Don't replace connections without a writing role

[eugeneius]

Followup to #40368, #40487, #40488; ultimately trying to unblock #40384.

If there's no corresponding writing connection, we currently overwrite the pool anyway, removing the connection. We should skip it instead.

[eileencodes]

So Rails kind of requires that there's a writing role, at least how it's configured currently. If you don't include one and boot a console, and try to query the db, you'll get an error because the default is to lookup on the writing role. I think this will be easier to change with granular connection management, but I haven't gotten around to doing that. Description of the problem is here #39580

[eugeneius]

Thanks for the context, I hadn't seen #39580.

My actual goal here is #40384, so that I can write tests for some code in our application that switches connections manually. Since that requires pool sharing to be reversible, I'm trying to understand all of the edge cases so that I'm confident they'll reverse correctly.

It's possible to write production code that works fine with no writing connection, as long as you wrap every query in a connected_to block, so I thought it was worthwhile to make that code behave the same during transactional tests. If you'd prefer not to add code to handle this case though, since it's not really supported, I'm fine with ignoring it.

[rails-bot]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
Thank you for your contributions.

[eileencodes]

I'm going to close this one because I prefer not to handle this case. If it's still problematic / an issue in your app let me know and we can talk about fixing this.