Unsupervised gen_server doesn't call terminate when it receives exit signal

Question

gen_server documentation on Module:terminate callback says:

Even if the gen_server process is not part of a supervision tree, this function is called if it receives an 'EXIT' message from its parent. Reason is the same as in the 'EXIT' message.

Here is my handle_info and terminate function:

handle_info(UnknownMessage, State) ->
    io:format("Got unknown message: ~p~n", [UnknownMessage]),
    {noreply, State}.

terminate(Reason, State) ->
    io:format("Terminating with reason: ~p~n", [Reason]).

I start this server using gen_server:start. I assume when I call erlang:exit(Pid, fuckoff), it should call terminate callback function. But it shows:

Got unknown message: {'EXIT',<0.33.0>,fuckoff}

Which means it is calling handle_info. But when I call gen_server:stop, everything works as mentioned in documentation. I'm calling my gen_server from shell. Would you please clarify this?

[UPDATE]

Here is source code of decode_msg function inside gen_server. If it receives any 'EXIT' message it should call terminate function:

decode_msg(Msg, Parent, Name, State, Mod, Time, Debug, Hib) ->
    case Msg of
    {system, From, Req} ->
        sys:handle_system_msg(Req, From, Parent, ?MODULE, Debug,
                  [Name, State, Mod, Time], Hib);
    {'EXIT', Parent, Reason} ->
        terminate(Reason, Name, Msg, Mod, State, Debug);
    _Msg when Debug =:= [] ->
        handle_msg(Msg, Parent, Name, State, Mod);
    _Msg ->
        Debug1 = sys:handle_debug(Debug, fun print_event/3,
                      Name, {in, Msg}),
        handle_msg(Msg, Parent, Name, State, Mod, Debug1)
end.

In my case it doesn't call terminate function.

[UPDATE]

When I start gen_server using gen_server:start_link(), sending an exit signal using erlang:exit(Pid, Reason) will result in calling terminate call back function which is an expected behaviour. It seems there is a difference in interpreting an exit signal whether a process is linked to its parent or not.

Answer 1

Short answer:

If you call the exit/2 function from inside the gen_server actor, it behaves as expected based on the documentation and the terminate/2 callback will be called.

Long answer:

When you send the exit message from the shell, the Parent value of exit tuple is set to the shell process id, on the other hand when you start the gen_server process form shell its Parent value is set to its own process id, not the shell process id, therefore when it gets the exit message it doesn't match the second clause of the receive block in decode_msg/8 function so the terminate/6 function is not called and finally the next clause is matched which is calling handle_msg/5 function.

Recommendation:

For getting the terminate/3 callback called even by sending an exit message to the gen_server process, you can trap the exit message in handle_info/2 and then return with stop tuple as follows:

init([]) ->
    process_flag(trap_exit, true),
    {ok, #state{}}.

handle_info({'EXIT', _From, Reason}, State) ->
    io:format("Got exit message: ~p~n", []),
    {stop, Reason, State}.

terminate(Reason, State) ->
    io:format("Terminating with reason: ~p~n", [Reason]),
    %% do cleanup ...
    ok.