MRI 1.8 thread fix to avoid blocking accept()

Rainbows! commit ee7fe220ccbc991e1e7cbe982caf48e3303274c7

Under MRI 1.8, listen sockets do not appear to have the
nonblocking I/O flag on by default, nor does it set the
nonblocking I/O flag when calling #accept (but it does
when using #accept_nonblock, of course).

Normally this is not a problem even when using green threads
since MRI will internally select(2) on the file descriptor
before attempting a blocking (and immediately successful)
accept(2).

However, when sharing a listen descriptor across multiple
processes, spurious wakeups are likely to occur, causing
multiple processes may be woken up when a single client
connects.

This causes a problem because accept(2)-ing on multiple
threads/processes for a single connection causes blocking accepts in
multiple processes, leading to stalled green threads.

This is not an issue under 1.9 where a blocking accept() call
unlocks the GVL to let other threads run.

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/zbatery.rb b/lib/zbatery.rb
index ce542d6..b2eb7ad 100644
--- a/lib/zbatery.rb
+++ b/lib/zbatery.rb
@@ -127,6 +127,13 @@ module Rainbows
       after_fork.call(self, worker)
       worker.user(*user) if user.kind_of?(Array) && ! worker.switched
       build_app! unless preload_app
+
+      # avoid spurious wakeups and blocking-accept() with 1.8 green threads
+      if RUBY_VERSION.to_f < 1.9
+        require "io/nonblock"
+        HttpServer::LISTENERS.each { |l| l.nonblock = true }
+      end
+
       logger.info "Zbatery #@use worker_connections=#@worker_connections"
     end
   end