BIND 10 #2851: extensions to ConfigurableClientList::getCachedZoneWriter

[BIND 10 Development]

#2851: extensions to ConfigurableClientList::getCachedZoneWriter
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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:         |
  shared memory data source          |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 Subtask of #2830.  Should be done after #2834.  Probably
 depend on #2850.

 We'll modify this method so we can create a zone writer even if the
 in-memory zone table is empty or if the specified zone isn't found in
 the "cache" (so the underlying data source should be searched).  This
 will be crucial for mapped-file type of segment.  And, I guess we can
 also use this extension to allow loading a zone that was previously
 rejected due to an error without requiring full rebuilding the data
 source (there may be a ticket for this).

 Also, if the corresponding memory segment is READ_ONLY, make sure this
 method fails with some error code.  It will be needed so b10-auth
 won't accidentally try to "reload" a zone in a mapped segment.

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Ticket URL: <http://bind10.isc.org/ticket/2851>
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BIND 10 Development