Unicorn in the jail

From: Tatsuya Ono <ononoma@gmail.com>
To: mongrel-unicorn <mongrel-unicorn@rubyforge.org>
Subject: Unicorn in the jail
Date: Wed, 3 Aug 2011 17:41:03 +0100
Message-ID: <CAHBuKRj09FdxAgzsefJWotexw-7JYZGJMtgUp_dhjPz9VbKD6Q@mail.gmail.com> (raw)

I ran into a problem when sending USR2 signal to master process which
is listening a TCP port. It shows the following error in log.

err: adding listener failed addr=0.0.0.0:3010 (in use)

I found similar error reported on this mailing list and yes, the host
I got the error was in the FreeBSD jail.
http://rubyforge.org/pipermail/mongrel-unicorn/2009-December/000205.html
http://rubyforge.org/pipermail/mongrel-unicorn/2009-December/000212.html

I dug into the problem and found some interesting thing. I want to
share it and the solution here.

* Why does it happen in the Jail?
When you bind 0.0.0.0 address to the socket on a Jail host, Jail
doesn't bind 0.0.0.0. Instead of it, it binds to the IP address of the
host such as 10.100.1.50, 192.168.1.50, etc.
Then the problem happens in Unicorn::HttpServer#inherit_listeners!
when it gets signal like USR2.
https://github.com/defunkt/unicorn/blob/406b8b0e2ed6e5be34d8ec3cd4b16048233c2856/lib/unicorn/http_server.rb#L703-731

Here you get 10.100.1.50 (for example) as inherited address from
ENV['UNICORN_FD'] but 0.0.0.0. So Unicorn tries to bind to 0.0.0.0
(again) at line 728. Then it fails because it is actually in use.

* How to solve the issue?
Thankfully unicorn.conf is ruby script so that I could write a logic
for Jail hosts without specifying IP address of those hosts (which we
want to avoid since we deploy it multiple servers). I share the
configuration file.
https://gist.github.com/1122965

Then Unicorn flies in the jail happily.

Tatsuya Ono
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