[erlang-questions] From whence do monitors come?

[Robert Virding]

It's the [flush] option to demonitor which saves you here. Apart from
removing the monitor it automatically removes any monitor message form
that process in the process mailbox. And yes the {'DOWN',...} message
does work as a normal message.

Robert

On 10 September 2010 15:21, Ulf Wiger <ulf.wiger@REDACTED> wrote:
> On 09/10/2010 03:07 PM, Matthew Sackman wrote:
>> gen.erl has the following code in do_call:
>>
>>           catch erlang:send(Process, {Label, {self(), Mref}, Request},
>>                 [noconnect]),
>>           receive
>>               {Mref, Reply} ->
>>                   erlang:demonitor(Mref, [flush]),
>>                   {ok, Reply};
>>               {'DOWN', Mref, _, _, noconnection} ->
>>                   exit({nodedown, Node});
>>               {'DOWN', Mref, _, _, Reason} ->
>>                   exit(Reason)
>>
>> If the callee is something like a gen_server, and on handling that msg
>> returns a {stop, Reply, Reason, State} quple, there seems to be a race
>> between the reply and the monitor. Or more simply, say the callee does
>> something like:
>>
>> receive {'$gen_call', From, _Msg} -> gen_server:reply(From, done) end.
>>
>> There seems to be a race - what's stopping the DOWN from the monitor
>> overtaking the reply from the callee?
>>
>> The only thing that I can think of is that the DOWN is considered to
>> come *from* the callee. Is that right?
>
> This is the only conclusion that would be compatible with the ordering
> guarantee, IMHO. The gen_server explicitly sends the reply before it
> dies, and it would be almost impossible to write an efficient and
> safe implementation of do_call if one couldn't be sure that the DOWN
> message will never be delivered before the last message sent to the
> same recipient.
>
> The ordering guarantee should be clearly documented in the reference
> manual, together with a clear statement on how EXIT signals and
> DOWN messages play along with that guarantee.
>
> BR,
> Ulf W
>
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