BIND 10 #327: early work on recursion

[BIND 10 Development]

#327: early work on recursion
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      Reporter:  each      |        Owner:  each                 
          Type:  defect    |       Status:  new                  
      Priority:  major     |    Milestone:  y2 12 month milestone
     Component:  recurser  |   Resolution:                       
      Keywords:            |    Sensitive:  0                    
Estimatedhours:  0.0       |        Hours:  0                    
      Billable:  1         |   Totalhours:  0                    
      Internal:  0         |  
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Comment(by each):

 One of the first things that will need to be done to implement a resolver
 (or any other server type other than authoritative) is to split off the
 asio_link module from !AuthSrv.

 In current code, the asio_link object is constructed with a pointer to the
 auth server object; when packets are received, it calls
 auth_srv->processMessage() and auth_srv->configSession().  I've replaced
 that approach with one in which the caller provides asio_link with a pair
 of callback objects, which do the same thing but can be provided by any
 server class (!AuthSrv, Resolver, Forwarder, or whatever.)

 While I was at it, in hopes of making it less painful to write the ASIO
 handlers for the more complicated state machine we'll need to have when we
 implement a full validating resolver, I changed the TCPServer and
 UDPServer classes to use the "stackless coroutine" pattern described at:
 http://blog.think-async.com/2010/03/potted-guide-to-stackless-
 coroutines.html

 The resulting code should be functionally identical to the previous code,
 but it is shorter and (IMHO) easier to read: instead of several different
 asynchronous response handlers, there's a single function for TCP and
 another for UDP, and the I/O operations are all laid out in logical order.

 The next step will be to move asio_link out of bin/auth and into src/lib
 somewhere, but I'm leaving it where it is for now to make it easier to
 read the diff.

 I'd very much appreciate another pair of eyes looking at what I've done so
 far...

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Ticket URL: <http://bind10.isc.org/ticket/327#comment:1>
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