I have a system which needs to act as a proxy to another of our servers, mainly for legacy support reasons. With the help of Dancer as a proxy, I've managed to get it mostly working with the following code: 
  return send_file(
    \'ignored', streaming => 1, callbacks => {
      override => sub {
        eval {
          my $client_connection = shift;
          my $ua = LWP::UserAgent->new;
          my $client;

          my $status = $ua->get($real_url, ':content_cb' => sub {
            my ($data, $resp) = @_;

            unless ($client) {
              my $headers_in = $resp->headers;
              my %headers_out = (
                'Content-Disposition' =>
                  sprintf('inline; filename="%s"', $file->{fileName}),
              );
              for (qw( Content-Type Content-Length Keep-Alive Last-Mod
+ified )) {
                $headers_out{$_} = $headers_in->header($_)
                  if $headers_in->header($_);
              }
              $client = $client_connection->([$resp->code, [%headers_o
+ut]]);
            }

            $client->write($data);
          });

          if ($status->is_error) {
            my $headers_in = $status->headers;
            my %headers_out;
            for (qw( Content-Type Content-Length Keep-Alive Last-Modif
+ied )) {
              $headers_out{$_} = $headers_in->header($_)
                if $headers_in->header($_);
            }

            my $client = $client_connection->([$status->code, [%header
+s_out]]);
            $client->write($status->error_as_HTML);
          }
          1;
        } or warn "Proxy failure: $@";

        return;
      },
    },
  );

[download]
I say "mostly working" because, while the files are streamed successfully and browsers will accept them without complaint, wget and curl are less forgiving. Both of these command-line programs issue errors after the file is (successfully) received: 
$ curl -sS -o rcvd https://foo/bar.pdf
curl: (18) transfer closed with outstanding read data remaining
$ wget https://foo/bar.pdf
<...>
2017-01-12 14:48:58 (6.10 MB/s) - Read error at byte 3316868 (Success.
+).Retrying.
<proceeds to loop endlessly>

[download]
The byte at which wget reports the read error is always the last byte of the file (i.e., equal to the file size), leading me to suspect that an EOF marker isn't being handled properly. Possibly also relevant is that, while the proxying code copies the Content-Length header from the original source, the original source does not provide that header, so Content-Length is not actually set.

Using curl/wget to download the file directly from the original source works perfectly with no error messages issued.

Does anyone have any insights as to what the cause of the problem might be?

Edit: On further investigation, the problem does not appear to be with Dancer itself. Testing with Dancer's internal mini-server (using bin/app.pl) does not exhibit this problem. It only shows up in the production environment, which has Apache <-> Starman <-> Dancer. So now to work out whether the issue is with one of the other components individually or with the interactions between them.

In reply to EOF problem with Dancer streaming proxy by dsheroh

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.