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

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

[Sean McAfee]

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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
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Windowing system distributor 'Apple', version 10.3.2487
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bug#74438: 29.1; global-mark-ring does not work as described

[Nikolay Kudryavtsev]

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Emacs won't allow you to set multiple marks in the same position, it's 
just going to activate the current mark. Hence your reproduction recipe 
can be simplified to:

 1. C-x C-f foo
 2. C-SPC => Mark set
 3. C-SPC => Mark deactivated
 4. C-SPC => Mark activated
 5. C-SPC => Mark deactivated
 6. C-SPC => Mark activated

See how you're not getting "Mark set" any time after the first message?

Maybe a couple of sentences should be added to the manual regarding this 
behavior, because mark ring is often a source of confusion for the new 
users.

Also note that the manual recommends you deactivate the mark before 
trying to jump to it:

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

"Instead of setting the mark in order to operate on a region, you can 
also use it to remember a position in the buffer (by typing ‘C-<SPC> 
C-<SPC>’), and later jump back there (by typing ‘C-u C-<SPC>’)."

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

[Eli Zaretskii]

> From: Nikolay Kudryavtsev <nikolay.kudryavtsev@gmail.com>
> Date: Wed, 20 Nov 2024 13:18:00 +0300
> 
> Emacs won't allow you to set multiple marks in the same position, it's just going to activate the current mark.
> Hence your reproduction recipe can be simplified to:
> 
> 1 C-x C-f foo
> 2 C-SPC => Mark set
> 3 C-SPC => Mark deactivated
> 4 C-SPC => Mark activated
> 5 C-SPC => Mark deactivated
> 6 C-SPC => Mark activated
> 
> See how you're not getting "Mark set" any time after the first message?

Yes, technically, it doesn't set a mark, it just activates the
existing mark.  And the global-mark-ring section says that Emacs
pushes a mark on the ring when it sets the mark in some buffer.  Which
is not what happens here.

> Maybe a couple of sentences should be added to the manual regarding this behavior, because mark ring is
> often a source of confusion for the new users.

If you can tell what are the confusions, I could work on clarifying
them.

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

[Sean McAfee]

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On Wed, Nov 20, 2024 at 2:18 AM Nikolay Kudryavtsev <
nikolay.kudryavtsev@gmail.com> wrote:

> Emacs won't allow you to set multiple marks in the same position, it's
> just going to activate the current mark.
>
I don't want to set multiple marks in the same position though.  I just
want to be able to know in which buffer I most recently set a mark.  I
expected that the first element in global-mark-ring would tell me, but it
doesn't, not reliably, because if the mark already happens to be at the
place I run set-mark-command, the ring is not updated, and the first
element may refer to some other buffer.  That may be the "correct" behavior
by some standard, but it doesn't match the documentation.

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

[Nikolay Kudryavtsev]

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Probably just needs a sentence to this end somewhere in "The Mark Ring" 
node, akin to this:

    If you set the mark at a position, you cannot immediately set
    another mark at the same position.

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

[Nikolay Kudryavtsev]

Using the mark ring for programming purposes is generally seen as a faux 
pas, see the docstrings for push-mark and set-mark, which explicitly 
warn against this.

If you still insist, then nothing is really stopping you from short 
circuiting this behavior by say doing a forward-char, set-mark, 
backward-char, set-mark again.

But I also don't understand why do you need buffer 1 mark to be at the 
front of the ring, because it's gonna reliably be as the second element 
in it anyway.

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

[Eli Zaretskii]

> From: Nikolay Kudryavtsev <nikolay.kudryavtsev@gmail.com>
> Date: Thu, 21 Nov 2024 10:51:14 +0300
> Cc: eefacm@gmail.com, 74438@debbugs.gnu.org
> 
> Probably just needs a sentence to this end somewhere in "The Mark Ring" node, akin to this:
> 
>  If you set the mark at a position, you cannot immediately set another mark at the same position.

Thanks.  I mentioned this where set-mark-command is described, as that
seemed like a better place.  I also added a node to Global Mark Ring
that merely activating a mark doesn't record it in the global ring.

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

[Sean McAfee]

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On Thu, Nov 21, 2024 at 12:09 AM Nikolay Kudryavtsev <
nikolay.kudryavtsev@gmail.com> wrote:

> Using the mark ring for programming purposes is generally seen as a faux
> pas, see the docstrings for push-mark and set-mark, which explicitly
> warn against this.
>

I've read those warnings, which seem to be about using the mark to keep
track of positions within a single function execution.  Here I'm writing a
command that wants to use information recorded by a previous command, which
seems like a legitimate use to me.


> If you still insist, then nothing is really stopping you from short
> circuiting this behavior by say doing a forward-char, set-mark,
> backward-char, set-mark again.
>

I could do that, but conceptually I just want to set the mark, and I don't
want to have to perpetually keep in mind that when I set the mark in this
one specific context, I need to go through an extended routine like that.


> But I also don't understand why do you need buffer 1 mark to be at the
> front of the ring, because it's gonna reliably be as the second element
> in it anyway.
>

But it won't; it could be anywhere in the global mark ring.

- Go to a new buffer foo and press C-SPC; now buffer foo is first in the
global mark ring.
- Go to a new buffer bar and press C-SPC; now buffer foo is second in the
ring.
- Go to a new buffer baz and press C-SPC; now buffer foo is third in the
ring.
- Go back to buffer foo and press C-SPC; buffer foo is still third in the
ring.

And buffer foo won't be in the ring at all if more than
global-mark-ring-max buffers are visited in this way.

Anyway, it seems like a consensus is emerging that it's the documentation
and not the code that needs to be updated.  At least I've thought of a way
to get the info I need without changing my workflow.  Something like:

    (defvar last-global-mark (make-marker))

    (defun my-set-mark-command (arg)
      (interactive "P")
      (set-mark-command arg)
      (unless (equal arg '(4))
        (set-marker last-global-mark (point))))

    (global-set-key [remap set-mark-command] #'my-set-mark-command)

I wish it weren't necessary, but at least it's not very long.

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

[Nikolay Kudryavtsev]

I still don't understand why do you need more than one mark for your use 
case?

You are comparing two sexps. You put the foo buffer sexp position into 
the mark ring. Now you go to the bar buffer and select the sexp there. 
Why do you need to put the bar into the mark ring? Why do you need the 
second foo mark? You can just use the first(and only) foo mark and be 
done with it.

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

[Sean McAfee]

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On Thu, Nov 21, 2024 at 12:32 PM Nikolay Kudryavtsev <
nikolay.kudryavtsev@gmail.com> wrote:

> I still don't understand why do you need more than one mark for your use
> case?
>
> You are comparing two sexps. You put the foo buffer sexp position into
> the mark ring. Now you go to the bar buffer and select the sexp there.
> Why do you need to put the bar into the mark ring? Why do you need the
> second foo mark? You can just use the first(and only) foo mark and be
> done with it.
>

Er...yeah.  That's exactly what I want to do.  Not sure how I gave a
different impression.

Here's what I have now, minus some defensive coding to keep it short:

    ;; Helper function:
    (defun write-sexp-at (pos dest-file)
      (write-region pos (save-excursion (goto-char pos) (forward-sexp)
(point)) dest-file))

    (defun json-diff ()
      (interactive)
      (let ((file-a (make-temp-file "json-diff"))
            (file-b (make-temp-file "json-diff")))
        (with-current-buffer (marker-buffer last-global-mark)
          (write-sexp-at (marker-position last-global-mark) file-a))
        (write-sexp-at (point) file-b)
        (shell-command (format "~/bin/json-diff %s %s" file-a file-b))))

Originally I tried using (car global-mark-ring) instead of
last-global-mark, but that doesn't work, per my original bug report.  I
also tried saying (mark t) instead of (marker-position (car
global-mark-ring)).  That would have returned the correct position, but it
doesn't matter because (marker-buffer (car global-mark-ring)) can't be
relied upon to return the right buffer.

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