Release D 2.089.0

[Manu]

On Mon, Nov 4, 2019 at 11:55 PM John Chapman via
Digitalmars-d-announce <digitalmars-d-announce at puremagic.com> wrote:
>
> On Tuesday, 5 November 2019 at 06:44:29 UTC, Manu wrote:
> > On Mon., 4 Nov. 2019, 2:05 am John Chapman via
> > Digitalmars-d-announce, < digitalmars-d-announce at puremagic.com>
> > wrote:
> >
> >> Something has changed with core.atomic.cas - it used to work
> >> with `null` as the `ifThis` argument, now it throws an AV. Is
> >> this intentional?
> >>
> >> If I use `cast(shared)null` it doesn't throw but if the change
> >> was deliberate shouldn't it be mentioned?
> >>
> >
> > Changes were made because there were a lot of problems with
> > that module...
> > but the (reasonably comprehensive) unit tests didn't reveal any
> > such
> > regressions. We also build+test many popular OSS projects via
> > buildkite,
> > and there weren't problems.
> > Can you show the broken code?
>
> Sure - this AVs on DMD 2.088 Windows:
>
> import core.atomic;
> void main() {
>    Object a, b = new Object;
>    cas(cast(shared)&a, null, cast(shared)b);
> }

Oh... a class.
Yeah, that's an interesting case that I actually noted had a low
testing surface area.
It's also theoretically broken; despite what's practical, I think it's
improperly spec-ed that shared classes can be used with atomics.
With a struct, you can declare `shared(T)* s_ptr`, but with classes
you can only `shared(C) c_ptr`, where the difference is that `s_ptr`
can be read/written... but `c_ptr` is typed such that the pointer
itself is shared (because classes are implicitly a pointer), so that
`c_ptr` can't be safely read/write-able...
So, I actually think that atomic API is mal-formed, and it should not
support `shared` arguments, but I tried to preserve existing
behaviour, while being more strict about what is valid. I obviously
missed something with `null` here.

Incidentally, in your sample above there, `a` and `b` are not
shared... why not just write: `cas(&a, null, b);` ?? If source data is
not shared, you shouldn't cast to shared.