async + threads:on + globals = error

[yglukhov]

import asyncdispatch
var s = "hi"
proc foo() {.async.} = echo s

nim c --threads:on test.nim

test.nim(3, 12) template/generic instantiation from here
Projects/nim/lib/pure/asyncmacro.nim(31, 8) Error: 'cb' is not GC-safe as it accesses 'nameIterVar' which is a global using GC'ed memory

Originated from dom96/jester#110

[Araq]

That works as expected. You cannot use global GC'ed memory with --threads:on.

[zielmicha]

Shouldn't this only apply to gcsafe procs?

We call a proc p GC safe when it doesn't access any global variable that contains GC'ed memory (string, seq, ref or a closure) either directly or indirectly through a call to a GC unsafe proc.

[yglukhov]

@Araq, are you saying that the following code should not compile with --threads:on?

var s = "hi"
proc foo() = echo s

Because now it compiles just fine ;)

[Araq]

No, I'm saying what the manual says and you ignore what the .async transformation does.

[yglukhov]

Well I know that async puts gcsafe everywhere. But what's the reason for that? As far as I understand async procs are perfectly safe to use globals in a single thread.

[andreaferretti]

@dom96 Do you have an idea why gcsafe is used in async?

[yglukhov]

Future defines it's callback as gcsafe, because one might want to make a gcsafe async proc. So every callback we store in the future should be gcsafe. I wonder if we can somehow allow both gcsafe and gcunsafe code in futures...