bug#29110: 25.2; Should push-mark allow duplicates?

[Pierre Neidhardt]

The `push-mark' function allows for duplicate marks.  I fail to see a
use case, but otherwise I think it's rather inconvenient:

- It makes traversing the ring tedious with respect to end-user
interaction.  (Think Ivy / Helm for the mark ring.)
Duplicates are probably not the expected behaviour for the end-user.

- Functions working with rings will probably want to remove the
duplicates, so they end up calling `remove' and the like over and over
again.

- It eats up more memory.

- It's counter-intuitive to developers who may in turn write code
without being careful that rings may contain duplicates.  This may
result in unexpected behaviour.


I got bitten hard by this while trying to figure out why
`helm-mark-ring` would randomly fail to follow the marks when Evil mode
was `require'd (not even turned on).

I reported the issues on both bug trackers:

- https://github.com/emacs-evil/evil/issues/845

- https://github.com/emacs-helm/helm/issues/1891

We could not find out the root of the issue, but we discovered that
advising `push-mark' so that it does not duplicate marks would do it.  I
know it's not a solution per se, but at least we've got a lead.

To reproduce, start emacs -Q and use "C-SPC C-SPC" a few times at
different spots.  Then move point somewhere and eval:

        (set-marker (mark-marker) (point))
        (push-mark)
        mark-ring

You should now have one duplicate in the ring.

Here is the proposed fix implemented in Helm:

https://github.com/emacs-helm/helm/commit/ffd2abf5c4bdfc998c09730387b11d2bf9ac1032




In GNU Emacs 25.2.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.16)
 of 2017-09-02 built on dhiov23k
Windowing system distributor 'The X.Org Foundation', version 11.0.11905000
System Description:     Gentoo Base System release 2.4.1

Configured using:
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 --localstatedir=/var
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 GENTOO_PACKAGE=app-editors/emacs-25.2 'CFLAGS=-march=ivybridge -O2
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Configured features:
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FREETYPE XFT ZLIB GTK3 X11

Important settings:
  value of $LANG: en_US.utf8
  locale-coding-system: utf-8-unix

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