Forked GC explained

[frame]

On Saturday, 3 September 2022 at 14:31:31 UTC, Steven 
Schveighoffer wrote:
> On 9/3/22 9:35 AM, frame wrote:

>> What happens if a manually `GC.free()` is called while the 
>> forked process marks the memory as free too but the GC 
>> immediately uses the memory again and then gets the 
>> notification to free it from the forked child? Can this happen?
>
> No, because if you can free it, you should have had a reference 
> to it when you forked, which should mean it's not garbage.

And what if the programmer has no actual reference but wrongly 
forced a `free()` through a pointer cast?

```
         | OP      | Memory M
-------------------------------------------
Parent: | -       | Unreferenced, marked in use
-------------------------------------------
Parent: | fork
-------------------------------------------
Parent: | -       | Unreferenced, marked in use
Child:  |         | Unreferenced, marked in use
-------------------------------------------
Parent: | -       | Unreferenced, marked in use
Child:  |         | Unreferenced, found M
-------------------------------------------
Parent: | free    | Unreferenced, marked not in use  <- error 
forced by programmer
Child:  |         | Unreferenced, found M
-------------------------------------------
Parent: | new     | Referenced, re-used because it was marked free
Child:  |         | Unreferenced, found M
-------------------------------------------
Parent: | -       | Referenced, used
Child:  |         | Done scanning. Please collect: M
-------------------------------------------
Parent: | collect | M
Child:  |         | exit
-------------------------------------------
```
@wjoe is the GC aware of this to exclude M from the child result 
set because it has changed while the child was running?

> There's a talk on it from the 2013 dconf by the inventor: 
> https://dconf.org/2013/talks/lucarella.html
>
> -Steve

Thanks for the link. The slides mentioning shared memory.