BIND 10 #2911: don't fallback from ixfr to axfr due to data source error

[BIND 10 Development]

#2911: don't fallback from ixfr to axfr due to data source error
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            Reporter:  jinmei        |                        Owner:
                Type:  defect        |  jinmei
            Priority:  medium        |                       Status:
           Component:  xfrin         |  accepted
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-20130528
         Sub-Project:  DNS           |                   Resolution:
Estimated Difficulty:  4             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by jinmei):

 trac2911 is ready for review.

 For the new config item, I named it "request_ixfr" following BIND 9's
 "request-ixfr".  And, what this branch basically does is:

 - introduce request_ixfr, and implement its logic as described in
   http://bind10.isc.org/ticket/2911#comment:10
 - as a result it now tries IXFR first (this is now safe)
 - deprecate use_ixfr (but for now I made it trigger an explicit
   error.  after a few releases later, or at the time of xfrin rewrite,
   we may want to completely remove it).

 unit tests and lettuce tests were updated accordingly.

 Also, while working on it, I noticed the "refresh" command wasn't
 actually supported even if the handling logic is written, because it's
 not listed in the spec file.  I also made it available in the spec
 (and it's actually necessary now I changed retransfer so it always
 uses AXFR, which is compatible with BIND 9's rndc).

 I also made some cleanups and refactoring at the beginning of this
 branch.  They were introduced with keeping in mind the later changes
 above, but they themselves are independent cleanups.  They should
 preserve the original behavior.  They are commits up to f081395
 (inclusive).  I suggest reviewing these one by one, and separately.

 The rest of the branch implements the main topic.  While it's done
 step by step, I think it's easier to read as a whole, with the
 understanding of the entire goal as described above.

 Proposed changelog entry:
 {{{
 621.?   [func]*         jinmei
         b10-xfrin now has tighter control on the choice of IXFR or AXFR
         through zones/request_ixfr configuration item.  It includes
         the new "IXFR only" behavior for some special cases.  b10-xfrin
         now also uses AXFR whenever necessary, so it is now safe to try
         IXFR by default and it's made the default.  The previous
         use_ixfr configuration item was deprecated and triggers startup
         failure if specified; configuration using use_ixfr should be
         updated.
         (Trac #2911, git TBD)
 }}}
 (while it was originally considered a bug fix, I think it's now
 actually a new feature in the end, so I chose "func").

-- 
Ticket URL: <https://bind10.isc.org/ticket/2911#comment:12>
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