gen_server that supervise and restarts event handlers (that implement the gen_event behavior) added with gen_event:add_sup_handler/3, suitable to be used in a standard otp supervision tree

event_handler_guard.erl

-module(event_handler_guard).
-author('[email protected]').

-behavior(gen_server).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% Types.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-record(state, {}).
-type state():: #state{}.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% Exports.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% Public API.
-export([start_link/1]).

%%% gen_server callbacks.
-export([
  init/1, terminate/2, code_change/3,
  handle_call/3, handle_cast/2, handle_info/2
]).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% Public API.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% @doc Starts the gen_server. Accepts a list of module names, which are the
%% event handlers needed to start and monitor.
-spec start_link([module()]) -> {ok, pid()} | ignore | {error, term()}.
start_link(EventHandlers) ->
  gen_server:start_link({local, ?MODULE}, ?MODULE, EventHandlers, []).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% gen_server API.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-spec init([module()]) -> {ok, state()}.
init(EventHandlers) ->
  [start(E) || E <- EventHandlers],
  {ok, #state{}, hibernate}.

-spec handle_cast(any(), state()) -> {noreply, state()}.
handle_cast(Msg, State) ->
  lager:error("Invalid cast: ~p", [Msg]),
  {noreply, State, hibernate}.

%% @doc Expects a gen_event_EXIT message and re-starts the event handler.
-spec handle_info(any(), state()) -> {noreply, state()}.
handle_info({gen_event_EXIT, Handler, Reason}, State) ->
  lager:error("Event Handler ~p died with: ~p", [Handler,Reason]),
  start(Handler),
  {noreply, State, hibernate};

handle_info(Msg, State) ->
  lager:error("Invalid msg: ~p", [Msg]),
  {noreply, State, hibernate}.

-spec handle_call(
  term(), {pid(), reference()}, state()
) -> {reply, term() | {invalid_request, term()}, state()}.
handle_call(Req, _From, State) ->
  lager:error("Invalid request: ~p", [Req]),
  {reply, invalid_request, State, hibernate}.

-spec terminate(atom(), state()) -> ok.
terminate(_Reason, _State) ->
  ok.

-spec code_change(string(), state(), any()) -> {ok, state()}.
code_change(_OldVsn, State, _Extra) ->
  {ok, State}.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% Private API.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% @doc Assuming that all the event handlers have a register/0 function, that
%% actually does the gen_event:add_sup_handler/3 call, just for convenience.
-spec start(module()) -> ok | {'EXIT', term()} | term().
start(EventHandler) ->
  EventHandler:register().

Lovely work. I am surprised I can't do this out of the box, but I can't see another way to do this given that supervision tree starts processes.