BIND 10 #2836: update in-memory zone loading so it can work with shmem segment

[BIND 10 Development]

#2836: update in-memory zone loading so it can work with shmem segment
-------------------------------------+-------------------------------------
                   Reporter:         |                 Owner:
  jinmei                             |                Status:  new
                       Type:  task   |             Milestone:  New Tasks
                   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.  depend on #2831.

 (according to my previous experiment) Most of the current
 implementation is sufficiently generic, so this task shouldn't be that
 difficult.

 The only tricky point is that `MemorySegment::allocate()` may now
 require internal region extension, which could change the base address
 of the mapped space.

 In my experiments, I addressed this issue by recording the latest
 pointer to `zone_data_` in the memory segment with a name at the
 beginning of `ZoneDataUpdater`:
 {{{#!cpp
         mem_sgmt_.setNamedAddress("updater_zone_data", zone_data_);
 }}}
 (type of zone_data_ has to be changed to pointer, not reference).

 and if `addRdataSet` failed, update the pointer from the segment:
 {{{#!cpp
     try {
         addRdataSet(rrset, sig_rrset);
     } catch (const util::MemorySegment::SegmentGrown&) {
         // If memory allocation failed due to SegmentGrown, zone_data_ may
         // have been invalidated due to possible memory remapping.  So we
         // refetch the address directly from the segment.
         zone_data_ = static_cast<ZoneData*>(
             mem_sgmt_.getNamedAddress("updater_zone_data"));

         // In theory, even the second attempt could fail, but it should be
         // very unlikely in practice.  Should that ever happen we
 propagate
         // the exception and let the application decide the next step.
         addRdataSet(rrset, sig_rrset);
     }
 }}}

 There may be some other places that need the same trick (this was the
 only case in my experiment, but I may have missed something).

 If there's a better way to address the issue, it doesn't have to be
 exactly this way, of course.

 Test this with both local and mapped segment.

-- 
Ticket URL: <http://bind10.isc.org/ticket/2836>
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