bug#27810: NS runtime feature detection

[Alan Third]

On Mon, Aug 07, 2017 at 09:23:10PM +0200, Charles A. Roelli wrote:
> On 06/08/2017 23:29, Alan Third wrote:
> > I believe we can make this slightly neater:
> > 
> >      enum NSScrollerStyle {
> >        NSScrollerStyleLegacy = 0,
> >        NSScrollerStyleOverlay = 1
> >      };
> 
> Strange, it doesn't work here:

We’ll just go with what works, then.

> > I’ve done a bit more reading up on this and I think I’ve misunderstood
> > how this works, and probably mislead you.
> > 
> > It seems these functions need to be declared as weak in the definition
> > of the library they’re supposed to be in. If we declare them in the
> > Emacs code‐base then the linker, reasonably, expects the functions to
> > be in the Emacs code‐base.
> 
> Maybe I'm also confused.  I thought we would be able to do this,
> since:
> 
>   - At link time, the symbol is marked as a weak reference, to be
>     resolved at runtime.
> 
>   - At runtime, the dynamic linker resolves the reference to the weak
>     symbol, setting it to NULL if it isn't available.  Normally the
>     definition of the function will be found in a dynamic library that
>     is part of macOS (as far as I understand).
> 
> The Apple compiler/linker should be capable of doing this, supposedly,
> as long as you give the magical -Wl,-U,_symbol command line arguments
> to the linker.  See also https://stackoverflow.com/a/34983229.

That’s quite a good description. I guess that we want to do what
you’re suggesting, then. I’m not sure how, though. I’ll try to have a
look through configure.ac to see if I can work it out sometime over
the weekend.

> I'd like to check, but wouldn't I need to either:
> 
> a) Statically link libraries Emacs depends on, or
> b) Include the dependent libraries in the app bundle?

Yes, I suppose so. I kind of assumed it would statically link at least
some of them, but I guess not.

I’ve had a look at the build scripts for emacsformacosx.com, but I
don’t understand what they’re doing.

I’ve attached what I have so far, which I think includes all your
changes except for the requirements for linker arguments.
Unfortunately master doesn’t build here now because of some other
problem so it’s untested.
-- 
Alan Third

0001-Allow-use-of-run-time-OS-version-checks-on-macOS-bug.patch
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