Fix deadlock issue in thread pool

[olivierbellone]

Description

Fix deadlock issue in thread pool.

This deadlock can happen when a thread is being trimmed, if the main server thread has already accepted a new incoming request and is waiting for the thread pool to not be full.

I was unable to write a test for this (race conditions are hard to reproduce!), but I was able to confirm this is a regression introduced in 5.0 by #2220. Previously, Puma did call not_full.signal when a thread was being trimmed: https://github.com/puma/puma/blob/v4.3.8/lib/puma/thread_pool.rb#L109.

I'm happy to add a test if anyone has suggestions for how to write one :)

Fixes #2655.

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added (or updated) appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.

[cjlarose]

@wjordan Any thoughts on if it'd be possible to write a test for this?

[olivierbellone]

@cjlarose Do you think there's any chance this could be merged and released this week? This bug is affecting our production servers and I'd much rather do a clean Puma upgrade rather than monkey patch my fix in. Totally understand if that doesn't work for you though.

[cjlarose]

I've reviewed the changes and this looks great. I'm comfortable merging without a regression test given the nature of the problem.

[cjlarose]

@olivierbellone I'm comfortable merging, but I can't guarantee a release this week. I'll bring it up with the team, but recall that you can always pull dependencies directly from git in your Gemfile.

[olivierbellone]

@cjlarose Thanks! Appreciate the quick merge. Using an untagged version is unfortunately not an option for us, but I'll figure something out.

[cjlarose]

@olivierbellone This change is available in Puma 5.4.0 now.

[olivierbellone]

Awesome! Thank you very much.