bug#38492: 27.0.50; Warn pdumper users when pure space has been overflowed

["Daniel Colascione" <dancol@dancol.org>]

>>> > We should probably consider just removing the pure space mechanism at
>>> > some point.
>>> Does this mean you agree with removing the warning in startup.el
>>> about overflowed pure space?
>> Yes.
>
> Why?
>
> This warning was placed because we needed to disable GC in dumps that
> had overflown the purespace, so the dumped Emacs was not
> fully functional.
>
> I see that with the current pdump we don't disable GC in dumps that
> overflew the purespace.  Why is that?
>
> IIRC, the problem with running GC in an Emacs with overflown purespace
> is that overflowing the purespace created pointers from the purespace to
> the heap and those aren't traced by the GC so it resulted in core dumps.
>
> I can't exactly remember the cases where this happened, but I can't see
> why the pdumper would be less affected than the unexec code.
>
> I see that the code has a slightly different explanation:
>
>     /* Can't GC if pure storage overflowed because we can't determine
>        if something is a pure object or not.  */
>     garbage_collection_inhibited++;
>
> but that comment similarly seems to apply to pdump just as much as it
> applies to unexec.

In a pdumped Emacs, we don't have anything in purespace on pdumper load,
so we can GC normally. We do disable GC during dump operation itself if we
overflow, true, and we shouldn't do that, but this problem affects only
the build.

We should just make sure Vpurify_flag is off if we're going to make a
pdumper image.