add Threads.@once

[chrstphrbrns]

It might be useful to have a Threads.@once macro that guarantees a line of code will only be executed once, either globally or in a given scope, a la C++ call_once or GCD dispatch_once

[nsajko]

xref C++, Go

[Seelengrab]

So something like a decrement-only semaphore that other tasks aren't blocked by if they can't decrement, guarding the function call?

[nsajko]

The go implementation actually uses a mutex:

https://github.com/golang/go/blob/be3d5fb6e6c9531e6c01dc909da710e1f596fb3a/src/sync/once.go#L18-L26

[Seelengrab]

Yes, you can use a lock and trylock as well. It would look like this:

l = Spinlock()
# ...
if trylock(l)
    # call f
end

You'd have to keep the done-ness around of course, which is where a "decrement only" semaphore saves some space, by having a smaller struct.

[adienes]

I think this was implemented in #55793

I guess we can leave the issue open if someone feels like also adding a macro to wrap those functions and use the feature more easily?