BIND 10 #1452: add UDP socket passing to fd_X handling

[BIND 10 Development]

#1452: add UDP socket passing to fd_X handling
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                   Reporter:  jelte  |                 Owner:  UnAssigned
                       Type:  task   |                Status:  reviewing
                   Priority:  major  |             Milestone:
                  Component:  DDNS   |  Sprint-20111220
                   Keywords:         |            Resolution:
            Defect Severity:  N/A    |             Sensitive:  0
Feature Depending on Ticket:  DDNS   |           Sub-Project:  DNS
        Add Hours to Ticket:  0      |  Estimated Difficulty:  5
                  Internal?:  0      |           Total Hours:  0
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Changes (by jinmei):

 * owner:  jinmei => UnAssigned
 * status:  accepted => reviewing


Comment:

 trac1452 is (finally) ready for review.

 Actually, this is only for the C++ implementation.  Since even this
 part is already quite big, I decided to defer the python wrapper
 part to a separate implementation/review cycle (or even a separate
 task).

 Unfortunately the diff is pretty large.  Basically, however, (I hope)
 it only does some straightforward things.  But the resulting code was
 this big due to the need for considering various corner cases and
 providing detailed tests.  My suggestion for review is to first read
 the overview doxygen documentation in util/io/socketsssion.h (in the
 `SocketSessionUtility` page), then look at the ForwardTest::pushAndPop
 test (in util/tests/socketsession_unittest.cc), which checks basic
 expected behaviors, along with the corresponding implementation.

 Other cases are generally for checking exceptional cases and error
 handling, and it will be easier to understand them once you get the
 overview of the design and implementation.

 If it's still too big to review in a single cycle, we can discuss
 whether/how to divide it into multiple chunks.

 Some other notes:
 - My long-term plan is to eventually replace the existing "xfr client"
   with this new interface.  But that will be a post ddns cleanup.
 - Likewise, I guess we might eventually hide fd_share details within
   the socket session implementation.  That will also be a subject of
   a post ddns discussion.

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Ticket URL: <https://bind10.isc.org/ticket/1452#comment:5>
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