BIND 10 #2895: delete record from DB datasrc should use actual column values for rdata

[BIND 10 Development]

#2895: delete record from DB datasrc should use actual column values for rdata
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                   Reporter:         |                 Owner:
  jinmei                             |                Status:  new
                       Type:         |             Milestone:  Next-Sprint-
  defect                             |  Proposed
                   Priority:         |              Keywords:
  medium                             |             Sensitive:  0
                  Component:  data   |           Sub-Project:  DNS
  source                             |  Estimated Difficulty:  0
               CVSS Scoring:         |           Total Hours:  0
            Defect Severity:  N/A    |
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 In `DatabaseUpdater::deleteRRset()`, we pass the underlying datasrc
 accessor textual form of each RDATA to be deleted.  The textual RDATA
 is converted from the `RRset` object passed to `deleteRRset()`.

 I'm afraid this is fragile, because some RDATA can be case insensitive
 in their textual representation while some others cannot, and so the
 text would be compared in case-sensitive manner at the DB layer (which
 is the case of our SQLite3 schema and implementation).  So, for
 example, if an NS RDATA is stored in the DB as "NS.EXAMPLE.COM." and
 we try to delete it via `deleteRRset()` specifying it as
 "ns.example.com.", this wouldn't work (and it actually doesn't for our
 SQLite3 data source).

 (Note: theoretically, it could happen for other columns (e.g. a single
 domain name can have several different representations), but I suspect
 it's much less likely to happen in practice for other fields.)

 So I propose revising `deleteRRset()` as follows:

 - For each RDATA to be deleted, it first gets the underlying textual
   representation in the DB using `DatabaseAccessor::getRecords()`
 - Then call `DatabaseAccessor::deleteRecordInZone()` with the string
   retrieved in the first step
 - We should probably report it as an exception if not all specified
   RDATA are found in the DB in the first step.

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Ticket URL: <http://bind10.isc.org/ticket/2895>
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