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This is utterly false in my experience. The popularity of Puma and Sidekiq mean that threading issues are quickly found and fixed.

Yeah I agree. Thread safety in ruby is a non issue these days. Maybe this is 10 years out of date experience?

How does Puma help with the libraries that are not thread safe?

I think he's saying threading issues are more likely to be found and fixed these days given the context in which most ruby libraries are running.