[erlang-questions] cowboy_static / file:sendfile

[Steve Strong]

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)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)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)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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