[erlang-questions] problems with cowboy hello world

Garry Hodgson <>
Thu Mar 12 05:18:39 CET 2015


i'm trying to port a webmachine application to cowboy,
with puzzling results so far. i've got all the plumbing set
up, and my handlers get called. i actually need to use
cowboy_rest_handler, but am trying first to get basic http
working. so i copied the example from the User Guide:

-module( hello_api ).

-export( [ init/3, terminate/3 ] ).

init( Transport, Req, Opts ) ->
     Reply = cowboy_req:reply(
                                 200,
                                 [ { <<"content-type">>, 
<<"text/plain">> } ],
                                 <<"Hello world!">>,
                                 Req
                             ),
     log:debug( "hello:init(): Req is ~p", [ Req ] ),
     { ok, Reply, Opts }.

terminate( Reason, Req, State) ->
     log:debug( "terminating for ~p", [ Reason ] ),
     ok.

and when i hit the server with:

--> curl k2v/v0/hello -v
* About to connect() to k2v port 80 (#0)
*   Trying 135.207.243.29... connected
* Connected to k2v (135.207.243.29) port 80 (#0)
 > GET /v0/hello HTTP/1.1
 > User-Agent: curl/7.19.7 (x86_64-redhat-linux-gnu) libcurl/7.19.7 
NSS/3.15.3 zlib/1.2.3 libidn/1.18 libssh2/1.4.2
 > Host: k2v
 > Accept: */*
 >
< HTTP/1.1 200 OK
< connection: keep-alive
< server: Cowboy
< date: Thu, 12 Mar 2015 03:52:39 GMT
< content-length: 12
< content-type: text/plain
<
* Connection #0 to host k2v left intact
Hello world!
* Closing connection #0


it works in that i get my response, but terminate() never gets called,
and i see this error report in erlang shell (included debug to show the 
req):

=INFO REPORT==== 11-Mar-2015::23:52:40 ===
{log,debug,
      "hello:init(): Req is 
{http_req,#Port<0.3094>,ranch_tcp,keepalive,<0.297.0>,\n 
<<\"GET\">>,'HTTP/1.1',\n {{135,207,243,29},36394},\n 
<<\"k2v\">>,undefined,80,<<\"/v0/hello\">>,\n 
undefined,<<>>,undefined,[],\n [{<<\"user-agent\">>,\n <<\"curl/7.19.7 
(x86_64-redhat-linux-gnu) libcurl/7.19.7 NSS/3.15.3 zlib/1.2.3 
libidn/1.18 libssh2/1.4.2\">>},\n {<<\"host\">>,<<\"k2v\">>},\n 
{<<\"accept\">>,<<\"*/*\">>}],\n 
[],undefined,[],waiting,<<>>,undefined,false,\n waiting,[],<<>>,undefined}",
      }

=ERROR REPORT==== 11-Mar-2015::23:52:40 ===
Ranch listener cowboy_http_server had connection process started with 
cowboy_protocol:start_link/4 at <0.297.0> exit with reason: 
{function_clause,[{lists,zip,[[transport,connection,pid,method,version,peer,host,host_info,port,path,path_info,qs,qs_vals,bindings,headers,p_headers,cookies,meta,body_state,buffer,multipart,resp_compress,resp_state,resp_headers,resp_body,onresponse],[]],[{file,"lists.erl"},{line,385}]},{lists,zip,2,[{file,"lists.erl"},{line,385}]},{cowboy_handler,handler_terminate,4,[{file,"src/cowboy_handler.erl"},{line,291}]},{cowboy_handler,handler_handle,4,[{file,"src/cowboy_handler.erl"},{line,118}]},{cowboy_protocol,execute,4,[{file,"src/cowboy_protocol.erl"},{line,442}]}]}

the error seems to be because cowboy_handler:handler_terminate() is
calling cowboy_req:to_list(Req) in this catch clause:

handler_terminate(Req, Handler, HandlerState, Reason) ->
     try
         Handler:terminate(Reason, cowboy_req:lock(Req), HandlerState)
     catch Class:Reason2 ->
         erlang:Class([
             {reason, Reason2},
             {mfa, {Handler, terminate, 3}},
             {stacktrace, erlang:get_stacktrace()},
             {req, cowboy_req:to_list(Req)},
             {state, HandlerState},
             {terminate_reason, Reason}
         ])
     end.

where cowboy_req:to_list(Req) is:

to_list(Req) ->
     lists:zip(record_info(fields, http_req), tl(tuple_to_list(Req))).

and that second expression yields [].

So, am i doing something wrong here? Has the api changed?
The documentation and examples don't seem to agree with
the code, let alone whatever questions/tutorials/etc i find
when searching online. I gather the api's are in flux
( init/3 vs. init/2 and rest_init/2, { upgrade, protocol, cowboy_rest }
vs { cowboy_rest, Req, #state{} } ), so i'm not sure what
code i should be running. i grabbed the latest using erlang.mk bootstrap
just a few days ago.

any suggestions would be greatly appreciated.

thanks.


-- 
Garry Hodgson
AT&T Chief Security Office (CSO)

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