Fix out_of_band hook never executed when multiple worker threads (clo…

[GuiTeK]

Description

Alternative solution to this PR: #2180
I believe this one should be merged and #2180 should be discarded but the maintainers are probably wiser than me on Puma!

Issue description

C.F. #2177

Fix description

Before this patch, the out_of_band hook was executed only when all worker threads were free:

pool << client
busy_threads = pool.wait_until_not_full
if busy_threads == 0
  @options[:out_of_band].each(&:call) if @options[:out_of_band]
end

In addition, due to how the function wait_until_not_full (in lib/puma/thread_pool.rb) works, namely:

• If not all threads are busy, return the number of busy threads
• Otherwise, wait (loop) for the condition above

The condition if busy_threads == 0 could never be satisfied if the number of worker threads was > 1.

EDIT: it could be satisfied if the worker finished its job before the call to wait_until_not_full, which is very unlikely because this function is called right after adding the client to the pool.

I believe that the out_of_band hook should be allowed to run if there is at least 1 available worker thread. It seems reasonable to me and provides an easy fix for this bug.

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[nateberkopec]

Well, the problem here is that if busy_threads != 0, it's not really out of band anymore, is it?

[nateberkopec]

I think this is a non-starter because it makes the OOB hook no longer OOB and just a background thread.