puma socket leak application

Puma 2.0.1 right now have a socket leak problem. It leaks socket/fd when it's restarted.

This application is a minimum set to show how puma leaks its sockets on restart. Same thing should happen on file descriptors.

How to use this application

First, this program tries to connect to a MySQL server, so please get one or some. Write the MySQL connection information to config.ru, such like

CONN = {
  :host = "localhost",
  :user = "user",
  :password = "password"
}

and run:

$ bundle install --path=.bundle/gems --binstubs=.bundle/bin

don't forget the binstubs option.

and then run puma by:

$ bundle exec puma

or:

$ bundle exec puma --daemon

Access http://localhost:9292/ to see how many sockets are opened. Access http://localhost:9292/reload to send USR2 to the process.

What this application does

When starting, the application simply opens a MySQL connection by using mysql2 gem. Connection is kept in a class variable.

By accessing the server, you will see the sockets opened. This information is taken from /proc filesystem, so this application only works on Linux with procfs mounted (or those compatible to it).

Accessing to /reload will send -USR2 to the server process itself. You will be seeing the sockets increasing than before.

Why it leaks

Puma server implements restart by exec-ing its process into a new one. On exec, file descriptors are not wasted by default. If you want it wasted, you must set close-on-exec on the file descriptor.

Ruby provides us an easy way to close the file descriptors (those higher than 0,1,2) when exec-ing, by giving an option :close_others to Kernel#exec.

In ruby 2.0.0 or higher, file descriptors are created with close-on-exec flag by default. Therefore, this problem doesn't appear on ruby 2.0.0 or later.