BIND 10 #419: Parallel xfrout session should be allowed

[BIND 10 Development]

#419: Parallel xfrout session should be allowed
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      Reporter:  zhanglikun  |        Owner:  zhanglikun
          Type:  defect      |       Status:  reviewing 
      Priority:  major       |    Milestone:            
     Component:  xfrout      |   Resolution:            
      Keywords:              |    Sensitive:  0         
Estimatedhours:  0.0         |        Hours:  0         
      Billable:  1           |   Totalhours:  0         
      Internal:  0           |  
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Comment(by zzchen_pku):

 Replying to [comment:3 zhanglikun]:
 > Hi jerry, I would to make UnixSockServer to be one simple tcpserver.
 Xfrout was designed to support the short connection from several auth
 processes, now the connection has been changed to long connection, we have
 to refactor the code of xfrout. Jerry, could you send one email to bind-
 dev list to talk about it?
 Done，please see below：
 ---
 Xfrout will maintain a tcp connection with each Auth process, until an
 error occurs or Xfrout/Auth shutdown.
                   |----------|
                   |  Xfrout  |
                   |----------|
                   /     |     \
                  /      |      \
                 /       |       \
          |----------|----------|-------|
          |   Auth1  |   Auth2  |  ...  |
          |----------|----------|-------|
           /    |    \
          /     |     \
         /      |      \
  |----------|----------|-------|
  | Request1 | Request2 |  ...  |
  |----------|----------|-------|

 * The connection between Xfrout and Auth needs to be created only on the
 first time or if an error occur.
 * On receiving a request, Auth will send the corresponding socket fd and
 request message to Xfrout Server via tcp connection, then Xfrout Server
 can handle the request and send xfrout response(AXFR/IXFR) via the socket
 fd in a new thread.
 * Xfrout server will maintain M + N tcp connections(If we have M Auth
 processes and handle N xfrout requests at the same time).

 I updated the code a bit to enable Xfrout support multi auth, committed on
 r4014.

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Ticket URL: <http://bind10.isc.org/ticket/419#comment:4>
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