Patches attached.

Of note:

I added a ring-resize function.

I wrote tests with names function-to-test/case-tested.  I'm not sure
if this violates style policies for tests.  It seems like existing
code tests multiple cases in a single test, but I find separate tests
easier to understand when they regress.  The / makes the test names
easier to read.

I based this off of 26 (as I think this is a minor bugfix/feature add).
On Thu, Sep 27, 2018 at 2:24 AM Eli Zaretskii <eliz@gnu.org> wrote:
>
> > From: Allen Li <darkfeline@felesatra.moe>
> > Date: Wed, 26 Sep 2018 16:14:15 -0700
> >
> > The xref-marker-ring-length user option doesn't have a setter, so
> > customizing it will not affect xref--marker-ring, except if/when user
> > customizations are loaded in a new Emacs process before xref is loaded.
> > Customizations after xref is loaded will have no effect.
>
> I think the setter should use ring-extend to enlarge the ring, and
> some custom code using ring-remove to make the ring smaller.
>
> Would you like to submit a patch along those lines?
>
> Thanks.