[erlang-questions] C-Node breakage with git master

[Sverker Eriksson]

Hi Andreas

Current master is broken with respect to monitor of c-node "processes".

I'm currently schratching my head trying to fix this mess. Nice to have
a shoulder to cry on.

The commit https://github.com/erlang/otp/commit/17e198d6ee60f7dec9abfed
272cf4226 is part of a work to make distributed operations (like send,
monitor, link, etc) to *not* wait for the connections setup to
complete. They should just enqueue their request and trigger a
connection setup to commence.

This can be done in a backward compatible manner as all of them have a
truly asynchronous interface.

All except erlang:monitor, when called toward c-nodes which do not
support process monitoring. Old behaviour is to throw badarg. But that
would force erlang:monitor to still be synchronous waiting for
connection setup in order to know if the node supports it or not.

So the idea is to slightly change the behavoir and instead of badarg go
ahead and create the monitor but only let it supervise the connection.
That is, you will only get 'DOWN' message with 'noconnection' from such
a monitor. This is similar to what gen:call implements today by cathing
badarg and using monitor_node.

Yes, erl_interface sets DFLAG_DIST_MONITOR but it does not implement
it. Why? I think it's an old sloppy fix to make erl_global_register()
work, where the c-node receives a monitor request as part of the reply
which it just ignores.


/Sverker



On tis, 2017-11-21 at 16:31 +0100, Andreas Schultz wrote:
> ----- On Nov 21, 2017, at 4:23 PM, Andreas Schultz aschultz@REDACTED
> wrote:
> 
> > 
> > Hi,
> > 
> > Something in the C-node handling changed between otp-20.1.3 and
> > today git master branch.
> Did sent to early, the behavior was changed by this commit:
> 
> https://github.com/erlang/otp/commit/17e198d6ee60f7dec9abfed272cf4226
> aea44535
> 
> I think the removal of the DFLAG_DIST_MONITOR and
> DFLAG_DIST_MONITOR_NAME
> in that cset is a mistake.
> 
> Also, the removed DFLAG_DIST_MONITOR test proves IMHO that a
> C-Node should not set that flag.
> 
> Comments?
> 
> Regards
> Andreas
> 
> > 
> > 
> > I have a C-node that implements the remote end of net_adm:ping/1.
> > To do that, it needs to answer to
> > 
> >  gen:call({net_kernel, Node}, '$gen_call', {is_auth, node()},
> > infinity)
> > 
> > Code is at [1].
> > 
> > In 20.1.3, the call is send as a plain message. With today's git
> > master
> > it first attempts to setup a monitor (a ERL_MONITOR_P message is
> > sent).
> > 
> > Monitors are not supported by C-node and ei_xreceive_msg therefor
> > returns
> > an error, causing a connection abort.
> > 
> > Reading through some the distribution code and gen.erl code, it
> > seems
> > that the C-node support is somewhat broken to begin with. There is
> > a comment in gen.erl [2] that suggests that a attempting to setup a
> > monitor on a C-node should return with an error. However, this
> > would
> > need to be done on sending side. ei_xreceive_msg has no support to
> > deal with it and the error code would not allow the consumer to
> > implement proper handling.
> > 
> > There are two flags in the distribution protocol
> > (DFLAG_DIST_MONITOR and
> > DFLAG_DIST_MONITOR_NAME). C-Nodes do set the DFLAG_DIST_MONITOR,
> > but not
> > the DFLAG_DIST_MONITOR_NAME flag. This seems to be wrong, IMHO the
> > DFLAG_DIST_MONITOR should be cleared.
> > 
> > But, I also can't find the place where erlang:monitor/1 would
> > actually
> > check and honor those flags. The monitor BIF seems to always send a
> > monitor request regardless of the node flags.
> > 
> > So my questions are:
> > 
> > 1. Has anyone an idea what changed to cause the ping/is_auth
> > change?
> > 2. What is the correct way to implement a gen_server in a C-Node or
> >   is it in deed currently not possible/broken?
> > 
> > Regards
> > Andreas
> > 
> > [1]: https://github.com/travelping/capwap-dp/blob/master/src/capwap
> > -dp.c#L943
> > [2]: https://github.com/erlang/otp/blob/master/lib/stdlib/src/gen.e
> > rl#L184
> > --
> > Dipl.-Inform. Andreas Schultz
> > 
> > email: as@REDACTED
> > phone: +49-391-819099-224
> > 
> > ----------------------- enabling your networks --------------------
> > --
> > 
> > Travelping GmbH                     phone:  +49-391-81 90 99 0
> > Roentgenstr.  13                    fax:    +49-391-81 90 99 299
> > 39108 Magdeburg                     email:  info@REDACTED
> > GERMANY                             web:    http://www.travelping.c
> > om
> > 
> > Company Registration: Amtsgericht Stendal        Reg No.:   HRB
> > 10578
> > Geschaeftsfuehrer: Holger Winkelmann          VAT ID No.:
> > DE236673780
> > -----------------------------------------------------------------
> > ----
> > _______________________________________________
> > erlang-questions mailing list
> > erlang-questions@REDACTED
> > http://erlang.org/mailman/listinfo/erlang-questions