Erlang gen_server: How to catch Errors?

Question

I'm new in Erlang development and I'm interested in processes relationship.

If we link two processes P1 and P2 with process_flag(trap_exit, true) and use construction like Pid ! msg and receive .. after .. end - it's possible to catch P1 errors like badarith by the second process P2.

But if we use gen_server process P1 , which is linked with P2, - P1 terminates after P2 fails.

So, How to catch exit() errors using gen_server?

Thanks in advance!

P.S. Testing code.

P1:

-module(test1).
-compile(export_all).
-behaviour(gen_server).

start() ->
  gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).

init([]) -> Link = self(),
            spawn(fun() ->
              process_flag(trap_exit, true),
              link(Link),
              test2:start(Link)
            end),
            {ok, "lol"}.

handle_call(stop, From, State) ->
  io:fwrite("Stop from ~p. State = ~p~n",[From, State]),
  {stop, normal, "stopped", State};

handle_call(MSG, From, State) ->
  io:fwrite("MSG ~p from ~p. State = ~p~n",[MSG, From, State]),
  {reply, "okay", State}.

handle_info(Info, State) -> io:fwrite("Info message ~p. State = ~p~n",[Info, State]), {noreply, State}.

terminate(Reason, State) -> io:fwrite("Reason ~p. State ~p~n",[Reason, State]), ok.

P2:

-module(test2).
-compile(export_all).

start(Mod) ->
  io:fwrite("test2: Im starting with Pid=~p~n",[self()]),
  receiver(Mod).

receiver(Mod)->
  receive
    stop ->
      Mod ! {goodbye},
      io:fwrite("Pid: I exit~n"),
      exit(badarith);
    X ->
      io:fwrite("Pid: I received ~p~n",[X])
  end.

Result: test1 process fails after test2 exits with badarith.

38> 
38> c(test1).                    
test1.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,test1}
39> c(test2).                    
test2.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,test2}
40> test1:start().               
test2: Im starting with Pid=<0.200.0>
{ok,<0.199.0>}
41> <0.200.0> ! stop.            
Pid: I exit
Info message {goodbye}. State = "lol"
stop
** exception exit: badarith
42> gen_server:call(test1, stop).
** exception exit: {noproc,{gen_server,call,[test1,stop]}}
     in function  gen_server:call/2 (gen_server.erl, line 215)
43> 

Answer 1

Linking is bidirectional, but trapping exits isn't; you need to call process_flag(trap_exit, true) in P1 (your current code does it in P2), and then handle messages of the form {'EXIT', FromPid, Reason} in handle_info (put P2's pid into State to help).

In this case it makes sense for P2 to stop if P1 does, otherwise I'd suggest using a monitor instead of a link.

Side notes:

1. Use spawn_link instead of spawn + link.

2. It's better style to move spawn into test2:start.