Warn when before_fork (and friends) called when running in single mode

[schneems]

Hello, beautiful people of Puma. I've seen a common problem relating to my docs for this feature https://devcenter.heroku.com/articles/language-runtime-metrics-ruby#debugging. I think it is affecting other puma users as well.

Problem

People will put some code in the before_fork block of their puma.rb but then accidentally boot their app in single mode instead of cluster mode. When this happens, before_fork never gets called.

Solution

My proposal is that when before_fork is set, but the app is booting in single mode, we emit a warning. Something like:

Warning: You specified code to run in a `before_fork` block, but Puma is configured to run without forking.

We could warn when code is specified in these configuration cases:

• after_worker_fork
• before_fork

Describe alternatives you've considered

Besides warning, we could make it configurable to error. Maybe conditionally based on PUMA_UNUSED_CONFIG=fail or something.

My only concern would be if it's too noisy (I don't know if people run single locally and clustered on prod or not). If that's the case we could possibly gate the feature behind an env var like PUMA_UNUSED_CONFIG=warn.

Additional context

You are all wonderful

[dentarg]

Warning sounds good to me, don't think we need/should make it fail (maybe you are just testing out the other mode temporary). If opt-in with PUMA_UNUSED_CONFIG=warn I don't think it will get much usage. We could make it possible to silence it, similar to silence_single_worker_warning.

[nateberkopec]

I don't know if people run single locally and clustered on prod or not

True, I have seen that often. Like Dentarg said, in that case we'd just make it opt-out.

[schneems]

I like it. To recap: Warn by default, no errors with an opt-out "escape valve" to disable the behavior.

We could either follow the silence_single_worker_warning pattern with a silence_fork_callback_warning or via env vars PUMA_UNUSED_CONFIG=disable in your bashrc (or something). Or something else.

[nateberkopec]

🤷 Why Not Both?