** exception exit: noconnection

[Lukas Larsson]

On Wed, Dec 4, 2019 at 10:59 AM Adam Lindberg <hello@REDACTED> wrote:

> Ah, thanks! That’s good to know. :-)
>
> Maybe I missed it but I can’t find this documented anywhere (in e.g.
> erlang:link/1 or erlang:monitor/2). The only place I can find it referenced
> is in an example in the Getting Started User’s Guide:
> http://erlang.org/doc/getting_started/robustness.html
>
>
It is mentioned under the Info section in the erlang:monitor/2
documentation.


> Perhaps it should be documented more prominently?
>

Yes it should, just as noproc is.


> Next question is I need to clarify is: can gen_server processes never
> receive exit messages as normal info messages? If I enable trap_exit I only
> receive a call to terminate with the noconnection error eventually...
>

I'm not sure I understand what you mean.

When trapping exits, a gen_server will either get the exit in the terminate
or handle_info callback. Which one depends on which process sends the exit
signal. If it is the "parent" process, i.e. the process that started the
gen_server, then the terminate callback will be called. If it some other
process it is the handle_info callback that is called. The assumption here
is that if the parent exits for any reason, you want to terminate your
gen_server, but if a child or peer exits, then you want to handle that and
possibly continue running.


>
> Cheers,
> Adam
>
> > On 4. Dec 2019, at 09:54, Adam Lindberg <hello@REDACTED> wrote:
> >
> > I have indeed linked processes. I realized that that is why the
> exception is “uncatchable” in the shell perhaps. Because the shell process
> dies because it is linked to my test processes, and the function running
> the test hasn’t encountered an error yet.
> >
> > Does links in Erlang always crash with {'EXIT', Pid, noconnection} when
> a node dies?
> >
> > Cheers,
> > Adam
> >
> >> On 4. Dec 2019, at 08:38, Lukas Larsson <lukas@REDACTED> wrote:
> >>
> >>
> >>
> >> On Tue, Dec 3, 2019 at 11:56 AM Adam Lindberg <hello@REDACTED> wrote:
> >> Hi!
> >>
> >> I’m running some tests using distributed Erlang. I set up a cluster of
> Erlang nodes doing Distributed Systems™ stuff, and a hidden node that have
> a connection to each of the nodes in that cluster. The hidden node
> orchestrates the test by starting all Erlang nodes as ports. It then starts
> a process (gen_server) on each node that manipulates stuff on that node. It
> also loads some mock modules among other things. The hidden node also has
> some managing gen_servers running locally, which some of the mocks makes
> RPC calls to from the cluster nodes (to simulate and orchestrate mocked
> hardware components).
> >>
> >> Now I wanted to test how my system behaves when killing some random
> nodes, chaos monkey style. So I picked the easiest option of using
> rpc:cast(RandomClusterNode, erlang, halt, [137]). However, now my test dies
> with the following obscure error: ** exception exit: noconnection. This
> even happens when first spawning a fun that then calls erlang:halt(137) (as
> to avoid the RPC connection somehow breaking).
> >>
> >> After searching a bit on the Internet it seems to be some internal
> uncatchable (!) error generated by Erlang [1][2], but it is not at all
> clear when it happens, and how to avoid it. After some debugging in the
> gen_servers running on the hidden node, I can see the error by setting
> process_flag(trap_exit, true) and printing it in terminate/2 but I still
> can’t catch it. I can’t even catch it in the shell by enclosing my run in a
> try-catch block! It’s almost not mentioned at all in the official
> documentation [3]. Most likely I’m setting up my test nodes and the
> application/test code in a way that generates this error, but I have no
> idea what exactly leads to it.
> >>
> >> I guess I have two problems:
> >>
> >> 1. What is the error, and how can I handle / avoid it?
> >>
> >> I'm not sure, but could it be that your process is linked to a process
> on the remote side? That what you are getting is a broken link error?
> >>
> >> 2. Why is it not documented?
> >>
> >> Cheers,
> >> Adam
> >>
> >>
> >> [1]:
> http://erlang.org/pipermail/erlang-questions/2012-April/066219.html
> >> [2]:
> http://erlang.org/pipermail/erlang-questions/2013-April/073246.html
> >> [3]: http://erlang.org/doc/getting_started/robustness.html
> >>
> >
>
>
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