Cowboy crashes when calling handling callback in REST flow

Question

I've got the following REST handler in an Erlang app using Cowboy 2.0 (latest RC). I've been reading the docs front to back and again, and I can't understand what's wrong with my code.

init(Req, State) ->
  {cowboy_rest, Req, State}.

content_types_provided(Req, State) ->
  error_logger:info_msg("Content negotiation~n"),
  {[
     {{<<"text">>, <<"html">>, '*'}, my_handler}
   ], Req, State}.

my_handler(Req, State) ->
  error_logger:info_msg("Got here~n"),      
  ...
  <handler logic>
  ...

And these are the logs. As you can see I get up to "Content negotiation", but don't reach the my_handler callback.

=INFO REPORT==== 31-Aug-2017::13:29:02 ===
Content negotiation

=CRASH REPORT==== 31-Aug-2017::13:29:02 ===
  crasher:
    initial call: cowboy_stream_h:proc_lib_hack/3
    pid: <0.258.0>
    registered_name: []
    exception exit: {{case_clause,no_call},
                     [{cowboy_rest,set_resp_body,2,
                          [{file,
                               "/app/_build/default/lib/cowboy/src/cowboy_rest.erl"},
                           {line,1019}]},
                      {cowboy_rest,upgrade,4,
                          [{file,
                               "/app/_build/default/lib/cowboy/src/cowboy_rest.erl"},
                           {line,238}]},
                      {cowboy_stream_h,execute,3,
                          [{file,
                               "/app/_build/default/lib/cowboy/src/cowboy_stream_h.erl"},
                           {line,179}]},
                      {cowboy_stream_h,proc_lib_hack,3,
                          [{file,
                               "/app/_build/default/lib/cowboy/src/cowboy_stream_h.erl"},
                           {line,164}]},
                      {proc_lib,init_p_do_apply,3,
                          [{file,"proc_lib.erl"},{line,247}]}]}
      in function  cowboy_stream_h:proc_lib_hack/3 (/app/_build/default/lib/cowboy/src/cowboy_stream_h.erl, line 169)
    ancestors: [<0.257.0>,<0.234.0>,<0.233.0>,ranch_sup,<0.222.0>]
    message_queue_len: 0
    messages: []
    links: [<0.257.0>]
    dictionary: []
    trap_exit: false
    status: running
    heap_size: 987
    stack_size: 27
    reductions: 596
  neighbours:

=ERROR REPORT==== 31-Aug-2017::13:29:02 ===
Ranch listener my_http_listener, connection process <0.257.0>, stream 1 had its request process <0.258.0> exit with reason {case_clause,no_call} and stacktrace [{cowboy_rest,set_resp_body,2,[{file,"/app/_build/default/lib/cowboy/src/cowboy_rest.erl"},{line,1019}]},{cowboy_rest,upgrade,4,[{file,"/app/_build/default/lib/cowboy/src/cowboy_rest.erl"},{line,238}]},{cowboy_stream_h,execute,3,[{file,"/app/_build/default/lib/cowboy/src/cowboy_stream_h.erl"},{line,179}]},{cowboy_stream_h,proc_lib_hack,3,[{file,"/app/_build/default/lib/cowboy/src/cowboy_stream_h.erl"},{line,164}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]

I'm not sure if there's any problem in the association of the content_types_provided/2 callback with the chosen handler function. I've written another REST handler which is very similar and it works correctly.

Also as a second question, is there a way in content_types_provided/2 to direct every request to the same handler, independent of the Accept: <type/sub-type> HTTP Header provided in the request?

Answer 1

Going through the cowboy_rest module code here, I think it's possible the error is generated because the function content_types_provided/2 is not exported.