MySQL won’t start after Ubuntu 11.10 upgrade

I upgrade my Ubuntu database server from 10.10 to 11.04 to 11.10 and suddenly my MySQL Server stopped responding.

Turned out the problem was that I didn’t accept one of the new apparmor configurations since it looked minimal and I had customized mine. Because of this my /var/log/mysql/error.log was filled with the following:

111020 17:31:44 [Note] Plugin 'FEDERATED' is disabled.
111020 17:31:44  InnoDB: Initializing buffer pool, size = 896.0M
111020 17:31:44  InnoDB: Completed initialization of buffer pool
111020 17:31:44  InnoDB: Started; log sequence number 0 172373406
111020 17:31:44 [ERROR] Can't start server : Bind on unix socket: Permission denied
111020 17:31:44 [ERROR] Do you already have another mysqld server running on socket: /var/run/mysqld/mysqld.sock ?
111020 17:31:44 [ERROR] Aborting

I had to edit the app armor file for MySQL

sudo nano /etc/apparmor.d/usr.sbin.mysqld

changed

  /run/mysqld/mysqld.pid w,
  /run/mysqld/mysqld.sock w,

to

  /{,var/}run/mysqld/mysqld.pid w,
  /{,var/}run/mysqld/mysqld.sock w,