Why not clean exit when there are no more goroutines?

[twp...@gmail.com]

tl;dr Why does Go choose not to exit cleanly when there are no more goroutines to run?

Background:

As part of a Go pub quiz, I asked the question:

  What always happens when this code runs?

  https://go.dev/play/p/opTvk1x-uAm

Specifically, the code calls runtime.Goexit in the main goroutine. At termination, the program prints:

  fatal error: no goroutines (main called runtime.Goexit) - deadlock!

Why is this considered a fatal error? Obviously there are no more goroutines to run, but why not exit cleanly in this case?

Obviously there's a very good reason for this behavior. I just can't work out what it is. 

Any insights or ideas?

Regards,

Tom

[Jan Mercl]

If the runtime cleanly exits on a deadlock then we no longer can
detect unexpected deadlocks. Neither can we say if all "goroutines
terminated" is expected or not. Hence "clean exit" iff main() returns.
(Let's ignore os.Exit(rc) for a second.)

[twp...@gmail.com]

Thank you Jan, that makes sense.