[erlang-questions] cowboy_static / file:sendfile

[Loïc Hoguin]

Hello,

This might have been fixed in OTP-21.2:

   OTP-15461    Application(s): erts
                Related Id(s): ERL-784

                Fixed bug in file:sendfile when the send operation
                failed. For sockets in active modes it could cause
                emulator crash or a hanging call. For sockets with
                {active,false} an unexpected {inet_reply, _, _} message
                could be sent to the calling process. The bug exists
                since OTP-21.0.

On 12/12/18 4:40 PM, Steve Strong wrote:
> Hi,
> 
> I’m seeing a potential issue with file:sendfile if the client shuts down 
> the connection.  My setup is a cowboy server with a simple cowboy_static 
> endpoint, and a rate-limited curl as the client - if I start the curl 
> download and then ctrl-c curl before it completes, cowboy sits hung 
> inside the call to sendfile.
> 
> I inspected the cowboy process as follows:
> 
> (file_encoder@REDACTED <mailto:file_encoder@REDACTED>)83> P = processes().
> [<0.0.0>,<0.1.0>,<0.2.0>,<0.3.0>,<0.4.0>,<0.5.0>,<0.8.0>,
>   <0.40.0>,<0.42.0>,<0.44.0>,<0.45.0>,<0.47.0>,<0.48.0>,
>   <0.50.0>,<0.51.0>,<0.52.0>,<0.53.0>,<0.54.0>,<0.55.0>,
>   <0.56.0>,<0.57.0>,<0.58.0>,<0.59.0>,<0.60.0>,<0.61.0>,
>   <0.62.0>,<0.63.0>,<0.64.0>,<0.65.0>|…]
> 
> %% Run curl and ctrl-c in a separate window…
> 
> (file_encoder@REDACTED <mailto:file_encoder@REDACTED>)84> 
> erlang:process_info(hd(processes() -- P), [current_function, messages]).
> [{current_function,{prim_inet,sendfile_1,4}},
>   {messages,[{'EXIT',<0.1798.0>,normal},
>              {tcp_closed,#Port<0.266>},
>              {inet_reply,#Port<0.266>,{error,closed}}]}]
> 
> 
> This process looks doomed to live forever - it is stuck in 
> prim_inet:sendfile_1 on the following receive:
> 
>        receive
>          {sendfile, S, {ok, SentLow, SentHigh}} ->
>            {ok, SentLow bor (SentHigh bsl 32)};
>          {sendfile, S, {error, Reason}} ->
>            {error, Reason};
>          {'EXIT', S, _Reason} ->
>            {error, closed}
>        end
> 
> The port that it had opened, 0.266, has closed so there will never be an 
> exit message:
> 
> (file_encoder@REDACTED <mailto:file_encoder@REDACTED>)89> erlang:ports().
> [#Port<0.0>,#Port<0.2>,#Port<0.4>,#Port<0.5>,#Port<0.6>,
>   #Port<0.8>,#Port<0.10>,#Port<0.11>,#Port<0.12>,#Port<0.13>,
>   #Port<0.14>,#Port<0.15>,#Port<0.16>,#Port<0.254>,
>   #Port<0.255>,#Port<0.256>,#Port<0.257>,#Port<0.258>,
>   #Port<0.259>,#Port<0.260>,#Port<0.261>,#Port<0.262>,
>   #Port<0.263>,#Port<0.264>]
> 
> I’m guessing the process wasn’t linked with the port, or it would have 
> received the exit message and all would have been well.  Unsure if this 
> is a cowboy or prim_inet bug, or indeed if I’ve done something wrong. 
>   Any ideas?
> 
> Cheers,
> 
> Steve
> 
> 
> 
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-- 
Loïc Hoguin
https://ninenines.eu