proxy_pass: more descriptive error messages

This should make it easier to figure out where certain
errors are coming from and perhaps fix problems with
upstreams, too.

This helped me track down the problem causing public-inbox WWW
component running under Perl v5.20.2 on my Debian jessie system
to break and drop connections going through
Plack::Middleware::Deflater with gzip:

  https://public-inbox.org/meta/20160607071401.29325-1-e@80x24.org/

Perl 5.14.2 on Debian wheezy did not detect this problem :x

1 files changed, 6 insertions, 2 deletions

diff --git a/lib/yahns/proxy_http_response.rb b/lib/yahns/proxy_http_response.rb
index 8de5b4f..9867da2 100644
--- a/lib/yahns/proxy_http_response.rb
+++ b/lib/yahns/proxy_http_response.rb
@@ -53,6 +53,8 @@ module Yahns::HttpResponse # :nodoc:
       logger.error('premature upstream EOF')
     when Kcar::ParserError
       logger.error("upstream response error: #{exc.message}")
+    when String
+      logger.error(exc)
     else
       Yahns::Log.exception(logger, 'upstream error', exc)
     end
@@ -167,7 +169,9 @@ module Yahns::HttpResponse # :nodoc:
       return proxy_unbuffer(wbuf) if Yahns::WbufLite === wbuf
     when nil # EOF
       # HTTP/1.1 upstream, unexpected premature EOF:
-      return proxy_err_response(nil, req_res, nil) if len || chunk
+      msg = "upstream EOF (#{len} bytes left)" if len
+      msg = 'upstream EOF (chunk)' if chunk
+      return proxy_err_response(nil, req_res, msg) if msg
 
       # HTTP/1.0 upstream:
       wbuf = proxy_write(wbuf, "0\r\n\r\n".freeze, req_res) if alive
@@ -198,7 +202,7 @@ module Yahns::HttpResponse # :nodoc:
       when :wait_readable
         return wait_on_upstream(req_res)
       when nil # premature EOF
-        return proxy_err_response(nil, req_res, nil)
+        return proxy_err_response(nil, req_res, 'upstream EOF (trailers)')
       end # no loop here
     end
     wbuf = proxy_write(wbuf, trailer_out(tlr), req_res)