Hi Eric,

Since we upgraded from 4.6.2 to 4.7.0 we're regularly having issues
where one or more unicorn master processes would not upgrade
correctly, resulting in an (old) master process.

What I notice is the following: when upgrading with 4.6.2 the file
unicorn.pid is copied to unicorn.pid.oldbin and the unicorn.pid file
is updated (the (new) master process). After a while the worker pid
files are updated and the unicorn.pid.oldbin file disappears, and all
is fine.

When upgrading with 4.7.0 though the file unicorn.pid.oldbin is
created, but there is no unicorn.pid file. After a while (when the new
master process has finished initialising, and is ready to fork the
workers) the worker pid files are updated and a unicorn.pid file
appears, and then the unicorn.pid.oldbin file disappears.

So there is a relatively long period where there is no unicorn.pid file.

I think the problem for us is caused by monit, our process monitor,
which monitors the unicorn.pid file:

check process unicorn with pidfile
/srv/app.itrp-staging.com/shared/pids/unicorn.pid
  start program = "/etc/init.d/unicorn start"
  stop program = "/etc/init.d/unicorn stop"
  ...

Because the unicorn.pid file is absent for a relatively long period
monit will try to start unicorn, while an upgrade is in progress.
(which might be another issue: the start action should recognise a
start or upgrade process is already underway; sadly this check too
relies on the existence of the unicorn.pid file)

I think the way 4.6.2 worked is better. There should be a pid file for
the new master process the moment it's created.

What do you think?

-- 
Jim
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