Begin writing HTTP request headers early to detect disconnected clients

This patch checks incoming connections and avoids calling the application
if the connection has been closed.

It works by sending the beginning of the HTTP response before calling
the application to see if the socket can successfully be written to.

By enabling this feature users can avoid wasting application rendering
time only to find the connection is closed when attempting to write, and
throwing out the result.

When a client disconnects while being queued or processed, Nginx will log
HTTP response 499 but the application will log a 200.

Enabling this feature will minimize the time window during which the problem
can arise.

The feature is disabled by default and can be enabled by adding
'check_client_connection true' to the unicorn config.

[ew: After testing this change, Tom Burns wrote:

  So we just finished the US Black Friday / Cyber Monday weekend running
  unicorn forked with the last version of the patch I had sent you.  It
  worked splendidly and helped us handle huge flash sales without
  increased response time over the weekend.

  Whereas in previous flash traffic scenarios we would see the number of
  HTTP 499 responses grow past the number of real HTTP 200 responses,
  over the weekend we saw no growth in 499s during flash sales.

  Unexpectedly the patch also helped us ward off a DoS attack where the
  attackers were disconnecting immediately after making a request.

ref: <CAK4qKG3rkfVYLyeqEqQyuNEh_nZ8yw0X_cwTxJfJ+TOU+y8F+w@mail.gmail.com>
]

Signed-off-by: Eric Wong <normalperson@yhbt.net>

1 files changed, 6 insertions, 0 deletions

diff --git a/examples/unicorn.conf.rb b/examples/unicorn.conf.rb
index 0238043..1f4c9c0 100644
--- a/examples/unicorn.conf.rb
+++ b/examples/unicorn.conf.rb
@@ -46,6 +46,12 @@ preload_app true
 GC.respond_to?(:copy_on_write_friendly=) and
   GC.copy_on_write_friendly = true
 
+# Enable this flag to have unicorn test client connections by writing the
+# beginning of the HTTP headers before calling the application.  This
+# prevents calling the application for connections that have disconnected
+# while queued.
+check_client_connection false
+
 before_fork do |server, worker|
   # the following is highly recomended for Rails + "preload_app true"
   # as there's no need for the master process to hold a connection