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I finally got around to reading "How does Sidekiq really work?" by Dan Svetlov dansvetlov.me/sidekiq-inte.... Dang, it did not disappoint. What an enjoyable and in-depth read! A few things that stuck out to me 1/11 🧵

1) @mike.contribsys.com you clever devil, that Signal.trap approach is ingenious! In my post on thread interrupts, I mentioned that you cannot use rails.logger inside of a signal.trap: bit.ly/43c23Ym. Similarly, you cannot use the Sidekiq logger in trap, which is inconvenient. 3/11 🧵

The full reasons for that are explained in docs.ruby-lang.org/en/3.4/signa..., and have to do with a concept called "reentrancy". But mike bypasses all of that by using using IO.pipe. The io writer is safe to write to in the trap, so that's all it does, it writes the signal. 4/11 🧵

Meanwhile, the main loop for sidekiq is always waiting on the io reader, calling a blocking method, `wait_readable` in the while loop. Signal received -> write the signal -> wait_readable returns truthy -> while loop runs and calls `handle_signal` with the signal read 5/11 🧵

That's called the "Self Pipe Trick" It's pretty much textbook how to handle signals in an application with a main IO loop.

It's so common Linux basically added an API to do it for you with `signalfd`: man7.org/linux/man-pa...