_prep_commands: |
     {{ lxc_cache_prep_pre_commands }}
     mkdir -p /etc/ansible/facts.d/
-    rm /etc/resolv.conf || true
+    if [ -f /etc/resolv.conf ] || [ -L /etc/resolv.conf ]; then
+      mv /etc/resolv.conf /etc/resolv.conf.org
+    fi
     {% for resolver in lxc_cache_prep_dns %}
     echo "nameserver {{ resolver }}" >> /etc/resolv.conf
     {% endfor %}
@@ -89,7 +91,11 @@ lxc_cache_map:
     echo -e '{{ lxc_container_default_interfaces }}' | tee /etc/network/interfaces
     mkdir -p /etc/network/interfaces.d
     chage -I -1 -d -1 -m 0 -M 99999 -E -1 root
-    echo "nameserver {{ lxc_net_address }}" > /etc/resolv.conf
+    if [ -f /etc/resolv.conf.org ] || [ -L /etc/resolv.conf.org ]; then
+      mv /etc/resolv.conf.org /etc/resolv.conf
+    else
+      rm -f /etc/resolv.conf
+    fi
     {{ lxc_cache_prep_post_commands }}
 
 # This forces any modified configurations to remain, and any unmodified configs to be replaced