BIND 10 #151: XfroutClient socket location and removal

[BIND 10 Development]

#151: XfroutClient socket location and removal
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 Reporter:  jreed   |        Owner:  shentingting                               
     Type:  defect  |       Status:  assigned                                   
 Priority:  major   |    Milestone:  04. 2nd Incremental Release: Early Adopters
Component:  xfrout  |   Resolution:                                             
 Keywords:          |    Sensitive:  0                                          
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Comment(by shane):

 Replying to [comment:12 zhanglikun]:
 > One Question: How to make sure there is only one xfrout process running
 on server? to check whether the socket file
 /usr/local/var/auth_xfrout_conn used by xfrout exist or not when starting?
 The problem is: when xfrout crashed last time, and /auth_xfrout_conn isn't
 deleted properly, you have to let user to delete /auth_xfrout_conn
 manually. Do this make sense?

 Likun and I talked about this. If you try to `bind()` to an AF_UNIX
 socket, it will fail if any file exists, even an unused AF_UNIX socket.
 So, what we will do is try to `connect()` first. In this case one of 3
 things happens:

   1. If there is no file, you get a socket.error with ENOENT
   1. If there is a file, but it is not a socket or nobody is listening,
 you get socket.err with ECONNREFUSED
   1. If there is a listener, then the `connect()` will succeed

 So if another copy of xfrout is running, the `connect()` will succeed, and
 we can exit with appropriate warning/error message at that time.

 Otherwise, we get an error and we can safely start.

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Ticket URL: <http://bind10.isc.org/ticket/151#comment:13>
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