[99s-extend] Cowboy handler linked processes

[Loïc Hoguin]

If you got time sure, I won't have much time until Monday. Have fun!

On 07/18/2013 12:37 PM, Adrian Roe wrote:
> That would be perfect!  Do you want me to make the change and issue a
> pull request?
>
> --
> Dr Adrian Roe
> Director
>
> On Thursday, 18 July 2013 at 11:36, Loïc Hoguin wrote:
>
>> I don't think the problem is that the handler is reused, we don't reuse
>> them if there's an error. However we do catch errors to print them in
>> the logs, and then the process stops normally. If you link without
>> trap_exit you receive a normal exit signal which is ignored and doesn't
>> kill your process. I suppose we should throw an exit signal when we got
>> an error, after logging everything, instead of stopping normally.
>>
>> On 07/18/2013 12:31 PM, Adrian Roe wrote:
>>> My issue is the other way round. My handler crashes - and terminate
>>> gets called, but the linked process is NOT stopped (unless I stop it in
>>> terminate having stashed any processes I need to stop in the process
>>> dictionary - this is what I'm currently doing, but yuck!)
>>>
>>> . My question is whether it wouldn't be better to no re-use the handler
>>> process that has crashed and replace it so that handler's can use the
>>> canonical erlang way of stopping related processes rather than having to
>>> do it by hand.
>>>
>>> Obviously if the handler does not crash there's no need to kill the
>>> process, so the current efficiency saving works in the "normal" case/
>>>
>>> --
>>> Dr Adrian Roe
>>> Director
>>>
>>> On Thursday, 18 July 2013 at 11:20, Loïc Hoguin wrote:
>>>
>>>> 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
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Extend mailing list
>>>>> Extend at lists.ninenines.eu <mailto:Extend at lists.ninenines.eu>
>>>>> http://lists.ninenines.eu:81/listinfo/extend
>>>>
>>>>
>>>> --
>>>> Loïc Hoguin
>>>> Erlang Cowboy
>>>> Nine Nines
>>>> http://ninenines.eu
>>
>>
>> --
>> Loïc Hoguin
>> Erlang Cowboy
>> Nine Nines
>> http://ninenines.eu
>


-- 
Loïc Hoguin
Erlang Cowboy
Nine Nines
http://ninenines.eu