bug#74438: 29.1; global-mark-ring does not work as described

[Sean McAfee <eefacm@gmail.com>]

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Starting from emacs -Q:

- Go to a new buffer named "foo".
- Set the mark with C-SPC.
- Go to a new buffer named "bar".
- Set the mark with C-SPC.
- Go back to buffer foo.
- Set the mark with C-SPC.
- Check the value of the global-mark-ring variable, as with C-h v
  or giving its name to the eval-expression command.

The first marker in the ring is the one in buffer bar, not in
buffer foo, even though buffer foo is where the mark was most
recently set.  This behavior contradicts what is described here:

https://www.gnu.org/software/emacs/manual/html_node/emacs/Global-Mark-Ring.html

To wit: "Each time you set a mark, this is recorded in the global mark
ring in addition to the current buffer’s own mark ring, if you have
switched buffers since the previous mark setting."

I've tracked the reason for this to the push-mark-command function,
which does not push the mark unless it's not at the same place as point,
or if it's given a true argument.  set-mark-command calls
push-mark-command, but passes nil to it unless, puzzlingly, the
command's name is not "set-mark-command".

For a bit of background, what I want to do is to set the mark at the
beginning of a sexp (actually JSON) in one buffer, then go to the start
of another sexp in a different buffer, and run a command that compares
the two sexps, using the marker at the front of the global mark ring to
find the first sexp.  But this is unreliable since setting the mark does
not necessarily update the global mark ring.  Workarounds I've
considered include:

- Call push-mark-command manually with M-x, or bind it to a key
  sequence, and call it with a prefix argument.
- Alias set-mark-command as a new command with a different name, and use
  that instead.
- Use a custom marker, updated from a custom command.

Obviously, none of these are as convenient as using the global mark
ring.  And in any event, there's still a bug in Emacs, even if only a
documentation bug.


In GNU Emacs 29.1 (build 1, aarch64-apple-darwin21.6.0, NS
 appkit-2113.60 Version 12.6.6 (Build 21G646)) of 2023-08-16 built on
 armbob.lan
Windowing system distributor 'Apple', version 10.3.2487
System Description:  macOS 14.7.1

Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules 'CFLAGS=-DFD_SETSIZE=10000
 -DDARWIN_UNLIMITED_SELECT' --with-x-toolkit=no'

Configured features:
ACL GLIB GMP GNUTLS JPEG JSON LIBXML2 MODULES NOTIFY KQUEUE NS PDUMPER
PNG RSVG SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER ZLIB

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

Major mode: ELisp/l

Minor modes in effect:
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Load-path shadows:
~/emacs/copilot.el/copilot hides ~/emacs/copilot
~/emacs/inf-ruby/inf-ruby hides
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3.app/Contents/Resources/lisp/use-package/use-package-diminish
/Users/sean/.emacs.d/elpa/use-package-20230426.2324/use-package hides
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/Users/sean/.emacs.d/elpa/use-package-20230426.2324/use-package-bind-key
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3.app/Contents/Resources/lisp/use-package/use-package-bind-key
/Users/sean/.emacs.d/elpa/bind-key-20230203.2004/bind-key hides
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/Users/sean/.emacs.d/elpa/use-package-20230426.2324/use-package-lint hides
/Applications/Emacs
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/Users/sean/.emacs.d/elpa/flim-20240221.1353/sasl hides /Applications/Emacs
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/Users/sean/.emacs.d/elpa/seq-2.24/seq hides /Applications/Emacs
3.app/Contents/Resources/lisp/emacs-lisp/seq
/Users/sean/.emacs.d/elpa/eldoc-1.15.0/eldoc hides /Applications/Emacs
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