BIND 10 #2008: remove equal additions and deletions from diff.py

[BIND 10 Development]

#2008: remove equal additions and deletions from diff.py
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            Reporter:  jelte         |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  New
           Component:  Unclassified  |  Tasks
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 As noted in the review of #1456
 (http://bind10.isc.org/ticket/1456?replyto=11#comment:11), it would be
 nice if the 'single update mode' of the isc.xfrin.diff.Diff class (or
 wherever we may move it to), would remove reverse duplicates (i.e. where
 the same exact record is both added and deleted).

 So when adding a record, if that exact record is in delete already, remove
 it from delete, and vice versa (or alternatively, if you also want to do
 duplicate checking, collect them all and filter them later). Care has to
 be taken around the SOA record though.

 This way, the class would also be usable for ixfr-from-differences, where
 usually most of the contents are the same. Otherwise it'd delete the
 entire zone data an add it again, which would make it unsuitable for later
 ixfr-out.

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Ticket URL: <http://bind10.isc.org/ticket/2008>
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