aboutsummaryrefslogtreecommitdiffstats
path: root/src/cowboy_rest.erl
blob: 49e194692ccac0db6589eed6536cdf98fc4fdede (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
%% Copyright (c) 2011-2017, Loïc Hoguin <[email protected]>
%%
%% 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.

%% Originally based on the Webmachine Diagram from Alan Dean and
%% Justin Sheehy.
-module(cowboy_rest).
-behaviour(cowboy_sub_protocol).

-export([upgrade/6]).

%% Common handler callbacks.

-callback init(Req, any())
	-> {ok | module(), Req, any()}
	| {module(), Req, any(), hibernate}
	| {module(), Req, any(), timeout()}
	| {module(), Req, any(), timeout(), hibernate}
	when Req::cowboy_req:req().

-callback terminate(any(), cowboy_req:req(), any()) -> ok.
-optional_callbacks([terminate/3]).

%% REST handler callbacks.

-callback allowed_methods(Req, State)
	-> {[binary()], Req, State}
	| {stop, 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}
	when Req::cowboy_req:req(), State::any().
-optional_callbacks([allow_missing_post/2]).

-callback charsets_provided(Req, State)
	-> {[binary()], Req, State}
	| {stop, 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}
	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}
	when Req::cowboy_req:req(), State::any().
-optional_callbacks([content_types_provided/2]).

-callback delete_completed(Req, State)
	-> {boolean(), Req, State}
	| {stop, 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}
	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}
	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}
	when Req::cowboy_req:req(), State::any().
-optional_callbacks([is_authorized/2]).

-callback is_conflict(Req, State)
	-> {boolean(), Req, State}
	| {stop, 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}
	when Req::cowboy_req:req(), State::any().
-optional_callbacks([known_methods/2]).

-callback languages_provided(Req, State)
	-> {[binary()], Req, State}
	| {stop, 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}
	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}
	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}
	when Req::cowboy_req:req(), State::any().
-optional_callbacks([moved_temporarily/2]).

-callback multiple_choices(Req, State)
	-> {boolean(), Req, State}
	| {stop, Req, State}
	when Req::cowboy_req:req(), State::any().
-optional_callbacks([multiple_choices/2]).

-callback options(Req, State)
	-> {ok, Req, State}
	| {stop, Req, State}
	when Req::cowboy_req:req(), State::any().
-optional_callbacks([options/2]).

-callback previously_existed(Req, State)
	-> {boolean(), Req, State}
	| {stop, 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}
	when Req::cowboy_req:req(), State::any().
-optional_callbacks([resource_exists/2]).

-callback service_available(Req, State)
	-> {boolean(), Req, State}
	| {stop, 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}
	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}
	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}
	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]).

%% End of REST callbacks. Whew!

-record(state, {
	method = undefined :: binary(),

	%% Handler.
	handler :: atom(),
	handler_state :: any(),

	%% Allowed methods. Only used for OPTIONS requests.
	allowed_methods :: [binary()] | undefined,

	%% Media type.
	content_types_p = [] ::
		[{binary() | {binary(), binary(), [{binary(), binary()}] | '*'},
			atom()}],
	content_type_a :: undefined
		| {binary() | {binary(), binary(), [{binary(), binary()}] | '*'},
			atom()},

	%% Language.
	languages_p = [] :: [binary()],
	language_a :: undefined | binary(),

	%% Charset.
	charsets_p = [] :: [binary()],
	charset_a :: undefined | binary(),

	%% Whether the resource exists.
	exists = false :: boolean(),

	%% Cached resource calls.
	etag :: undefined | no_call | {strong | weak, binary()},
	last_modified :: undefined | no_call | calendar:datetime(),
	expires :: undefined | no_call | calendar:datetime() | binary()
}).

-spec upgrade(Req, Env, module(), any(), infinity, run)
	-> {ok, Req, Env} when Req::cowboy_req:req(), Env::cowboy_middleware:env().
upgrade(Req0, Env, Handler, HandlerState, infinity, run) ->
	Method = cowboy_req:method(Req0),
	{ok, Req, Result} = service_available(Req0, #state{method=Method,
		handler=Handler, handler_state=HandlerState}),
	{ok, Req, Env#{result => Result}}.

service_available(Req, State) ->
	expect(Req, State, service_available, true, fun known_methods/2, 503).

%% known_methods/2 should return a list of binary methods.
known_methods(Req, State=#state{method=Method}) ->
	case call(Req, State, known_methods) of
		no_call when Method =:= <<"HEAD">>; Method =:= <<"GET">>;
				Method =:= <<"POST">>; Method =:= <<"PUT">>;
				Method =:= <<"PATCH">>; Method =:= <<"DELETE">>;
				Method =:= <<"OPTIONS">> ->
			next(Req, State, fun uri_too_long/2);
		no_call ->
			next(Req, State, 501);
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		{List, Req2, HandlerState} ->
			State2 = State#state{handler_state=HandlerState},
			case lists:member(Method, List) of
				true -> next(Req2, State2, fun uri_too_long/2);
				false -> next(Req2, State2, 501)
			end
	end.

uri_too_long(Req, State) ->
	expect(Req, State, uri_too_long, false, fun allowed_methods/2, 414).

%% allowed_methods/2 should return a list of binary methods.
allowed_methods(Req, State=#state{method=Method}) ->
	case call(Req, State, allowed_methods) of
		no_call when Method =:= <<"HEAD">>; Method =:= <<"GET">> ->
			next(Req, State, fun malformed_request/2);
		no_call when Method =:= <<"OPTIONS">> ->
			next(Req, State#state{allowed_methods=
				[<<"HEAD">>, <<"GET">>, <<"OPTIONS">>]},
				fun malformed_request/2);
		no_call ->
			method_not_allowed(Req, State,
				[<<"HEAD">>, <<"GET">>, <<"OPTIONS">>]);
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		{List, Req2, HandlerState} ->
			State2 = State#state{handler_state=HandlerState},
			case lists:member(Method, List) of
				true when Method =:= <<"OPTIONS">> ->
					next(Req2, State2#state{allowed_methods=List},
						fun malformed_request/2);
				true ->
					next(Req2, State2, fun malformed_request/2);
				false ->
					method_not_allowed(Req2, State2, List)
			end
	end.

method_not_allowed(Req, State, []) ->
	Req2 = cowboy_req:set_resp_header(<<"allow">>, <<>>, Req),
	respond(Req2, State, 405);
method_not_allowed(Req, State, Methods) ->
	<< ", ", Allow/binary >> = << << ", ", M/binary >> || M <- Methods >>,
	Req2 = cowboy_req:set_resp_header(<<"allow">>, Allow, Req),
	respond(Req2, State, 405).

malformed_request(Req, State) ->
	expect(Req, State, malformed_request, false, fun is_authorized/2, 400).

%% is_authorized/2 should return true or {false, WwwAuthenticateHeader}.
is_authorized(Req, State) ->
	case call(Req, State, is_authorized) of
		no_call ->
			forbidden(Req, State);
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		{true, Req2, HandlerState} ->
			forbidden(Req2, State#state{handler_state=HandlerState});
		{{false, AuthHead}, Req2, HandlerState} ->
			Req3 = cowboy_req:set_resp_header(
				<<"www-authenticate">>, AuthHead, Req2),
			respond(Req3, State#state{handler_state=HandlerState}, 401)
	end.

forbidden(Req, State) ->
	expect(Req, State, forbidden, false, fun valid_content_headers/2, 403).

valid_content_headers(Req, State) ->
	expect(Req, State, valid_content_headers, true,
		fun valid_entity_length/2, 501).

valid_entity_length(Req, State) ->
	expect(Req, State, valid_entity_length, true, fun options/2, 413).

%% If you need to add additional headers to the response at this point,
%% you should do it directly in the options/2 call using set_resp_headers.
options(Req, State=#state{allowed_methods=Methods, method= <<"OPTIONS">>}) ->
	case call(Req, State, options) of
		no_call when Methods =:= [] ->
			Req2 = cowboy_req:set_resp_header(<<"allow">>, <<>>, Req),
			respond(Req2, State, 200);
		no_call ->
			<< ", ", Allow/binary >>
				= << << ", ", M/binary >> || M <- Methods >>,
			Req2 = cowboy_req:set_resp_header(<<"allow">>, Allow, Req),
			respond(Req2, State, 200);
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		{ok, Req2, HandlerState} ->
			respond(Req2, State#state{handler_state=HandlerState}, 200)
	end;
options(Req, State) ->
	content_types_provided(Req, State).

%% content_types_provided/2 should return a list of content types and their
%% associated callback function as a tuple: {{Type, SubType, Params}, Fun}.
%% Type and SubType are the media type as binary. Params is a list of
%% Key/Value tuple, with Key and Value a binary. Fun is the name of the
%% callback that will be used to return the content of the response. It is
%% given as an atom.
%%
%% An example of such return value would be:
%%    {{<<"text">>, <<"html">>, []}, to_html}
%%
%% Note that it is also possible to return a binary content type that will
%% then be parsed by Cowboy. However note that while this may make your
%% resources a little more readable, this is a lot less efficient.
%%
%% An example of such return value would be:
%%    {<<"text/html">>, to_html}
content_types_provided(Req, State) ->
	case call(Req, State, content_types_provided) of
		no_call ->
			State2 = State#state{
				content_types_p=[{{<<"text">>, <<"html">>, '*'}, to_html}]},
			try cowboy_req:parse_header(<<"accept">>, Req) of
				undefined ->
					languages_provided(
						Req#{media_type => {<<"text">>, <<"html">>, []}},
						State2#state{content_type_a={{<<"text">>, <<"html">>, []}, to_html}});
				Accept ->
					choose_media_type(Req, State2, prioritize_accept(Accept))
			catch _:_ ->
				respond(Req, State2, 400)
			end;
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		{[], Req2, HandlerState} ->
			not_acceptable(Req2, State#state{handler_state=HandlerState});
		{CTP, Req2, HandlerState} ->
			CTP2 = [normalize_content_types(P) || P <- CTP],
			State2 = State#state{
				handler_state=HandlerState, content_types_p=CTP2},
			try cowboy_req:parse_header(<<"accept">>, Req2) of
				undefined ->
					{PMT, _Fun} = HeadCTP = hd(CTP2),
					languages_provided(
						Req2#{media_type => PMT},
						State2#state{content_type_a=HeadCTP});
				Accept ->
					choose_media_type(Req2, State2, prioritize_accept(Accept))
			catch _:_ ->
				respond(Req2, State2, 400)
			end
	end.

normalize_content_types({ContentType, Callback})
		when is_binary(ContentType) ->
	{cow_http_hd:parse_content_type(ContentType), Callback};
normalize_content_types(Normalized) ->
	Normalized.

prioritize_accept(Accept) ->
	lists:sort(
		fun ({MediaTypeA, Quality, _AcceptParamsA},
			 {MediaTypeB, Quality, _AcceptParamsB}) ->
				%% Same quality, check precedence in more details.
				prioritize_mediatype(MediaTypeA, MediaTypeB);
			({_MediaTypeA, QualityA, _AcceptParamsA},
			 {_MediaTypeB, QualityB, _AcceptParamsB}) ->
				%% Just compare the quality.
				QualityA > QualityB
		end, Accept).

%% Media ranges can be overridden by more specific media ranges or
%% specific media types. If more than one media range applies to a given
%% type, the most specific reference has precedence.
%%
%% We always choose B over A when we can't decide between the two.
prioritize_mediatype({TypeA, SubTypeA, ParamsA}, {TypeB, SubTypeB, ParamsB}) ->
	case TypeB of
		TypeA ->
			case SubTypeB of
				SubTypeA -> length(ParamsA) > length(ParamsB);
				<<"*">> -> true;
				_Any -> false
			end;
		<<"*">> -> true;
		_Any -> false
	end.

%% Ignoring the rare AcceptParams. Not sure what should be done about them.
choose_media_type(Req, State, []) ->
	not_acceptable(Req, State);
choose_media_type(Req, State=#state{content_types_p=CTP},
		[MediaType|Tail]) ->
	match_media_type(Req, State, Tail, CTP, MediaType).

match_media_type(Req, State, Accept, [], _MediaType) ->
	choose_media_type(Req, State, Accept);
match_media_type(Req, State, Accept, CTP,
		MediaType = {{<<"*">>, <<"*">>, _Params_A}, _QA, _APA}) ->
	match_media_type_params(Req, State, Accept, CTP, MediaType);
match_media_type(Req, State, Accept,
			CTP = [{{Type, SubType_P, _PP}, _Fun}|_Tail],
			MediaType = {{Type, SubType_A, _PA}, _QA, _APA})
		when SubType_P =:= SubType_A; SubType_A =:= <<"*">> ->
	match_media_type_params(Req, State, Accept, CTP, MediaType);
match_media_type(Req, State, Accept, [_Any|Tail], MediaType) ->
	match_media_type(Req, State, Accept, Tail, MediaType).

match_media_type_params(Req, State, _Accept,
		[Provided = {{TP, STP, '*'}, _Fun}|_Tail],
		{{_TA, _STA, Params_A}, _QA, _APA}) ->
	PMT = {TP, STP, Params_A},
	languages_provided(Req#{media_type => PMT},
		State#state{content_type_a=Provided});
match_media_type_params(Req, State, Accept,
		[Provided = {PMT = {_TP, _STP, Params_P}, _Fun}|Tail],
		MediaType = {{_TA, _STA, Params_A}, _QA, _APA}) ->
	case lists:sort(Params_P) =:= lists:sort(Params_A) of
		true ->
			languages_provided(Req#{media_type => PMT},
				State#state{content_type_a=Provided});
		false ->
			match_media_type(Req, State, Accept, Tail, MediaType)
	end.

%% languages_provided should return a list of binary values indicating
%% which languages are accepted by the resource.
%%
%% @todo I suppose we should also ask the resource if it wants to
%% set a language itself or if it wants it to be automatically chosen.
languages_provided(Req, State) ->
	case call(Req, State, languages_provided) of
		no_call ->
			charsets_provided(Req, State);
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		{[], Req2, HandlerState} ->
			not_acceptable(Req2, State#state{handler_state=HandlerState});
		{LP, Req2, HandlerState} ->
			State2 = State#state{handler_state=HandlerState, languages_p=LP},
			case cowboy_req:parse_header(<<"accept-language">>, Req2) of
				undefined ->
					set_language(Req2, State2#state{language_a=hd(LP)});
				AcceptLanguage ->
					AcceptLanguage2 = prioritize_languages(AcceptLanguage),
					choose_language(Req2, State2, AcceptLanguage2)
			end
	end.

%% A language-range matches a language-tag if it exactly equals the tag,
%% or if it exactly equals a prefix of the tag such that the first tag
%% character following the prefix is "-". The special range "*", if
%% present in the Accept-Language field, matches every tag not matched
%% by any other range present in the Accept-Language field.
%%
%% @todo The last sentence probably means we should always put '*'
%% at the end of the list.
prioritize_languages(AcceptLanguages) ->
	lists:sort(
		fun ({_TagA, QualityA}, {_TagB, QualityB}) ->
			QualityA > QualityB
		end, AcceptLanguages).

choose_language(Req, State, []) ->
	not_acceptable(Req, State);
choose_language(Req, State=#state{languages_p=LP}, [Language|Tail]) ->
	match_language(Req, State, Tail, LP, Language).

match_language(Req, State, Accept, [], _Language) ->
	choose_language(Req, State, Accept);
match_language(Req, State, _Accept, [Provided|_Tail], {'*', _Quality}) ->
	set_language(Req, State#state{language_a=Provided});
match_language(Req, State, _Accept, [Provided|_Tail], {Provided, _Quality}) ->
	set_language(Req, State#state{language_a=Provided});
match_language(Req, State, Accept, [Provided|Tail],
		Language = {Tag, _Quality}) ->
	Length = byte_size(Tag),
	case Provided of
		<< Tag:Length/binary, $-, _Any/bits >> ->
			set_language(Req, State#state{language_a=Provided});
		_Any ->
			match_language(Req, State, Accept, Tail, Language)
	end.

set_language(Req, State=#state{language_a=Language}) ->
	Req2 = cowboy_req:set_resp_header(<<"content-language">>, Language, Req),
	charsets_provided(Req2#{language => Language}, State).

%% charsets_provided should return a list of binary values indicating
%% which charsets are accepted by the resource.
charsets_provided(Req, State) ->
	case call(Req, State, charsets_provided) of
		no_call ->
			set_content_type(Req, State);
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		{[], Req2, HandlerState} ->
			not_acceptable(Req2, State#state{handler_state=HandlerState});
		{CP, Req2, HandlerState} ->
			State2 = State#state{handler_state=HandlerState, charsets_p=CP},
			case cowboy_req:parse_header(<<"accept-charset">>, Req2) of
				undefined ->
					set_content_type(Req2, State2#state{charset_a=hd(CP)});
				AcceptCharset ->
					AcceptCharset2 = prioritize_charsets(AcceptCharset),
					choose_charset(Req2, State2, AcceptCharset2)
			end
	end.

%% The special value "*", if present in the Accept-Charset field,
%% matches every character set (including ISO-8859-1) which is not
%% mentioned elsewhere in the Accept-Charset field. If no "*" is present
%% in an Accept-Charset field, then all character sets not explicitly
%% mentioned get a quality value of 0, except for ISO-8859-1, which gets
%% a quality value of 1 if not explicitly mentioned.
prioritize_charsets(AcceptCharsets) ->
	AcceptCharsets2 = lists:sort(
		fun ({_CharsetA, QualityA}, {_CharsetB, QualityB}) ->
			QualityA > QualityB
		end, AcceptCharsets),
	case lists:keymember(<<"*">>, 1, AcceptCharsets2) of
		true -> AcceptCharsets2;
		false ->
			case lists:keymember(<<"iso-8859-1">>, 1, AcceptCharsets2) of
				true -> AcceptCharsets2;
				false -> [{<<"iso-8859-1">>, 1000}|AcceptCharsets2]
			end
	end.

choose_charset(Req, State, []) ->
	not_acceptable(Req, State);
choose_charset(Req, State=#state{charsets_p=CP}, [Charset|Tail]) ->
	match_charset(Req, State, Tail, CP, Charset).

match_charset(Req, State, Accept, [], _Charset) ->
	choose_charset(Req, State, Accept);
match_charset(Req, State, _Accept, [Provided|_], {Provided, _}) ->
	set_content_type(Req, State#state{charset_a=Provided});
match_charset(Req, State, Accept, [_|Tail], Charset) ->
	match_charset(Req, State, Accept, Tail, Charset).

set_content_type(Req, State=#state{
		content_type_a={{Type, SubType, Params}, _Fun},
		charset_a=Charset}) ->
	ParamsBin = set_content_type_build_params(Params, []),
	ContentType = [Type, <<"/">>, SubType, ParamsBin],
	ContentType2 = case Charset of
		undefined -> ContentType;
		Charset -> [ContentType, <<"; charset=">>, Charset]
	end,
	Req2 = cowboy_req:set_resp_header(<<"content-type">>, ContentType2, Req),
	encodings_provided(Req2#{charset => Charset}, State).

set_content_type_build_params('*', []) ->
	<<>>;
set_content_type_build_params([], []) ->
	<<>>;
set_content_type_build_params([], Acc) ->
	lists:reverse(Acc);
set_content_type_build_params([{Attr, Value}|Tail], Acc) ->
	set_content_type_build_params(Tail, [[Attr, <<"=">>, Value], <<";">>|Acc]).

%% @todo Match for identity as we provide nothing else for now.
%% @todo Don't forget to set the Content-Encoding header when we reply a body
%% and the found encoding is something other than identity.
encodings_provided(Req, State) ->
	variances(Req, State).

not_acceptable(Req, State) ->
	respond(Req, State, 406).

%% variances/2 should return a list of headers that will be added
%% to the Vary response header. The Accept, Accept-Language,
%% Accept-Charset and Accept-Encoding headers do not need to be
%% specified.
%%
%% @todo Do Accept-Encoding too when we handle it.
%% @todo Does the order matter?
variances(Req, State=#state{content_types_p=CTP,
		languages_p=LP, charsets_p=CP}) ->
	Variances = case CTP of
		[] -> [];
		[_] -> [];
		[_|_] -> [<<"accept">>]
	end,
	Variances2 = case LP of
		[] -> Variances;
		[_] -> Variances;
		[_|_] -> [<<"accept-language">>|Variances]
	end,
	Variances3 = case CP of
		[] -> Variances2;
		[_] -> Variances2;
		[_|_] -> [<<"accept-charset">>|Variances2]
	end,
	try variances(Req, State, Variances3) of
		{Variances4, Req2, State2} ->
			case [[<<", ">>, V] || V <- Variances4] of
				[] ->
					resource_exists(Req2, State2);
				[[<<", ">>, H]|Variances5] ->
					Req3 = cowboy_req:set_resp_header(
						<<"vary">>, [H|Variances5], Req2),
					resource_exists(Req3, State2)
			end
	catch Class:Reason ->
		error_terminate(Req, State, Class, Reason)
	end.

variances(Req, State, Variances) ->
	case unsafe_call(Req, State, variances) of
		no_call ->
			{Variances, Req, State};
		{HandlerVariances, Req2, HandlerState} ->
			{Variances ++ HandlerVariances, Req2,
				State#state{handler_state=HandlerState}}
	end.

resource_exists(Req, State) ->
	expect(Req, State, resource_exists, true,
		fun if_match_exists/2, fun if_match_must_not_exist/2).

if_match_exists(Req, State) ->
	State2 = State#state{exists=true},
	case cowboy_req:parse_header(<<"if-match">>, Req) of
		undefined ->
			if_unmodified_since_exists(Req, State2);
		'*' ->
			if_unmodified_since_exists(Req, State2);
		ETagsList ->
			if_match(Req, State2, ETagsList)
	end.

if_match(Req, State, EtagsList) ->
	try generate_etag(Req, State) of
		%% Strong Etag comparison: weak Etag never matches.
		{{weak, _}, Req2, State2} ->
			precondition_failed(Req2, State2);
		{Etag, Req2, State2} ->
			case lists:member(Etag, EtagsList) of
				true -> if_none_match_exists(Req2, State2);
				%% Etag may be `undefined' which cannot be a member.
				false -> precondition_failed(Req2, State2)
			end
	catch Class:Reason ->
		error_terminate(Req, State, Class, Reason)
	end.

if_match_must_not_exist(Req, State) ->
	case cowboy_req:header(<<"if-match">>, Req) of
		undefined -> is_put_to_missing_resource(Req, State);
		_ -> precondition_failed(Req, State)
	end.

if_unmodified_since_exists(Req, State) ->
	try cowboy_req:parse_header(<<"if-unmodified-since">>, Req) of
		undefined ->
			if_none_match_exists(Req, State);
		IfUnmodifiedSince ->
			if_unmodified_since(Req, State, IfUnmodifiedSince)
	catch _:_ ->
		if_none_match_exists(Req, State)
	end.

%% If LastModified is the atom 'no_call', we continue.
if_unmodified_since(Req, State, IfUnmodifiedSince) ->
	try last_modified(Req, State) of
		{LastModified, Req2, State2} ->
			case LastModified > IfUnmodifiedSince of
				true -> precondition_failed(Req2, State2);
				false -> if_none_match_exists(Req2, State2)
			end
	catch Class:Reason ->
		error_terminate(Req, State, Class, Reason)
	end.

if_none_match_exists(Req, State) ->
	case cowboy_req:parse_header(<<"if-none-match">>, Req) of
		undefined ->
			if_modified_since_exists(Req, State);
		'*' ->
			precondition_is_head_get(Req, State);
		EtagsList ->
			if_none_match(Req, State, EtagsList)
	end.

if_none_match(Req, State, EtagsList) ->
	try generate_etag(Req, State) of
		{Etag, Req2, State2} ->
			case Etag of
				undefined ->
					precondition_failed(Req2, State2);
				Etag ->
					case is_weak_match(Etag, EtagsList) of
						true -> precondition_is_head_get(Req2, State2);
						false -> method(Req2, State2)
					end
			end
	catch Class:Reason ->
		error_terminate(Req, State, Class, Reason)
	end.

%% Weak Etag comparison: only check the opaque tag.
is_weak_match(_, []) ->
	false;
is_weak_match({_, Tag}, [{_, Tag}|_]) ->
	true;
is_weak_match(Etag, [_|Tail]) ->
	is_weak_match(Etag, Tail).

precondition_is_head_get(Req, State=#state{method=Method})
		when Method =:= <<"HEAD">>; Method =:= <<"GET">> ->
	not_modified(Req, State);
precondition_is_head_get(Req, State) ->
	precondition_failed(Req, State).

if_modified_since_exists(Req, State) ->
	try cowboy_req:parse_header(<<"if-modified-since">>, Req) of
		undefined ->
			method(Req, State);
		IfModifiedSince ->
			if_modified_since_now(Req, State, IfModifiedSince)
	catch _:_ ->
		method(Req, State)
	end.

if_modified_since_now(Req, State, IfModifiedSince) ->
	case IfModifiedSince > erlang:universaltime() of
		true -> method(Req, State);
		false -> if_modified_since(Req, State, IfModifiedSince)
	end.

if_modified_since(Req, State, IfModifiedSince) ->
	try last_modified(Req, State) of
		{undefined, Req2, State2} ->
			method(Req2, State2);
		{LastModified, Req2, State2} ->
			case LastModified > IfModifiedSince of
				true -> method(Req2, State2);
				false -> not_modified(Req2, State2)
			end
	catch Class:Reason ->
		error_terminate(Req, State, Class, Reason)
	end.

not_modified(Req, State) ->
	Req2 = cowboy_req:delete_resp_header(<<"content-type">>, Req),
	try set_resp_etag(Req2, State) of
		{Req3, State2} ->
			try set_resp_expires(Req3, State2) of
				{Req4, State3} ->
					respond(Req4, State3, 304)
			catch Class:Reason ->
				error_terminate(Req, State2, Class, Reason)
			end
	catch Class:Reason ->
		error_terminate(Req, State, Class, Reason)
	end.

precondition_failed(Req, State) ->
	respond(Req, State, 412).

is_put_to_missing_resource(Req, State=#state{method= <<"PUT">>}) ->
	moved_permanently(Req, State, fun is_conflict/2);
is_put_to_missing_resource(Req, State) ->
	previously_existed(Req, State).

%% moved_permanently/2 should return either false or {true, Location}
%% with Location the full new URI of the resource.
moved_permanently(Req, State, OnFalse) ->
	case call(Req, State, moved_permanently) of
		{{true, Location}, Req2, HandlerState} ->
			Req3 = cowboy_req:set_resp_header(
				<<"location">>, Location, Req2),
			respond(Req3, State#state{handler_state=HandlerState}, 301);
		{false, Req2, HandlerState} ->
			OnFalse(Req2, State#state{handler_state=HandlerState});
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		no_call ->
			OnFalse(Req, State)
	end.

previously_existed(Req, State) ->
	expect(Req, State, previously_existed, false,
		fun (R, S) -> is_post_to_missing_resource(R, S, 404) end,
		fun (R, S) -> moved_permanently(R, S, fun moved_temporarily/2) end).

%% moved_temporarily/2 should return either false or {true, Location}
%% with Location the full new URI of the resource.
moved_temporarily(Req, State) ->
	case call(Req, State, moved_temporarily) of
		{{true, Location}, Req2, HandlerState} ->
			Req3 = cowboy_req:set_resp_header(
				<<"location">>, Location, Req2),
			respond(Req3, State#state{handler_state=HandlerState}, 307);
		{false, Req2, HandlerState} ->
			is_post_to_missing_resource(Req2, State#state{handler_state=HandlerState}, 410);
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		no_call ->
			is_post_to_missing_resource(Req, State, 410)
	end.

is_post_to_missing_resource(Req, State=#state{method= <<"POST">>}, OnFalse) ->
	allow_missing_post(Req, State, OnFalse);
is_post_to_missing_resource(Req, State, OnFalse) ->
	respond(Req, State, OnFalse).

allow_missing_post(Req, State, OnFalse) ->
	expect(Req, State, allow_missing_post, true, fun accept_resource/2, OnFalse).

method(Req, State=#state{method= <<"DELETE">>}) ->
	delete_resource(Req, State);
method(Req, State=#state{method= <<"PUT">>}) ->
	is_conflict(Req, State);
method(Req, State=#state{method=Method})
		when Method =:= <<"POST">>; Method =:= <<"PATCH">> ->
	accept_resource(Req, State);
method(Req, State=#state{method=Method})
		when Method =:= <<"GET">>; Method =:= <<"HEAD">> ->
	set_resp_body_etag(Req, State);
method(Req, State) ->
	multiple_choices(Req, State).

%% delete_resource/2 should start deleting the resource and return.
delete_resource(Req, State) ->
	expect(Req, State, delete_resource, false, 500, fun delete_completed/2).

%% delete_completed/2 indicates whether the resource has been deleted yet.
delete_completed(Req, State) ->
	expect(Req, State, delete_completed, true, fun has_resp_body/2, 202).

is_conflict(Req, State) ->
	expect(Req, State, is_conflict, false, fun accept_resource/2, 409).

%% content_types_accepted should return a list of media types and their
%% associated callback functions in the same format as content_types_provided.
%%
%% The callback will then be called and is expected to process the content
%% pushed to the resource in the request body.
%%
%% content_types_accepted SHOULD return a different list
%% for each HTTP method.
accept_resource(Req, State) ->
	case call(Req, State, content_types_accepted) of
		no_call ->
			respond(Req, State, 415);
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		{CTA, Req2, HandlerState} ->
			CTA2 = [normalize_content_types(P) || P <- CTA],
			State2 = State#state{handler_state=HandlerState},
			try cowboy_req:parse_header(<<"content-type">>, Req2) of
				ContentType ->
					choose_content_type(Req2, State2, ContentType, CTA2)
			catch _:_ ->
				respond(Req2, State2, 415)
			end
	end.

%% The special content type '*' will always match. It can be used as a
%% catch-all content type for accepting any kind of request content.
%% Note that because it will always match, it should be the last of the
%% list of content types, otherwise it'll shadow the ones following.
choose_content_type(Req, State, _ContentType, []) ->
	respond(Req, State, 415);
choose_content_type(Req, State, ContentType, [{Accepted, Fun}|_Tail])
		when Accepted =:= '*'; Accepted =:= ContentType ->
	process_content_type(Req, State, Fun);
%% The special parameter '*' will always match any kind of content type
%% parameters.
%% Note that because it will always match, it should be the last of the
%% list for specific content type, otherwise it'll shadow the ones following.
choose_content_type(Req, State, {Type, SubType, Param},
		[{{Type, SubType, AcceptedParam}, Fun}|_Tail])
		when AcceptedParam =:= '*'; AcceptedParam =:= Param ->
	process_content_type(Req, State, Fun);
choose_content_type(Req, State, ContentType, [_Any|Tail]) ->
	choose_content_type(Req, State, ContentType, Tail).

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});
		{true, Req2, HandlerState2} when Exists ->
			State2 = State#state{handler_state=HandlerState2},
			next(Req2, State2, fun has_resp_body/2);
		{true, Req2, HandlerState2} ->
			State2 = State#state{handler_state=HandlerState2},
			next(Req2, State2, fun maybe_created/2);
		{false, Req2, HandlerState2} ->
			State2 = State#state{handler_state=HandlerState2},
			respond(Req2, State2, 400);
		{{true, ResURL}, Req2, HandlerState2} when Method =:= <<"POST">> ->
			State2 = State#state{handler_state=HandlerState2},
			Req3 = cowboy_req:set_resp_header(
				<<"location">>, ResURL, Req2),
			if
				Exists -> respond(Req3, State2, 303);
				true -> respond(Req3, State2, 201)
			end
	end catch Class:Reason = {case_clause, no_call} ->
		error_terminate(Req, State, Class, Reason)
	end.

%% If PUT was used then the resource has been created at the current URL.
%% Otherwise, if a location header has been set then the resource has been
%% created at a new URL. If not, send a 200 or 204 as expected from a
%% POST or PATCH request.
maybe_created(Req, State=#state{method= <<"PUT">>}) ->
	respond(Req, State, 201);
maybe_created(Req, State) ->
	case cowboy_req:has_resp_header(<<"location">>, Req) of
		true -> respond(Req, State, 201);
		false -> has_resp_body(Req, State)
	end.

has_resp_body(Req, State) ->
	case cowboy_req:has_resp_body(Req) of
		true -> multiple_choices(Req, State);
		false -> respond(Req, State, 204)
	end.

%% Set the Etag header if any for the response provided.
set_resp_body_etag(Req, State) ->
	try set_resp_etag(Req, State) of
		{Req2, State2} ->
			set_resp_body_last_modified(Req2, State2)
	catch Class:Reason ->
		error_terminate(Req, State, Class, Reason)
	end.

%% Set the Last-Modified header if any for the response provided.
set_resp_body_last_modified(Req, State) ->
	try last_modified(Req, State) of
		{LastModified, Req2, State2} ->
			case LastModified of
				LastModified when is_atom(LastModified) ->
					set_resp_body_expires(Req2, State2);
				LastModified ->
					LastModifiedBin = cowboy_clock:rfc1123(LastModified),
					Req3 = cowboy_req:set_resp_header(
						<<"last-modified">>, LastModifiedBin, Req2),
					set_resp_body_expires(Req3, State2)
			end
	catch Class:Reason ->
		error_terminate(Req, State, Class, Reason)
	end.

%% Set the Expires header if any for the response provided.
set_resp_body_expires(Req, State) ->
	try set_resp_expires(Req, State) of
		{Req2, State2} ->
			set_resp_body(Req2, State2)
	catch Class:Reason ->
		error_terminate(Req, State, Class, Reason)
	end.

%% Set the response headers and call the callback found using
%% content_types_provided/2 to obtain the request body and add
%% it to the response.
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});
		{Body, Req2, HandlerState2} ->
			State2 = State#state{handler_state=HandlerState2},
			Req3 = cowboy_req:set_resp_body(Body, Req2),
			multiple_choices(Req3, State2)
	end catch Class:Reason = {case_clause, no_call} ->
		error_terminate(Req, State, Class, Reason)
	end.

multiple_choices(Req, State) ->
	expect(Req, State, multiple_choices, false, 200, 300).

%% Response utility functions.

set_resp_etag(Req, State) ->
	{Etag, Req2, State2} = generate_etag(Req, State),
	case Etag of
		undefined ->
			{Req2, State2};
		Etag ->
			Req3 = cowboy_req:set_resp_header(
				<<"etag">>, encode_etag(Etag), Req2),
			{Req3, State2}
	end.

-spec encode_etag({strong | weak, binary()}) -> iolist().
encode_etag({strong, Etag}) -> [$",Etag,$"];
encode_etag({weak, Etag}) -> ["W/\"",Etag,$"].

set_resp_expires(Req, State) ->
	{Expires, Req2, State2} = expires(Req, State),
	case Expires of
		Expires when is_atom(Expires) ->
			{Req2, State2};
		Expires when is_binary(Expires) ->
			Req3 = cowboy_req:set_resp_header(
				<<"expires">>, Expires, Req2),
			{Req3, State2};
		Expires ->
			ExpiresBin = cowboy_clock:rfc1123(Expires),
			Req3 = cowboy_req:set_resp_header(
				<<"expires">>, ExpiresBin, Req2),
			{Req3, State2}
	end.

%% Info retrieval. No logic.

generate_etag(Req, State=#state{etag=no_call}) ->
	{undefined, Req, State};
generate_etag(Req, State=#state{etag=undefined}) ->
	case unsafe_call(Req, State, generate_etag) of
		no_call ->
			{undefined, Req, State#state{etag=no_call}};
		{Etag, Req2, HandlerState} when is_binary(Etag) ->
			Etag2 = cow_http_hd:parse_etag(Etag),
			{Etag2, Req2, State#state{handler_state=HandlerState, etag=Etag2}};
		{Etag, Req2, HandlerState} ->
			{Etag, Req2, State#state{handler_state=HandlerState, etag=Etag}}
	end;
generate_etag(Req, State=#state{etag=Etag}) ->
	{Etag, Req, State}.

last_modified(Req, State=#state{last_modified=no_call}) ->
	{undefined, Req, State};
last_modified(Req, State=#state{last_modified=undefined}) ->
	case unsafe_call(Req, State, last_modified) of
		no_call ->
			{undefined, Req, State#state{last_modified=no_call}};
		{LastModified, Req2, HandlerState} ->
			{LastModified, Req2, State#state{handler_state=HandlerState,
				last_modified=LastModified}}
	end;
last_modified(Req, State=#state{last_modified=LastModified}) ->
	{LastModified, Req, State}.

expires(Req, State=#state{expires=no_call}) ->
	{undefined, Req, State};
expires(Req, State=#state{expires=undefined}) ->
	case unsafe_call(Req, State, expires) of
		no_call ->
			{undefined, Req, State#state{expires=no_call}};
		{Expires, Req2, HandlerState} ->
			{Expires, Req2, State#state{handler_state=HandlerState,
				expires=Expires}}
	end;
expires(Req, State=#state{expires=Expires}) ->
	{Expires, Req, State}.

%% REST primitives.

expect(Req, State, Callback, Expected, OnTrue, OnFalse) ->
	case call(Req, State, Callback) of
		no_call ->
			next(Req, State, OnTrue);
		{stop, Req2, HandlerState} ->
			terminate(Req2, State#state{handler_state=HandlerState});
		{Expected, Req2, HandlerState} ->
			next(Req2, State#state{handler_state=HandlerState}, OnTrue);
		{_Unexpected, Req2, HandlerState} ->
			next(Req2, State#state{handler_state=HandlerState}, OnFalse)
	end.

call(Req, State=#state{handler=Handler, handler_state=HandlerState},
		Callback) ->
	case erlang:function_exported(Handler, Callback, 2) of
		true ->
			try
				Handler:Callback(Req, HandlerState)
			catch Class:Reason ->
				error_terminate(Req, State, Class, Reason)
			end;
		false ->
			no_call
	end.

unsafe_call(Req, #state{handler=Handler, handler_state=HandlerState},
		Callback) ->
	case erlang:function_exported(Handler, Callback, 2) of
		true -> Handler:Callback(Req, HandlerState);
		false -> no_call
	end.

next(Req, State, Next) when is_function(Next) ->
	Next(Req, State);
next(Req, State, StatusCode) when is_integer(StatusCode) ->
	respond(Req, State, StatusCode).

respond(Req, State, StatusCode) ->
	terminate(cowboy_req:reply(StatusCode, Req), State).

-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),
	erlang:raise(Class, Reason, erlang:get_stacktrace()).

terminate(Req, #state{handler=Handler, handler_state=HandlerState}) ->
	Result = cowboy_handler:terminate(normal, Req, HandlerState, Handler),
	{ok, Req, Result}.