Question

From what I’ve read in the docs, gen_servers don’t trap exits. Moreover, my understanding is that if a process starts another process with spawn_link, and the child process crashes, the parent crashes too.

However, this is not what I’m seeing. I’ve got a gen_server that spawn_links a process. I set up a function in the child process like so:

test(JobIsHalfDone) -> 
    case JobIsHalfDone of
        true -> exit(test);
        false -> ok
    end.

When this function sends the exit signal, I get a message:

** exception exit: test

Yet its parent gen_server keeps right on ticking. Why?

Answer 1

Lets compare experience, I have the following behaviour that says that it does die when a linked process dies.

1> {ok, Pid} = gen_server:start(server, [], []).
{ok,<0.33.0>}
2> gen_server:call(Pid, nice_to_see_you).
thanks
3> gen_server:call(Pid, nice_to_see_you).
thanks
4> gen_server:call(Pid, twap).           
oh_noes
5> gen_server:call(Pid, nice_to_see_you).
** exception exit: {noproc,{gen_server,call,[<0.33.0>,nice_to_see_you]}}
     in function  gen_server:call/2

That is, I made it crash by spawn_link:ing a process that does not much but dying, bringing the server down with it.

-module(server).
-compile(export_all).
init(_) ->
  {ok, []}.
handle_call(twap, _From, State) ->
   spawn_link(fun suicidal/0),
   {reply, oh_noes, State};
handle_call(_, _From, State) ->
   {reply, thanks, State}.
suicidal() ->
   exit(kaboom).

The fact that you see the “*** exception exit: test” seems to indicate that you used gen_server:start_link/3 from the shell, so that you gen server is linked to the shell process. That might introduce additional confusion, but nothing explaining why you would think the server didnt die.