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

Tue Nov 21 18:31:04 CET 2017

Hi,

----- On Nov 21, 2017, at 6:19 PM, Sverker Eriksson sverker.eriksson@REDACTED wrote:

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

Good to know ;-)

> 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.

I would be nice if we could have to way to support monitors in a
C-node as well. It would have to be the responsibility of the
C-node to implement whatever kind of monitoring it sees fit when such
a request comes in.

And since I'm dreaming, cached atoms in C-nodes would be great ;-)

> 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.

Sounds good to me.

Andreas

> 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
>> > 
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