Always try to send a valid HTTP response back

This reworks error handling throughout the entire stack to be
more Ruby-ish.  Exceptions are raised instead of forcing the
us to check return values.

  If a client is sending us a bad request, we send a 400.

  If unicorn or app breaks in an unexpected way, we'll
  send a 500.

Both of these last-resort error responses are sent using
IO#write_nonblock to avoid tying Unicorn up longer than
necessary and all exceptions raised are ignored.

Sending a valid HTTP response back should reduce the chance of
us from being marked as down or broken by a load balancer.
Previously, some load balancers would mark us as down if we close
a socket without sending back a valid response; so make a best
effort to send one.  If for some reason we cannot write a valid
response, we're still susceptible to being marked as down.

A successful HttpResponse.write() call will now close the socket
immediately (instead of doing it higher up the stack).  This
ensures the errors will never get written to the socket on a
successful response.

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/unicorn/const.rb b/lib/unicorn/const.rb
index 4e78171..f2143bf 100644
--- a/lib/unicorn/const.rb
+++ b/lib/unicorn/const.rb
@@ -79,6 +79,10 @@ module Unicorn
     # Maximum request body size before it is moved out of memory and into a tempfile for reading.
     MAX_BODY=MAX_HEADER
 
+    # common errors we'll send back
+    ERROR_400_RESPONSE = "HTTP/1.1 400 Bad Request\r\n\r\n".freeze
+    ERROR_500_RESPONSE = "HTTP/1.1 500 Internal Server Error\r\n\r\n".freeze
+
     # A frozen format for this is about 15% faster
     CONTENT_LENGTH="CONTENT_LENGTH".freeze
     REMOTE_ADDR="REMOTE_ADDR".freeze