erlang's EXIT and DOWN signal - erlang

Question

The following code is also from rabbitmq's supervisor2.erl. The code's function is to kill supervisor's children, by for every child:
monitor child send an trappable exit signal
start timer
if timer's arrive, send an untrappable exit signal (kill).
My question about EXIT and DOWN signal.
If the child doesn't trap the exit signal, the supervisor will receive 2 signal, first is exit signal, and then is DOWN signal, is it right? Is the signal sequence is strictly guaranteed?
If the child traps the exit signal, the supervisor will receive only 1 signal, just down signal, is it right?
terminate_simple_children(Child, Dynamics, SupName) ->
Pids = dict:fold(fun (Pid, _Args, Pids) ->
erlang:monitor(process, Pid),
unlink(Pid),
exit(Pid, child_exit_reason(Child)),
[Pid | Pids]
end, [], Dynamics),
TimeoutMsg = {timeout, make_ref()},
TRef = timeout_start(Child, TimeoutMsg),
{Replies, Timedout} =
lists:foldl(
fun (_Pid, {Replies, Timedout}) ->
{Reply, Timedout1} =
receive
TimeoutMsg ->
Remaining = Pids -- [P || {P, _} <- Replies],
[exit(P, kill) || P <- Remaining],
receive {'DOWN', _MRef, process, Pid, Reason} ->
{{error, Reason}, true}
end;
{'DOWN', _MRef, process, Pid, Reason} ->
{child_res(Child, Reason, Timedout), Timedout};
{'EXIT', Pid, Reason} -> %%<==== strict signal, first EXIT, then DOWN.
receive {'DOWN', _MRef, process, Pid, _} ->
{{error, Reason}, Timedout}
end
end,
{[{Pid, Reply} | Replies], Timedout1}
end, {[], false}, Pids),
timeout_stop(Child, TRef, TimeoutMsg, Timedout),
ReportError = shutdown_error_reporter(SupName),
[case Reply of
{_Pid, ok} -> ok;
{Pid, {error, R}} -> ReportError(R, Child#child{pid = Pid})
end || Reply <- Replies],
ok.

Answer 1

There are two things you are confusing here:
First, the child is the one trapping exits, but you are looking at the supervisor code. What the child does with exit signals does not directly affect the supervisor.
the kill exit signal cannot be trapped. It always kills the child.
The supervisor2 has a monitor on the child. This means it is guaranteed to get a 'DOWN' message and this code is concerned about getting that kind of message. If supervisor2 is also trapping exits, it will get the 'EXIT' message in addition.