Internal DNS server. Failure, when a client a) registers an IP b) deletes that IP c) registers again

[Günter Kukkukk]

I've just started again to work on a DNS failure, which
i called myself "the zombie (Records=0, Children=0)" issue.

This bug is probably related to bugzilla 9559 and many other
user reports to the samba mailing lists.

Testcase: Recent git tree
Assuming a valid kinit has been done already.
------
nsupdate -g
> update add mytest.intranet01.hom 3600 A 192.168.200.233
> send
> update delete mytest.intranet01.hom A 192.168.200.233
> send
> update add mytest.intranet01.hom 3600 A 192.168.200.233
> send
; TSIG error with server: tsig verify failure
update failed: SERVFAIL
------ 
The TSIG error should be _ignored_ here atm, it is a different issue.
Many other clients programs will run the same sequence
when updating a record.

When we now run
samba-tool dns query linux300 intranet01.hom mytest ALL
  Name=, Records=0, Children=0

This zombie entry _cannot_ be removed by both samba-tool
and any dns requests!
(But samba-tool can be used to a) assign a new IP record again,
and then b) delete it completely)
I've talked to some users which see lots of those zombie records!
Care must been taken cause e.g.
  Name=_msdcs, Records=0, Children=0
also contains those zero records.
---------

I've have prepared a very first patch (see attachment), which
addresses this issue.
Please comment whether this is the right approach.
Sure, the DEBUG() statements - beside one - should be removed.

With the patch applied all works as expected. 

Comments welcome. :-)

Cheers, Günter
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