[zeromq-dev] Closed dealer socket may cause context termination to hang

[Michi Henning]

A doc patch for this would be really nice. I've been bitten by this as well.

Michi.


On 6 Nov 2014, at 17:56 , Pieter Hintjens <ph at imatix.com> wrote:

> You have to set linger=0 on the socket before closing it.
> 
> There are two old design mistakes in libzmq which cause this. One is
> the default linger of infinity. I'd have changed that ages ago except
> would break compatibility. The other is libzmq's shutdown logic, which
> can't handle even the simple case you showed. It's worse... try just
> creating the socket and connecting, then terminating, and it blocks.
> 
> The good news is you can trivially fix this. Set linger=0 on all
> sockets before destroying them, and destroy all sockets before
> terminating. Or use a binding like CZMQ which does this for you.
> 
> There is IME no value in "waiting for messages to be sent" in normal
> operation. That happens only in apps which fire up, send a message,
> then exit. Rare. Normal ZeroMQ apps run forever and message loss at
> the end of a connection is always dealt with at a higher level.
> 
> -Pieter
> 
> On Thu, Nov 6, 2014 at 1:13 AM, André Caron <andre.l.caron at gmail.com> wrote:
>> Hi there,
>> 
>> I've been bit by this nasty little bugger and I thought I'd share.
>> 
>> In this little snippet, terminating the context hangs unless the connection
>> was established, even though we're closing the socket properly before
>> terminating the context.
>> 
>>    import zmq
>>    c = zmq.Context()
>>    s = c.socket(zmq.DEALER)
>>    s.connect('tcp://127.0.0.1:66')
>>    s.send_multipart(['foo', 'bar', 'meh'])
>>    s.close()
>>    c.term()  # hangs (until someone starts listening on port 666).
>> 
>> It happens that if you put a linger on the socket when closing it, the
>> program shuts down nicely.
>> 
>> Took me a while to figure out why, but it seems this is a symptom of that
>> little subtlety between bind and connect in ZeroMQ.  I remember reading
>> about this somewhere in the guide, but I probably thought I'd come back to
>> this concept and never did.  So apparently, the dealer socket immediately
>> creates a queue for messages when you call "zmq_connect()", even if the TCP
>> connection is never established and somehow this queue that never gets
>> flushed if you don't establish a connection.
>> 
>> I'm really surprised by this behavior.  I would think that closing the
>> dealer socket would stop the reconnection attempts, and then the socket
>> would drop the queue and allow the context to terminate (as when I set the
>> linger on the socket).
>> 
>> I know the queue creation on "zmq_connect()" is by design, but is the
>> attempt to connect to the peer after the dealer socket is closed desired as
>> well?
>> 
>> If it is, I'd like to know if there is a place to document these kinds of
>> gotchas.  If so, I'd probably send in a documentation patch for this
>> particular one.
>> 
>> Cheers,
>> 
>> André
>> 
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