Handle supervisor calls properly everywhere

src/cowboy_children.erl

@@ -20,8 +20,7 @@
 -export([shutdown/2]).
 -export([shutdown_timeout/3]).
 -export([terminate/1]).
--export([which_children/2]).
--export([count_children/1]).
+-export([handle_supervisor_call/4]).
 
 -record(child, {
 	pid :: pid(),
@@ -160,6 +159,24 @@ longest_shutdown_time([#child{shutdown=ChildTime}|Tail], Time) when ChildTime >
 longest_shutdown_time([_|Tail], Time) ->
 	longest_shutdown_time(Tail, Time).
 
+-spec handle_supervisor_call(any(), {pid(), any()}, children(), module()) -> ok.
+handle_supervisor_call(which_children, {From, Tag}, Children, Module) ->
+	From ! {Tag, which_children(Children, Module)},
+	ok;
+handle_supervisor_call(count_children, {From, Tag}, Children, _) ->
+	From ! {Tag, count_children(Children)},
+	ok;
+%% We disable start_child since only incoming requests
+%% end up creating a new process.
+handle_supervisor_call({start_child, _}, {From, Tag}, _, _) ->
+	From ! {Tag, {error, start_child_disabled}},
+	ok;
+%% All other calls refer to children. We act in a similar way
+%% to a simple_one_for_one so we never find those.
+handle_supervisor_call(_, {From, Tag}, _, _) ->
+	From ! {Tag, {error, not_found}},
+	ok.
+
 -spec which_children(children(), module()) -> [{module(), pid(), worker, [module()]}].
 which_children(Children, Module) ->
 	[{Module, Pid, worker, [Module]} || #child{pid=Pid} <- Children].

src/cowboy_http.erl

@@ -202,14 +202,8 @@ loop(State=#state{parent=Parent, socket=Socket, transport=Transport, opts=Opts,
 		Msg = {'EXIT', Pid, _} ->
 			loop(down(State, Pid, Msg), Buffer);
 		%% Calls from supervisor module.
-		{'$gen_call', {From, Tag}, which_children} ->
-			From ! {Tag, cowboy_children:which_children(Children, ?MODULE)},
-			loop(State, Buffer);
-		{'$gen_call', {From, Tag}, count_children} ->
-			From ! {Tag, cowboy_children:count_children(Children)},
-			loop(State, Buffer);
-		{'$gen_call', {From, Tag}, _} ->
-			From ! {Tag, {error, ?MODULE}},
+		{'$gen_call', From, Call} ->
+			cowboy_children:handle_supervisor_call(Call, From, Children, ?MODULE),
 			loop(State, Buffer);
 		%% Unknown messages.
 		Msg ->

src/cowboy_http2.erl

@@ -247,14 +247,8 @@ loop(State=#state{parent=Parent, socket=Socket, transport=Transport,
 		Msg = {'EXIT', Pid, _} ->
 			loop(down(State, Pid, Msg), Buffer);
 		%% Calls from supervisor module.
-		{'$gen_call', {From, Tag}, which_children} ->
-			From ! {Tag, cowboy_children:which_children(Children, ?MODULE)},
-			loop(State, Buffer);
-		{'$gen_call', {From, Tag}, count_children} ->
-			From ! {Tag, cowboy_children:count_children(Children)},
-			loop(State, Buffer);
-		{'$gen_call', {From, Tag}, _} ->
-			From ! {Tag, {error, ?MODULE}},
+		{'$gen_call', From, Call} ->
+			cowboy_children:handle_supervisor_call(Call, From, Children, ?MODULE),
 			loop(State, Buffer);
 		Msg ->
 			error_logger:error_msg("Received stray message ~p.", [Msg]),

src/cowboy_websocket.erl

@@ -250,6 +250,10 @@ handler_loop(State=#state{socket=Socket, messages={OK, Closed, Error},
 			websocket_close(State, HandlerState, timeout);
 		{timeout, OlderTRef, ?MODULE} when is_reference(OlderTRef) ->
 			handler_loop(State, HandlerState, SoFar);
+		%% Calls from supervisor module.
+		{'$gen_call', From, Call} ->
+			cowboy_children:handle_supervisor_call(Call, From, [], ?MODULE),
+			handler_loop(State, HandlerState, SoFar);
 		Message ->
 			handler_call(State, HandlerState,
 				SoFar, websocket_info, Message, fun handler_before_loop/3)