Why puma workers constantly hung, and how we fixed by discovering the bug of Ruby v2.5.8 and v2.6.6

While running Rails puma servers in production, we were seeing the incident that some old worker processes suddenly got stuck regardless of no change in the amount or trend of requests.I found out the root cause and reported it to the upstream. This issue still exists in Ruby 2.6.0 and can be found as far back as Ruby 2.5.0. If you just want a summary of the bug, see ruby-lang.