[erlang-questions] question on 'EXIT' messages delivered to exit-trapping gen_servers

[Chandru]

Hi Adam,

It looks like if a gen_server process was spawned using proc_lib:spawn and
an EXIT signal is received indicating that the Parent process has died, the
terminate() function is called!

The documentation should probably clarify this.

Here is the extract from gen_server.erl in R11B-5

loop(Parent, Name, State, Mod, Time, Debug) ->
    Msg = receive
          Input ->
            Input
      after Time ->
          timeout
      end,
    case Msg of
    {system, From, Req} ->
        sys:handle_system_msg(Req, From, Parent, ?MODULE, Debug,
                  [Name, State, Mod, Time]);
    {'EXIT', Parent, Reason} ->
        terminate(Reason, Name, Msg, Mod, State, Debug);
    _Msg when Debug =:= [] ->
        handle_msg(Msg, Parent, Name, State, Mod, Time);
    _Msg ->
        Debug1 = sys:handle_debug(Debug, {?MODULE, print_event},
                      Name, {in, Msg}),
        handle_msg(Msg, Parent, Name, State, Mod, Time, Debug1)
    end.

cheers
Chandru


2009/4/3 Adam Kocoloski <adam.kocoloski@REDACTED>

> Hi Erlangers, I've been doing a little hacking on CouchDB and running
> into some behavior that I don't understand.  We have a standalone
> gen_server (let's call it A) that traps exits and was started by
> another gen_server (B) using start_link.  B does not trap exits, and
> is linked to some other process C.  Now we have the following sequence
> of events:
>
> * exit(C, kill)
> * B exits immediately with Reason killed
> * A calls terminate with Reason killed
>
> That last line surprised me.  I was expecting to receive {'EXIT', B,
> killed} in A's mailbox, and in fact I had written a handle_info to
> process all 'EXIT' messages, but none arrived.  I checked that
> trap_exit is still true in A's terminate.  {trap_exit, true} clearly
> has some effect, as without it terminate isn't even called.
>
> I tried changing the exit signal to some other atom, but I still get
> the same basic results (just with goodbye_cruel_world instead of kill/
> killed in the Reasons).  I'm stumped.  Regards, Adam
> _______________________________________________
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