[ale] Python (or other) socket identification

[Ed Cashin]

You can use the IP as the key in a dict, and any object you want can be the
value.  I'm not sure what the identifiers are doing in your example, though.

On Fri, Feb 3, 2017 at 3:37 AM, Alex Carver <agcarver+ale at acarver.net>
wrote:

> I've been trying to figure out a method of tracking some sockets but I
> keep running into roadblocks.
>
> I've got a small daemon python script that runs a thread to accept
> connections from remote devices.  All the basic stuff is working so no
> problems there.  What I'm trying to do is make an asynchronous queue for
> the thread so I can send data back towards those devices.
>
> The queue idea was to put the device IP address and the data to send
> into the queue (all IPs are static and known).  The daemon thread loops
> continuously looking for status updates from the remote devices (via
> select() ).  At the end of the loop, it would check the queue for new
> items and send the data to the appropriate device.
>
> I currently keep a list of the socket objects that get created for every
> new connection.  This list gets fed into select() during each loop to
> tell me where there's data waiting to be received from some device.  The
> devices don't send regular updates on a rapid basis.  Instead they
> maintain an open connection and send updates once every couple minutes
> unless some event occurs and then they send an immediate update.
>
> Problem: I don't want to wait for one of the devices to send its message
> so I can send the data back.  I want to send as soon as the data is in
> the queue.
>
> First brute force solution is to loop through all the available sockets,
> check to see if the remote IP matches the one in the queue, then send
> data to it.  Works but can get bogged down as it does a getpeer() (to
> get the remote IP) on each socket until it finds a match.
>
> I didn't like that idea so I thought about two related lists.  One was
> keyed on the IP and the other keyed on the socket.  The IP key works
> fine but you can't use an object as a key in Python.  The reason for the
> second list keyed on socket is to avoid having to run through the IP
> keyed array looking for a matching value when trying to purge a dead
> socket (i.e. looping through the list of socket objects that select()
> returned, if one is dead you can't do a getpeer() on it so go to the
> second list, search for the socket ID and get its IP)
>
> Ok, so next idea was to use the file descriptor of the socket to get
> around the key problem.  I thought that was going to work until I ran
> into the hiccup of the socket possibly disappearing (timed out
> connection, device powers off, etc.) When the socket closes for whatever
> reason, the file descriptor goes away although the object persists until
> it's garbage collected after explicitly close()d.  If the file
> descriptor vanishes, I can't use it to purge the lists.
>
> So I'm trying to figure out if there's a more persistent identifier
> available that can be used as a key and will persist through a socket
> termination and can also be used to reference the socket it came from
>
>
>
> The gist of the daemon (this runs in a loop) in rough Python (some
> pseduo-Python for brevity):
>
> read,write,error = select(socketlist, [], [], 0) # no blocking
>
> for s in read:
>         if s == the_server_socket:
>                 client, address = s.accept()
>                 socketlist.append(client)
>                 #right here I would want to add:
>                 client_track_list_by_ip[address] = client.identifier
>                 client_track_list_by_identifier[client.identifier] = ip
>
>         else:
>                 #this was a client device, do things
>
>
>         #this bit of code runs in various places
>         #the idea is to remove the socket from the
>         #socketlist if it's dead
>
>         (if the socket is dead or caused an error):
>                 socketlist.remove(s)
>                 #here I would attempt to purge the two tracking lists
>                 #get peer won't work here because the socket
>                 #isn't connected, go find it in the other list
>                 ip = client_track_list_by_identifier[s.identifier]
>                 del client_track_list_by_ip[ip]
>                 del client_track_list_by_identifier[s.identifier]
>                 s.close()
>
>
> #then the queue handling:
> while ( queue is not empty ):
>         queue_item = getfromqueue()
>
>         send_to_socket(client_track_list_by_ip[queue_item.ip],
> queue_item.data)
>
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-- 
  Ed Cashin <ecashin at noserose.net>
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