BIND 10 #2273: in-memory load should allow RRSIG without covered RRset

[BIND 10 Development]

#2273: in-memory load should allow RRSIG without covered RRset
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            Reporter:  jinmei        |                        Owner:
                Type:  defect        |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           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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 In #2267 I noticed the new in-memory zone loader doesn't support
 the case where RRSIG is added without having the corresponding
 covered RRset.  It's almost a broken setup, but I actually intended to
 allow such cases originally.  That may become a more practical issue
 when we support incremental updates (after IXFR-in or DDNS), where
 there may be some intermediate state that only RRSIGs are added first
 and we cannot predict whether/when the covered RRset comes.

 But I didn't support that case in #2267 because from a quick look it
 would break some assumption in the finder implementation.

 So this ticket will fix both the loader and the finder:

 - the loader just allows this case.  It should be an easy update to
   `InMemoryClient::Loader` class (of #2267).
 - handle this case in the finder.  If, for example, a lookup of
   example.com/A results in an RRSIG-only rdataset (which covers type
   A), it should return NXRRSET.

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Ticket URL: <http://bind10.isc.org/ticket/2273>
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