[99s-extend] Cowboy handler linked processes

[Loïc Hoguin]

I don't know what happens but there's two things I know:

  *  Handlers don't trap_exit, so if the linked process crashes, they 
crash too
  *  If the handler crashes, we close the connection and stop the 
handler; if not this is a bug

After your log message the handler should stop unless there's a bug 
somewhere.

On 07/18/2013 12:15 PM, Adrian Roe wrote:
> We have been using spawn_linked workers to handle tasks that live for
> the lifetime of a single HTTP request
>
> Although in the cowboy guide it is clear that Cowboy can use "One
> Process of Many Requests" I am surprised that this is the case even if
> the handler crashes.  For example, our use case is to copy a large file
> to the server over HTTP where a worker process relays the file contents
> to long term storage.  The worker process is spawn_linked from the HTTP
> handler and (for our use case) should die if the handler stops.
>
> If the client stops the upload (for example by browsing away, or losing
> connectivity) we correctly receive an error (see sample Lager trace
> below), but what we are seeing is that spawn_linked processes are NOT
> being killed.
>
> Is this intended behaviour - I accept it makes sense to reuse the
> processes but should this continue to be the case even if the previous
> use of the process crashed?  If it is intended behaviour I think the
> docs should highlight this as we've been leaking processes for some time
> now, but I've always seen it as erlang's job to look after related
> process trees in the event of error.  Our current workaround is to hold
> a list of linked processes in process storage and then kill them in the
> terminate handler which is ugly in the extreme!!  We don't know the PIDS
> of the linked processes until it is too late to return State to Cowboy
> (i.e. we are already in our handle code)...
>
> Kind regards
>
> Adrian
>
> 16:09:32.347 [info] Trailer upload failed with reason
> {case_clause,{error,closed}}
> 16:09:32.348 [error] ** Cowboy handler upload_trailer_resource
> terminating in handle/2
>     for the reason error:{case_clause,{error,closed}}
> ** Handler state was {state,undefined,0,undefined,undefined,undefined}
> ** Request was
> [{socket,#Port<0.11230>},{transport,ranch_tcp},{connection,keepalive},{pid,<0.1987.0>},{method,<<"POST">>},{version,'HTTP/1.1'},{peer,{{84,92,32,116},64136}},{host,<<"54.225.117.108">>},{host_info,undefined},{port,8000},{path,<<"/upload_trailer">>},{path_info,undef
> ined},{qs,<<"name=linux-7.4.21.zip&size=54015414">>},{qs_vals,undefined},{bindings,[]},{headers,[{<<"host">>,<<"54.225.117.108:8000">>},{<<"connection">>,<<"keep-alive">>},{<<"content-length">>,<<"54015414">>},{<<"origin">>,<<"http://54.225.117.108:8000">>},{<<"user-agent">>,<<"M
> ozilla/5.0 (Macintosh; Intel Mac OS X 10_8_4) AppleWebKit/537.36 (KHTML,
> like Gecko) Chrome/28.0.1500.71
> Safari/537.36">>},{<<"content-type">>,<<>>},{<<"accept">>,<<"*/*">>},{<<"referer">>,<<"http://54.225.117.108:8000/">>},{<<"accept-encoding">>,<<"gzip,deflate,sdch">>},{<<"acce
> pt-language">>,<<"en-US,en;q=0.8">>},{<<"cookie">>,<<"__jwpusr=cbc133d7-1b49-443c-8a13-364660cc93e5;
> id3as_manager=f4803c004d71dde3b64394f6e6f44faa54970e93">>}]},{p_headers,[{<<"connection">>,[<<"keep-alive">>]}]},{cookies,undefined},{meta,[]},{body_state,waiting},{multipart,unde
> fined},{buffer,<<>>},{resp_compress,true},{resp_state,waiting},{resp_headers,[]},{resp_body,<<>>},{onresponse,undefined}]
> ** Stacktrace:
> [{i_cowboy,stream_body,0,[{file,"src/i_cowboy.erl"},{line,76}]},{upload_trailer_resource,stream_upload_file,4,[{file,"src/endpoints/upload_trailer_resource.erl"},{line,247}]},{upload_trailer_resource,upload_file,1,[{file,"src/endpoints/upload_trailer_resource.erl"}
> ,{line,237}]},{upload_trailer_resource,head_or_post,1,[{file,"src/endpoints/upload_trailer_resource.erl"},{line,202}]},{upload_trailer_resource,sequence,2,[{file,"src/endpoints/upload_trailer_resource.erl"},{line,106}]},{upload_trailer_resource,process_request,1,[{file,"src/endpo
> ints/upload_trailer_resource.erl"},{line,212}]},{i_cowboy,do,3,[{file,"src/i_cowboy.erl"},{line,29}]},{cowboy_handler,handler_handle,4,[{file,"src/cowboy_handler.erl"},{line,119}]}]
>
>
> --
> Dr Adrian Roe
> Director
>
>
>
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-- 
Loïc Hoguin
Erlang Cowboy
Nine Nines
http://ninenines.eu