Move advisory lock to it's own connection
This PR moves advisory lock to it's own connection instead of
`ActiveRecord::Base` to fix #37748. As a note the issue is present on
both mysql and postgres. We don't see it on sqlite3 because sqlite3
doesn't support advisory locks.
The underlying problem only appears if:
1) the app is using multiple databases, and therefore establishing a new
connetion in the abstract models
2) the app has a migration that loads a model (ex `Post.update_all`)
which causes that new connection to get established.
This is because when Rails runs migrations the default connections are
established, the lock is taken out on the `ActiveRecord::Base`
connection. When the migration that calls a model is loaded, a new
connection will be established and the lock will automatically be
released.
When Rails goes to release the lock in the ensure block it will find
that the connection has been closed. Even if the connection wasn't
closed the lock would no longer exist on that connection.
We originally considered checking if the connection was active, but
ultimately that would hide that the advisory locks weren't working
correctly because there'd be no lock to release.
We also considered making the lock more granular - that it only blocked
on each migration individually instead of all the migrations for that
connection. This might be the right move going forward, but right now
multi-db migrations that load models are very broken in Rails 6.0 and
master.
John and I don't love this fix, it requires a bit too much knowledge of
internals and how Rails picks up connections. However, it does fix the
issue, makes the lock more global, and makes the lock more resilient to
changing connections.
Co-authored-by: NJohn Crepezzi <john.crepezzi@gmail.com>
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