Eric Bergen

To continue the pid file theme I’ve found another slight issue. This was unrelated to the testing which I found the previous pid file issues. I was working on an unmonitored development mysql system. While working on it I ran it out of disk space in /. The box has it’s mysql datadir in a separate partition which had plenty of space. The pid file is in a dir on /. When I started mysqld_safe mysqld exited because it couldn’t create the pid file. mysqld_safe continued to restart mysqld until I saw the problem and killed it a few minutes later. I’m not sure exactly why, I didn’t spend very much time digging into a failure that I caused by filling up the disk. mysqld was exiting because it was trying to create a pid file in a full partition.

Note: This was a stock mysqld, not one running my pid file patch.

Previously I wrote about how late the mysql pid file is created in the startup process. At first glance it seemed like a relatively easy thing to fix. In main() there is a call to start_signal_handler(). The first instance of static void start_signal_handler() does only one thing. It checks !opt_bootstrap to make sure mysqld isn’t being called by mysql_install_db. I’m not sure why mysql_install_db can’t have a pid file created but that’s getting outside the scope of my investigation. It seems simple enough to move the call to start_signal_handler() above the call to init_server_components() in main() and have the pidfile created earlier. It turns out pidfile creation happens differently on different arches.

For windows and netware start_signal_handler simply creates the pid file. For __EMX__ (I’m not sure what that is) start signal handler does nothing. By default start_signal_handler starts a signal handler thread. This thread then creates the pid file. I think this can be cleaned up by removing the start_signal_handler functions that either do nothing or only create a pid file and handle the pid file creation for arches that need it directly in main with some good self documenting ifdefs right around it.

I don’t have all the environments to test that this patch really works. I’ve tested it on linux and it does create a pid file and deletes it on shutdown. The pid file is created just after argument parsing and before the heavy weight storage engine initialization.

[Update 2009-12-07: I think the old patch broke embedded. I updated it to ifdef out the call to start_signal_handler]
Here is the patch Create pid file sooner patch.

mysql_safe is responsible for restarting mysqld if it crashes and will exit cleanly if mysqld was shutdown. The way it determines if mysqld shutdown correctly is if the pid file is cleaned up correctly. MySQL does quite a few things before creating the pid file like initializing storage engines. It can take quite a while to initialize innodb if it has to roll forward the transaction log. During this time if mysqld crashes mysqld_safe won’t restart it. Normally this is ok because the server would just crash again but it can mess with your head a bit if you’re testing changes to mysqld_safe. Especially if those changes involve what mysqld_safe does if mysqld crashes. I think it makes sense to create the pidfile earlier and there is a bug for it. Chime in on the bug if this has burned you.