TCP servers in Python - two processes want to use same port

[Nowan]

Pif Paf wrote:
> I am writing programs that will run as TCP servers. Briefly, I want to 
> set up a TCP server on a port in such a way that if another server is 
> already sitting on that port (both servers are Python programs I will be 
> writing), the old one is booted off (and its process ended).
> 
> My master process is a test harness, which tests a script which as part 
> of its functionality sends http GET messages. My test harness sets up 
> an http server as a child process, by creating an instance of 
> popen2.Popen3().
> 
> The child process is a very simple program using SimpleHTTPServer. The 
> problem is when I run my test harness, break it with Ctrl-C then try to
> run it again. The http server complains that it cannot bind to the socket,
> as the address is already in use.
> 
> My production system will use several http servers (I'm using http GET
> messages as a simple form of inter-process communication), and I don't 
> want to have a situation where a program runs one time (because it's 
> asking to bind to a port that isn't already in use), but the next time, 
> it doesn't run because the port is being used.
> 
> Now of course I could use files to describe what ports are currently in
> use and what process ids are using them, and kill the process before
> trying to re-use the port, but that solution strikes me as inelegant.
> Is there a better way? For example, is there a way of writing a program
> that will force binding to a port, and if an existing process has that
> port will kick it off (and possibly destroy it)?
> 
> --
> Pif Paf
I think you might have better luck in a TCP networking group, as this 
isn't really a python question (unless python has some nify TCP api). 
you might check into Twisted to see what's there for nifty apis.

If you're talking unix, you might want to spawn both servers as children 
of the same parent.  after launching both servers, the parent just waits 
as a manager process.  when the second child is unable to acquire the 
port, it can signal the parent.  the parent can send a stop/kill signal 
to the  child.

signaling a process requires the process id (pid).  the children don't 
know each others process ids.  but they have their parent's process id. 
  The parent gets the process ids of all children.

not sure how that would apply in windows.  don't think windows even has 
signals.  if so, you would have to substitute some other form of 
interprocess communications.