[PATCH 4/5] proxy_pass: more descriptive error messages

[Eric Wong <e@80x24.org>]

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
---
 lib/yahns/proxy_http_response.rb | 8 ++++++--
 lib/yahns/req_res.rb             | 6 ++++--
 test/test_proxy_pass.rb          | 4 ++--
 3 files changed, 12 insertions(+), 6 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 @@ def proxy_err_response(code, req_res, exc)
       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 @@ def proxy_read_body(tip, kcar, req_res)
       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 @@ def proxy_read_trailers(kcar, req_res)
       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)
diff --git a/lib/yahns/req_res.rb b/lib/yahns/req_res.rb
index 9bb8f35..041b908 100644
--- a/lib/yahns/req_res.rb
+++ b/lib/yahns/req_res.rb
@@ -42,7 +42,8 @@ def yahns_step # yahns event loop entry point
           # continue looping in middle "case @resbuf" loop
         when :wait_readable
           return rv # spurious wakeup
-        when nil then return c.proxy_err_response(502, self, nil)
+        when nil
+          return c.proxy_err_response(502, self, 'upstream EOF (headers)')
         end # NOT looping here
 
       when String # continue reading trickled response headers from upstream
@@ -50,7 +51,8 @@ def yahns_step # yahns event loop entry point
         case rv = kgio_tryread(0x2000, buf)
         when String then res = req.headers(@hdr, resbuf << rv) and break
         when :wait_readable then return rv
-        when nil then return c.proxy_err_response(502, self, nil)
+        when nil
+          return c.proxy_err_response(502, self, 'upstream EOF (big headers)')
         end while true
         @resbuf = false
 
diff --git a/test/test_proxy_pass.rb b/test/test_proxy_pass.rb
index 5dd8058..4c4b53a 100644
--- a/test/test_proxy_pass.rb
+++ b/test/test_proxy_pass.rb
@@ -485,7 +485,7 @@ def check_truncated_upstream(host, port)
     assert_equal exp, res
     errs = File.readlines(@err.path).grep(/\bERROR\b/)
     assert_equal 1, errs.size
-    assert_match(/premature upstream EOF/, errs[0])
+    assert_match(/upstream EOF/, errs[0])
     @err.truncate(0)
 
     # truncated headers or no response at all...
@@ -501,7 +501,7 @@ def check_truncated_upstream(host, port)
       s.close
       errs = File.readlines(@err.path).grep(/\bERROR\b/)
       assert_equal 1, errs.size
-      assert_match(/premature upstream EOF/, errs[0])
+      assert_match(/upstream EOF/, errs[0])
       @err.truncate(0)
     end
   end