Support for server-side trailer headers with chunk-encoded responses

[clemep-sios]

I am using Starman as the backend HTTP server for a PSGI app, and need to use a Trailer header, e.g.:

<Response Headers>
Trailer: X-Payload-Signature
<Response Body>
X-Payload-Signature: 330cce9978d4682f

Right now, Starman does not support trailer headers (https://tools.ietf.org/html/rfc7230#section-4.1.2) with chunk-encoded server responses.

Would there be any interest in a simple addition to support this?

A very rough idea for a patch (minimally tested, so far):

diff --git a/lib/Starman/Server.pm b/lib/Starman/Server.pm
index c2c6d77..468a778 100644
--- a/lib/Starman/Server.pm
+++ b/lib/Starman/Server.pm
@@ -550,8 +550,17 @@ sub _finalize_response {
                 }
                 _syswrite($conn, \$buffer);
             },
+            trailer => sub {
+                $self->{trailer} = shift;
+            },
             close => sub {
-                _syswrite($conn, \"0$CRLF$CRLF") if $chunked;
+                if ($chunked) {
+                    _syswrite($conn, \"0$CRLF");
+                    if (my $trailer = delete $self->{trailer}) {
+                        _syswrite($conn, \"$trailer$CRLF");
+                    }
+                    _syswrite($conn, \"$CRLF");
+                }
             };
     }
 }

I will work on a PR and unit tests, if there is interest.