BIND 10 #2459: call to getCachedZoneWriter must be protected by mutex

[BIND 10 Development]

#2459: call to getCachedZoneWriter must be protected by mutex
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                   Reporter:         |                 Owner:  jinmei
  jinmei                             |                Status:  accepted
                       Type:         |             Milestone:
  defect                             |  Sprint-20121120
                   Priority:         |            Resolution:
  medium                             |             Sensitive:  0
                  Component:         |           Sub-Project:  DNS
  b10-auth                           |  Estimated Difficulty:  3
                   Keywords:         |           Total Hours:  0
            Defect Severity:  High   |
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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Comment (by jinmei):

 trac2459 is ready for review.

 The branch is exactly what's proposed in the ticket's description
 (and it's small).

 I also confirmed my theory was (seemingly) correct by the following
 setup:
 - run b10-auth with an sqlite3-only data source
 - keep sending queries to b10-auth at a high volume using
   queryperf(-like tool)
 - at the same time, repeat "Auth loadzone" from bindctl in a shell
   loop

 Eventually b10-auth crashed due to segmentation fault (normally within
 a minute or so).  It died somewhere around sqlite3 API, so (while I
 didn't looked into it any further) it's quite likely to be a race I
 suspected.  After applying this change, it didn't happen.

 I don't think we need a changelog for this fix as it's an intermediate
 regression.

-- 
Ticket URL: <https://bind10.isc.org/ticket/2459#comment:5>
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