[zeromq-dev] Clean shutdown of malamute client zactor in python binding?

[Pieter Hintjens]

Yes, it's not simple yet. We're slowly building threadsafe sockets,
which will let us deal with actors from any thread.

On Wed, Feb 3, 2016 at 6:34 PM, Michal Vyskocil
<michal.vyskocil at gmail.com> wrote:
> Hi,
>
> In czmq things are stopped when zsys_interrupted is 1. This apply for
> malamute as well. It is modified by default signal handler, so simple ctrl-c
> ends your client cleanly, even if in recv call.
>
> I assume that the default signal handler of camp is not setup correctly in
> your case. Consult zsys man page for details.
>
> Dne 3. 2. 2016 6:07 PM napsal uživatel "Aaron Sokoloski"
> <asokoloski at gmail.com>:
>>
>> Hi folks,
>>
>> I'm trying to make the python malamute binding as easy to use as possible.
>> I'm having trouble making sure that everything gets cleaned up correctly on
>> exit, though, when the client is running in a thread other than the main
>> thread, and the client is blocking on a call to recv (aka mlm_client_recv).
>>
>> So far, I have something that works, but isn't ideal, which is to create a
>> "shutdown" inproc socket pair whose only purpose is to notify the recieving
>> end when the process gets interrupted, using a python signal handler.  Then
>> the application code can poll both that receiving socket and the malamute
>> client msgpipe, and exit the receive loop when it gets a message on the
>> shutdown socket.
>>
>> Here's the gist of that approach:
>> https://gist.github.com/asokoloski/02ab5affeca9be2bebdb
>>
>> Although conceptually simple, this is a bit awkward, especially for
>> application code that may want to create multiple clients.  So what I'm
>> trying to do is figure out a way to make everything get cleaned up
>> automatically.  Ideally, what would happen is that the zactor thread
>> finishes whatever it was doing and the zactor gets destroyed, then an
>> exception is raised in the python thread.
>>
>> Destroying the zactor from another thread is a big no-no, because zeromq
>> sockets are not thread safe, correct?  So the thread that is doing the recv
>> has to wake up.  We could terminate the context directly, which would make
>> the recv return, but that's quite abrupt, and doesn't give the zactor a
>> chance to shut down.  We could make the recv call poll the socket
>> internally, periodically waking up to check if it's been interrupted, but
>> that seems gross, especially as it would have to poll rather quickly -- the
>> shutdown timeout is 200ms.
>>
>> One options is to take the same approach of having a shutdown socket pair,
>> but try to hide it inside the python library code.  Maybe the application
>> code just calls recv(), but inside recv() the object polls the two sockets.
>> But that doesn't really play nicely in the case where, say, the application
>> code wants to poll the malamute client and some other sockets at the same
>> time.  It's doable, but feels complicated.
>>
>> So I have lots of questions now.  Is my reasoning above valid?  Is this
>> sort of thing a problem with pure c programs as well?  Does it even matter
>> if the zactor shuts down cleanly, aside from avoiding ugly warnings?  Is
>> there another approach that I haven't even considered?
>>
>> If there isn't a better way to do this, I can bite the bullet and make the
>> application code handle it, but it sure feels a bit lacking.
>>
>> Thanks in advance for any help anyone can offer,
>> Aaron
>>
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>>
>
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