BIND 10 #2460: make ConfigurableClientList::getCachedZoneWriter thread safe

[BIND 10 Development]

#2460: make ConfigurableClientList::getCachedZoneWriter thread safe
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            Reporter:  jinmei       |                        Owner:
                Type:  defect       |                       Status:  new
            Priority:  medium       |                    Milestone:
           Component:  data source  |                   Resolution:
            Keywords:               |                 CVSS Scoring:
           Sensitive:  0            |              Defect Severity:  N/A
         Sub-Project:  DNS          |  Feature Depending on Ticket:
Estimated Difficulty:  7            |          Add Hours to Ticket:  0
         Total Hours:  0            |                    Internal?:  0
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Comment (by jinmei):

 See https://lists.isc.org/pipermail/bind10-dev/2013-January/004215.html

 I believe this task solves the issue I explained in that message.
 While the allowable scalability for SQLite3 may be moot, I suspect
 the same discussion will apply to other databases because opening a
 new DB connection could be time consuming or even blocking operation.

 So I'm proposing it for the next sprint.

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Ticket URL: <http://bind10.isc.org/ticket/2460#comment:2>
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