IF_* variables are not defined (for static IP) in post-up phase

[Per Olav Kroka]

SYMPTOM
My computer is set up with static IP on interface eno1 and running 'sudo apt update' fails. Adding nameserver aaa.bbb.ccc.ddd in resolv.conf helps, as a workaround, but this is reset on reboot. Another workaround is to create a secondary IP on the same interface using DHCP (it works), but I don't want that.

ANALYSIS
I have traced the problem to the IF_* variables not being set at "post-up" phase, where resolvconf tries to get the values. (see 000resolvconf file in /etc/network/if-up.d.)

To verify this I created a script named 0000dbg which I placed in all the if-*.d folders:
#---- Start
DBGDIR=/etc/test
mkdir -p $DBGDIR
DEBUGFILE=$DBGDIR/$PHASE

set | grep -E "^(IF_|(IFACE|LOGICAL|ADDRFAM|METHOD|MODE|PHASE|VERBOSITY|PATH)\b)" > $DEBUGFILE
#--- end

(For some reason the mkdir command did not work, so I created the directory manually.)

These where the resulting files (listed with tail -n20 ...):
==> /etc/test/post-down <==
ADDRFAM='meta'
IFACE='--all'
LOGICAL='auto'
METHOD='none'
MODE='stop'
PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
PHASE='post-down'
VERBOSITY='0'

==> /etc/test/post-up <==
ADDRFAM='meta'
IFACE='--all'
LOGICAL='auto'
METHOD='none'
MODE='start'
PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
PHASE='post-up'
VERBOSITY='0'

==> /etc/test/pre-down <==
ADDRFAM='inet'
IFACE='eno1'
IF_ADDRESS='192.168.0.3'
IF_BROADCAST='192.168.0.255'
IF_DNSNAMESERVERS='193.213.112.4
IF_GATEWAY='192.168.0.1'
IF_HOSTNAME='hopper'
IF_NETMASK='255.255.255.0'
LOGICAL='eno1'
METHOD='static'
MODE='stop'
PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
PHASE='pre-down'
VERBOSITY='0'

==> /etc/test/pre-up <==
ADDRFAM='inet'
IFACE='lo'
LOGICAL='lo'
METHOD='loopback'
MODE='start'
PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
PHASE='pre-up'
VERBOSITY='0'

As you can see the IF_* variables were created in the 'pre-down' phase but not in the 'post-up' as expected.

I would expect that some of the code in the ifdown program should have been in the ifup program.

VERSIONS
Distribution: Ubuntu 16.04 (LTS) (latest and greatest)
ifupdown 0.8.10ubuntu1.1 amd64

I also found this one <http://irclogs.thegrebs.com/debian/2014/02/03> (search for "IF_" )