Ensure Puma runs in 0:1 configuration

[maschwenk]

Summary

Ensure Puma always runs in 0:1 configuration when running feature specs.

Other Information

#28083
#30712
#30638

It seems at some point Rails was told to point to the default Puma server config baked into Capybara, but that never enforced the required 0:1 Puma thread config that's necessary for transactional fixtures.

#30638 (comment)

This comment above gets at the issue.

@eileencodes and @twalpole have context here, @eileencodes I see you have done a lot of work since then on DB connection sharing for Rails 6. Unsure what the correct configuration is now.

[twalpole]

I'm unclear why 0:1 is required for this. No one I know has had issues running with the default setup here since the DB connection is shared between the threads. If you're modifying the actual DB connection in one of your threads then you're going to have issues even with 0:1 because you're still sharing the DB connection between the app thread and the test thread, and probably shouldn't be using DB connection sharing at all.

[maschwenk]

@twalpole Fair point. It was introduced here I believe. I had assumed that was a technical choice rather than a sensible default. If it was not indeed a technical choice, then I'll close this PR. I did notice that when my app runs:

1. 0:1 we get a lot of flakiness in specs
2. 0:4 (the capybara default) we get a bunch of PG::ProtocolViolation: ERROR: bind message supplies 0 parameters, but prepared statement "" requires 5

This is on Ruby 2.5.8, Rails 5.2.4.3, Capybara 3.24.0. Obviously that's a secondary issue, but just noting.

[eileencodes]

This change isn't correct, however I looked for your specific exception in the Rails issues tracker and got this #36763.

Looks like this was fixed in master and 6.0 in #36949

[maschwenk]

@eileencodes 👍🏼 and I appreciate you pointing to the bugfix. Closing, apologies for confusion.