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<H1>[99s-extend] Cowboy handler linked processes</H1>
<B>Loïc Hoguin</B>
<A HREF="mailto:extend%40lists.ninenines.eu?Subject=Re%3A%20%5B99s-extend%5D%20Cowboy%20handler%20linked%20processes&In-Reply-To=%3C51E7C174.2030501%40ninenines.eu%3E"
TITLE="[99s-extend] Cowboy handler linked processes">essen at ninenines.eu
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<I>Thu Jul 18 12:20:36 CEST 2013</I>
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<PRE>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:
><i> We have been using spawn_linked workers to handle tasks that live for
</I>><i> the lifetime of a single HTTP request
</I>><i>
</I>><i> Although in the cowboy guide it is clear that Cowboy can use "One
</I>><i> Process of Many Requests" I am surprised that this is the case even if
</I>><i> the handler crashes. For example, our use case is to copy a large file
</I>><i> to the server over HTTP where a worker process relays the file contents
</I>><i> to long term storage. The worker process is spawn_linked from the HTTP
</I>><i> handler and (for our use case) should die if the handler stops.
</I>><i>
</I>><i> If the client stops the upload (for example by browsing away, or losing
</I>><i> connectivity) we correctly receive an error (see sample Lager trace
</I>><i> below), but what we are seeing is that spawn_linked processes are NOT
</I>><i> being killed.
</I>><i>
</I>><i> Is this intended behaviour - I accept it makes sense to reuse the
</I>><i> processes but should this continue to be the case even if the previous
</I>><i> use of the process crashed? If it is intended behaviour I think the
</I>><i> docs should highlight this as we've been leaking processes for some time
</I>><i> now, but I've always seen it as erlang's job to look after related
</I>><i> process trees in the event of error. Our current workaround is to hold
</I>><i> a list of linked processes in process storage and then kill them in the
</I>><i> terminate handler which is ugly in the extreme!! We don't know the PIDS
</I>><i> of the linked processes until it is too late to return State to Cowboy
</I>><i> (i.e. we are already in our handle code)...
</I>><i>
</I>><i> Kind regards
</I>><i>
</I>><i> Adrian
</I>><i>
</I>><i> 16:09:32.347 [info] Trailer upload failed with reason
</I>><i> {case_clause,{error,closed}}
</I>><i> 16:09:32.348 [error] ** Cowboy handler upload_trailer_resource
</I>><i> terminating in handle/2
</I>><i> for the reason error:{case_clause,{error,closed}}
</I>><i> ** Handler state was {state,undefined,0,undefined,undefined,undefined}
</I>><i> ** Request was
</I>><i> [{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
</I>><i> 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">>,<<"<A HREF="http://54.225.117.108:8000">http://54.225.117.108:8000</A>">>},{<<"user-agent">>,<<"M
</I>><i> ozilla/5.0 (Macintosh; Intel Mac OS X 10_8_4) AppleWebKit/537.36 (KHTML,
</I>><i> like Gecko) Chrome/28.0.1500.71
</I>><i> Safari/537.36">>},{<<"content-type">>,<<>>},{<<"accept">>,<<"*/*">>},{<<"referer">>,<<"<A HREF="http://54.225.117.108:8000/">http://54.225.117.108:8000/</A>">>},{<<"accept-encoding">>,<<"gzip,deflate,sdch">>},{<<"acce
</I>><i> pt-language">>,<<"en-US,en;q=0.8">>},{<<"cookie">>,<<"__jwpusr=cbc133d7-1b49-443c-8a13-364660cc93e5;
</I>><i> id3as_manager=f4803c004d71dde3b64394f6e6f44faa54970e93">>}]},{p_headers,[{<<"connection">>,[<<"keep-alive">>]}]},{cookies,undefined},{meta,[]},{body_state,waiting},{multipart,unde
</I>><i> fined},{buffer,<<>>},{resp_compress,true},{resp_state,waiting},{resp_headers,[]},{resp_body,<<>>},{onresponse,undefined}]
</I>><i> ** Stacktrace:
</I>><i> [{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"}
</I>><i> ,{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
</I>><i> 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}]}]
</I>><i>
</I>><i>
</I>><i> --
</I>><i> Dr Adrian Roe
</I>><i> Director
</I>><i>
</I>><i>
</I>><i>
</I>><i> _______________________________________________
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</I>><i>
</I>
--
Loïc Hoguin
Erlang Cowboy
Nine Nines
<A HREF="http://ninenines.eu">http://ninenines.eu</A>
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