BIND 10 #2850: Define and implement ZoneTableSegmentMapped

[BIND 10 Development]

#2850: Define and implement ZoneTableSegmentMapped
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            Reporter:  jinmei        |                        Owner:  muks
                Type:  task          |                       Status:
            Priority:  medium        |  assigned
           Component:  data source   |                    Milestone:
            Keywords:                |  Sprint-20130423
           Sensitive:  0             |                   Resolution:
         Sub-Project:  DNS           |                 CVSS Scoring:
Estimated Difficulty:  5             |              Defect Severity:  N/A
         Total Hours:  0             |  Feature Depending on Ticket:
                                     |  shared memory data source
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by jinmei):

 Some notes, based on the latest shape of other related tasks:

 - READ_WRITE would actually mean if the file doesn't exist a new one
   is created; otherwise the existing one will be opened in read-write
   mode without initial modification.
 - `getHeader()` before any `reset()` would need a special care.  the
   implementation should probably define a special singleton zone table
   header (with a NULL `table_`) and refer to it until the real segment
   is set.
 - if the branch hasn't become too big, consider adding one more
   virtual method:

 {{{#!cpp
 class ZoneTableSegment {
 public:
     /// return true iff the segment is writable.
     ///
     /// The user of the zone table segment will load or update zones
     /// into the segment only for writable ones.  "local" segments
     /// will always be writable.  a "mapped" segment will be writable if
     /// a mapped memory segment in read-write mode has been set.
     virtual bool isWritable() const = 0;
 };

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Ticket URL: <http://bind10.isc.org/ticket/2850#comment:3>
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