[zeromq-dev] What is the canonical handling of zeromq sockets when fork+exec?

[Luca Boccassi]

On Fri, 2016-11-25 at 12:22 +0100, zmqdev wrote:
> On 25.11.2016 11:50, Luca Boccassi wrote:
> > What I can say is that we have a unit test for this situation:
> >
> > https://github.com/zeromq/libzmq/blob/master/tests/test_fork.cpp
> >
> > And the child closes the (TCP) socket explicitly before the context.
> > Which is in fact what should happen in all cases.
> >
> > The parent then can receive messages on the sockets just fine.
> >
> > Maybe it's a linger issue? By default a socket has 30s of linger grace
> > period.
> >
> > Try setting ZMQ_LINGER to 0 in the socket in the child, close the socket
> > and then terminate the context perhaps.
> 
> thanks. Formatted differently:
> 
> 	1. zmq_close sockets in child (perhaps setting ZMQ_LINGER to 0 beforehand)
> 	2. zmq_term context in child
> 
> and only then
> 
> 	3. close rest of file descriptors in child
> 
> The reason I went directly to point 3 is this line from the man page of 
> fork(2):
> 
>      The child process is created with a single thread—the one
>      that called fork().
> 
> (see http://man7.org/linux/man-pages/man2/fork.2.html)
> 
> Michael Kerrisk in "The Linux Programming Interface" insists:
> 
>      When a multithreaded process calls fork(), only the calling thread is
>      replicated in the child process. (The ID of the thread in the child 
> is the
>      same as the ID of the thread that called fork() in the parent.) All 
> of the
>      other threads vanish in the child; no thread-specific data 
> destructors or
>      cleanup handlers are executed for those threads.
>      (...)
> 
> Of course, that's where I run into the problem?!

Yes I suspect the background I/O thread suddenly going missing might
have caused issues.

Did setting the linger and closing the socket help?

Kind regards,
Luca Boccassi
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