Add {switch_handler, Module} return value to cowboy_rest

Also {switch_handler, Module, Opts}.

Allows switching to a different handler type. This is
particularly useful for processing most of the request
with cowboy_rest and then streaming the response body
using cowboy_loop.

doc/src/guide/rest_handlers.asciidoc

@@ -43,8 +43,12 @@ you need.
 All callbacks take two arguments, the Req object and the State,
 and return a three-element tuple of the form `{Value, Req, State}`.
 
-All callbacks can also return `{stop, Req, State}` to stop execution
-of the request.
+Nearly all callbacks can also return `{stop, Req, State}` to
+stop execution of the request, and
+`{{switch_handler, Module}, Req, State}` or
+`{{switch_handler, Module, Opts}, Req, State}` to switch to
+a different handler type. The exceptions are `expires`
+`generate_etag`, `last_modified` and `variances`.
 
 The following table summarizes the callbacks and their default values.
 If the callback isn't defined, then the default value will be used.

doc/src/manual/cowboy_rest.asciidoc

@@ -25,11 +25,15 @@ init(Req, State)
 Callback(Req, State)
     -> {Result, Req, State}
      | {stop, Req, State}
+     | {{switch_handler, Module}, Req, State}
+     | {{switch_handler, Module, Opts}, Req, State}
 
 terminate(Reason, Req, State) -> ok  %% optional
 
 Req    :: cowboy_req:req()
 State  :: any()
+Module :: module()
+Opts   :: any()
 Reason :: normal
         | {crash, error | exit | throw, any()}
 
@@ -51,7 +55,14 @@ implemented. They otherwise all follow the same interface.
 
 The `stop` tuple can be returned to stop REST processing.
 If no response was sent before then, Cowboy will send a
-'204 No Content'.
+'204 No Content'. The `stop` tuple can be returned from
+any callback, excluding `expires`, `generate_etag`,
+`last_modified` and `variances`.
+
+A `switch_handler` tuple can be returned from these same
+callbacks to stop REST processing and switch to a different
+handler type. This is very useful to, for example, to stream
+the response body.
 
 The optional `terminate/3` callback will ultimately be called
 with the reason for the termination of the handler.

src/cowboy_rest.erl

@@ -20,6 +20,9 @@
 -export([upgrade/4]).
 -export([upgrade/5]).
 
+-type switch_handler() :: {switch_handler, module()}
+	| {switch_handler, module(), any()}.
+
 %% Common handler callbacks.
 
 -callback init(Req, any())
@@ -35,162 +38,181 @@
 -callback allowed_methods(Req, State)
 	-> {[binary()], Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([allowed_methods/2]).
 
 -callback allow_missing_post(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([allow_missing_post/2]).
 
 -callback charsets_provided(Req, State)
 	-> {[binary()], Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([charsets_provided/2]).
 
 -callback content_types_accepted(Req, State)
 	-> {[{binary() | {binary(), binary(), '*' | [{binary(), binary()}]}, atom()}], Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([content_types_accepted/2]).
 
 -callback content_types_provided(Req, State)
 	-> {[{binary() | {binary(), binary(), '*' | [{binary(), binary()}]}, atom()}], Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([content_types_provided/2]).
 
 -callback delete_completed(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([delete_completed/2]).
 
 -callback delete_resource(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([delete_resource/2]).
 
 -callback expires(Req, State)
 	-> {calendar:datetime() | binary() | undefined, Req, State}
-	| {stop, Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([expires/2]).
 
 -callback forbidden(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([forbidden/2]).
 
 -callback generate_etag(Req, State)
 	-> {binary() | {weak | strong, binary()}, Req, State}
-	| {stop, Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([generate_etag/2]).
 
 -callback is_authorized(Req, State)
 	-> {true | {false, iodata()}, Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([is_authorized/2]).
 
 -callback is_conflict(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([is_conflict/2]).
 
 -callback known_methods(Req, State)
 	-> {[binary()], Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([known_methods/2]).
 
 -callback languages_provided(Req, State)
 	-> {[binary()], Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([languages_provided/2]).
 
 -callback last_modified(Req, State)
 	-> {calendar:datetime(), Req, State}
-	| {stop, Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([last_modified/2]).
 
 -callback malformed_request(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([malformed_request/2]).
 
 -callback moved_permanently(Req, State)
 	-> {{true, iodata()} | false, Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([moved_permanently/2]).
 
 -callback moved_temporarily(Req, State)
 	-> {{true, iodata()} | false, Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([moved_temporarily/2]).
 
 -callback multiple_choices(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([multiple_choices/2]).
 
 -callback options(Req, State)
 	-> {ok, Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([options/2]).
 
 -callback previously_existed(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([previously_existed/2]).
 
 -callback resource_exists(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([resource_exists/2]).
 
 -callback service_available(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([service_available/2]).
 
 -callback uri_too_long(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([uri_too_long/2]).
 
 -callback valid_content_headers(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([valid_content_headers/2]).
 
 -callback valid_entity_length(Req, State)
 	-> {boolean(), Req, State}
 	| {stop, Req, State}
+	| {switch_handler(), Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([valid_entity_length/2]).
 
 -callback variances(Req, State)
 	-> {[binary()], Req, State}
-	| {stop, Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([variances/2]).
 
@@ -233,11 +255,17 @@
 
 -spec upgrade(Req, Env, module(), any())
 	-> {ok, Req, Env} when Req::cowboy_req:req(), Env::cowboy_middleware:env().
-upgrade(Req0, Env, Handler, HandlerState) ->
+upgrade(Req0, Env, Handler, HandlerState0) ->
 	Method = cowboy_req:method(Req0),
-	{ok, Req, Result} = service_available(Req0, #state{method=Method,
-		handler=Handler, handler_state=HandlerState}),
-	{ok, Req, Env#{result => Result}}.
+	case service_available(Req0, #state{method=Method,
+			handler=Handler, handler_state=HandlerState0}) of
+		{ok, Req, Result} ->
+			{ok, Req, Env#{result => Result}};
+		{Mod, Req, HandlerState} ->
+			Mod:upgrade(Req, Env, Handler, HandlerState);
+		{Mod, Req, HandlerState, Opts} ->
+			Mod:upgrade(Req, Env, Handler, HandlerState, Opts)
+	end.
 
 -spec upgrade(Req, Env, module(), any(), any())
 	-> {ok, Req, Env} when Req::cowboy_req:req(), Env::cowboy_middleware:env().
@@ -260,6 +288,8 @@ known_methods(Req, State=#state{method=Method}) ->
 			next(Req, State, 501);
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{List, Req2, HandlerState} ->
 			State2 = State#state{handler_state=HandlerState},
 			case lists:member(Method, List) of
@@ -285,6 +315,8 @@ allowed_methods(Req, State=#state{method=Method}) ->
 				[<<"HEAD">>, <<"GET">>, <<"OPTIONS">>]);
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{List, Req2, HandlerState} ->
 			State2 = State#state{handler_state=HandlerState},
 			case lists:member(Method, List) of
@@ -316,6 +348,8 @@ is_authorized(Req, State) ->
 			forbidden(Req, State);
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{true, Req2, HandlerState} ->
 			forbidden(Req2, State#state{handler_state=HandlerState});
 		{{false, AuthHead}, Req2, HandlerState} ->
@@ -346,6 +380,8 @@ options(Req, State=#state{allowed_methods=Methods, method= <<"OPTIONS">>}) ->
 				= << << ", ", M/binary >> || M <- Methods >>,
 			Req2 = cowboy_req:set_resp_header(<<"allow">>, Allow, Req),
 			respond(Req2, State, 200);
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
 		{ok, Req2, HandlerState} ->
@@ -387,6 +423,8 @@ content_types_provided(Req, State) ->
 			end;
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{[], Req2, HandlerState} ->
 			not_acceptable(Req2, State#state{handler_state=HandlerState});
 		{CTP, Req2, HandlerState} ->
@@ -489,6 +527,8 @@ languages_provided(Req, State) ->
 			charsets_provided(Req, State);
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{[], Req2, HandlerState} ->
 			not_acceptable(Req2, State#state{handler_state=HandlerState});
 		{LP, Req2, HandlerState} ->
@@ -549,6 +589,8 @@ charsets_provided(Req, State) ->
 			set_content_type(Req, State);
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{[], Req2, HandlerState} ->
 			not_acceptable(Req2, State#state{handler_state=HandlerState});
 		{CP, Req2, HandlerState} ->
@@ -832,6 +874,8 @@ moved_permanently(Req, State, OnFalse) ->
 			OnFalse(Req2, State#state{handler_state=HandlerState});
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		no_call ->
 			OnFalse(Req, State)
 	end.
@@ -853,6 +897,8 @@ moved_temporarily(Req, State) ->
 			is_post_to_missing_resource(Req2, State#state{handler_state=HandlerState}, 410);
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		no_call ->
 			is_post_to_missing_resource(Req, State, 410)
 	end.
@@ -903,6 +949,8 @@ accept_resource(Req, State) ->
 			respond(Req, State, 415);
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{CTA, Req2, HandlerState} ->
 			CTA2 = [normalize_content_types(P) || P <- CTA],
 			State2 = State#state{handler_state=HandlerState},
@@ -938,6 +986,8 @@ process_content_type(Req, State=#state{method=Method, exists=Exists}, Fun) ->
 	try case call(Req, State, Fun) of
 		{stop, Req2, HandlerState2} ->
 			terminate(Req2, State#state{handler_state=HandlerState2});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{true, Req2, HandlerState2} when Exists ->
 			State2 = State#state{handler_state=HandlerState2},
 			next(Req2, State2, fun has_resp_body/2);
@@ -1019,6 +1069,8 @@ set_resp_body(Req, State=#state{content_type_a={_, Callback}}) ->
 	try case call(Req, State, Callback) of
 		{stop, Req2, HandlerState2} ->
 			terminate(Req2, State#state{handler_state=HandlerState2});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{Body, Req2, HandlerState2} ->
 			State2 = State#state{handler_state=HandlerState2},
 			Req3 = cowboy_req:set_resp_body(Body, Req2),
@@ -1114,6 +1166,8 @@ expect(Req, State, Callback, Expected, OnTrue, OnFalse) ->
 			next(Req, State, OnTrue);
 		{stop, Req2, HandlerState} ->
 			terminate(Req2, State#state{handler_state=HandlerState});
+		{Switch, Req2, HandlerState} when element(1, Switch) =:= switch_handler ->
+			switch_handler(Switch, Req2, HandlerState);
 		{Expected, Req2, HandlerState} ->
 			next(Req2, State#state{handler_state=HandlerState}, OnTrue);
 		{_Unexpected, Req2, HandlerState} ->
@@ -1148,6 +1202,11 @@ next(Req, State, StatusCode) when is_integer(StatusCode) ->
 respond(Req, State, StatusCode) ->
 	terminate(cowboy_req:reply(StatusCode, Req), State).
 
+switch_handler({switch_handler, Mod}, Req, HandlerState) ->
+	{Mod, Req, HandlerState};
+switch_handler({switch_handler, Mod, Opts}, Req, HandlerState) ->
+	{Mod, Req, HandlerState, Opts}.
+
 -spec error_terminate(cowboy_req:req(), #state{}, atom(), any()) -> no_return().
 error_terminate(Req, #state{handler=Handler, handler_state=HandlerState}, Class, Reason) ->
 	cowboy_handler:terminate({crash, Class, Reason}, Req, HandlerState, Handler),

test/handlers/switch_handler_h.erl

@@ -0,0 +1,36 @@
+-module(switch_handler_h).
+
+-export([init/2]).
+-export([content_types_provided/2]).
+-export([provide/2]).
+-export([info/3]).
+
+init(Req, State) ->
+	{cowboy_rest, Req, State}.
+
+content_types_provided(Req, State) ->
+	{[{<<"text/plain">>, provide}], Req, State}.
+
+provide(Req0, run) ->
+	Req = cowboy_req:stream_reply(200, Req0),
+	send_after(0),
+	{{switch_handler, cowboy_loop}, Req, 0};
+provide(Req0, hibernate) ->
+	Req = cowboy_req:stream_reply(200, Req0),
+	send_after(0),
+	{{switch_handler, cowboy_loop, hibernate}, Req, 0}.
+
+send_after(N) ->
+	erlang:send_after(100, self(), {stream, msg(N)}).
+
+msg(0) -> <<"Hello\n">>;
+msg(1) -> <<"streamed\n">>;
+msg(2) -> <<"world!\n">>;
+msg(3) -> stop.
+
+info({stream, stop}, Req, State) ->
+	{stop, Req, State};
+info({stream, What}, Req, State) ->
+	cowboy_req:stream_body(What, nofin, Req),
+	send_after(State + 1),
+	{ok, Req, State + 1}.

test/rest_handler_SUITE.erl

@@ -0,0 +1,70 @@
+%% Copyright (c) 2017, Loïc Hoguin <essen@ninenines.eu>
+%%
+%% Permission to use, copy, modify, and/or distribute this software for any
+%% purpose with or without fee is hereby granted, provided that the above
+%% copyright notice and this permission notice appear in all copies.
+%%
+%% THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+%% WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+%% MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+%% ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+%% WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+%% ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+%% OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+-module(rest_handler_SUITE).
+-compile(export_all).
+
+-import(ct_helper, [config/2]).
+-import(ct_helper, [doc/1]).
+-import(cowboy_test, [gun_open/1]).
+
+%% ct.
+
+all() ->
+	cowboy_test:common_all().
+
+groups() ->
+	cowboy_test:common_groups(ct_helper:all(?MODULE)).
+
+init_per_group(Name, Config) ->
+	cowboy_test:init_common_groups(Name, Config, ?MODULE).
+
+end_per_group(Name, _) ->
+	cowboy:stop_listener(Name).
+
+%% Dispatch configuration.
+
+init_dispatch(_) ->
+	cowboy_router:compile([{'_', [
+		{"/switch_handler", switch_handler_h, run},
+		{"/switch_handler_opts", switch_handler_h, hibernate}
+	]}]).
+
+%% Internal.
+
+do_decode(Headers, Body) ->
+	case lists:keyfind(<<"content-encoding">>, 1, Headers) of
+		{_, <<"gzip">>} -> zlib:gunzip(Body);
+		_ -> Body
+	end.
+
+%% Tests.
+
+switch_handler(Config) ->
+	doc("Switch REST to loop handler for streaming the response body."),
+	ConnPid = gun_open(Config),
+	Ref = gun:get(ConnPid, "/switch_handler", [{<<"accept-encoding">>, <<"gzip">>}]),
+	{response, nofin, 200, Headers} = gun:await(ConnPid, Ref),
+	{ok, Body} = gun:await_body(ConnPid, Ref),
+	<<"Hello\nstreamed\nworld!\n">> = do_decode(Headers, Body),
+	ok.
+
+switch_handler_opts(Config) ->
+	doc("Switch REST to loop handler for streaming the response body; with options."),
+	ConnPid = gun_open(Config),
+	Ref = gun:get(ConnPid, "/switch_handler_opts", [{<<"accept-encoding">>, <<"gzip">>}]),
+	{response, nofin, 200, Headers} = gun:await(ConnPid, Ref),
+	{ok, Body} = gun:await_body(ConnPid, Ref),
+	<<"Hello\nstreamed\nworld!\n">> = do_decode(Headers, Body),
+	ok.