Yanking to ?+ register does not work in Emacs >=29

[dvzubarev]

Issue type

Bug report

Environment

Emacs version: 30.0.50
Operating System: Ubuntu 22.04
Evil version: 1.15.0
Evil installation type: MELPA
Graphical:
Tested in a make emacs session: No

Reproduction steps

1. emacs -Q -L . -l evil.el
2. M-x evil-mode
3. copy any text outside of Emacs
4. i C-r + (so far so good)
5. yank something inside Emacs: ygE
6. i C-r + or "+p - evil-get-register: Register ‘+’ is empty
7. try to yank to + register "+ygE
8. i C-r + or "+p - evil-get-register: Register ‘+’ is empty

Expected behavior

I would expect that text is pasted after yanking text inside Emacs, like it works in Emacs 28.

Actual behavior

evil-get-register: Register ‘+’ is empty

Further notes

It is related to #1677
After yanking text inside Emacs try to eval (gui--selection-value-internal 'CLIPBOARD). It will return nil.
I extracted the code that works for Emacs <29:

(defun old-x-selection (what)
  (let ((request-type (or (bound-and-true-p x-select-request-type)
                          '(UTF8_STRING COMPOUND_TEXT STRING)))
        text)
    (unless (consp request-type)
      (setq request-type (list request-type)))
    (while (and request-type (not text))
      (setq text (ignore-errors
                   (evil-get-selection what (pop request-type)))))
    (when text
      (remove-text-properties 0 (length text) '(foreign-selection nil) text))
    text))

Eval (old-x-selection 'CLIPBOARD) it will return text that was yanked inside Emacs.
Skimming over code of gui--selection-value-internal It seems that it can't pass the first unless.

(unless
(and gui-last-cut-in-clipboard
               ;; `gui-backend-selection-owner-p' might be unreliable on
               ;; some other window systems.
               (memq window-system '(x haiku))
               (eq type 'CLIPBOARD)
               ;; Should we unify this with gui--clipboard-selection-unchanged-p?
               (gui-backend-selection-owner-p type))
...
)

[Elliott Shugerman]

I'm confused, do you hit this bug on Emacs 29? Or Only 30? I can't reproduce it on 29, FWIW.

[dvzubarev]

I can reproduce this on:

1. Emacs 30.0.50 on Ubuntu 22.04, Emacs was built from sources, with native compilation
2. Emacs 29.1 on ArchLinux, installed via pacman (package emacs-nativecomp)

[dvzubarev]

What are your values of variables:

• window-system
• select-enable-clipboard

Mine is:

• x
• t
  If your window-system is not in '(x haiku) or the second variable is nil, then that's why it is working for you.

[dvzubarev]

A comment from gui-last-cut-in-clipboard

;; The doc string of `interprogram-paste-function' says to return
  ;; nil if no other program has provided text to paste.

I think it means that if you copied text to clipboard from Emacs then this function should return nil.
unless statement that I pasted in the first message checks exactly this.
It may be useful for something, but evil uses this function to get the last value from the clipboard.
So I think that it is pretty safe to override gui-last-cut-in-clipboard locally, before invocation of this function.

(let ((gui-last-cut-in-clipboard nil))
        (gui--selection-value-internal what))

unless statement will always succeed in this case.

[Elliott Shugerman]

os	emacs version	window-system	select-enable-clipboard
MacOS 13.4	29.1	ns	nil
NixOS 23.05	29.1	x	nil

Also, up until a couple months ago I ran Emacs 29 on Arch Linux with window-system == pgtk, never noticed the issue you describe.

I can reproduce the "Register '+' is empty" error by setting select-enable-clipboard to t on my NixOS system (but not on MacOS).

I'm confused as to why you would bother with specifying the + register at all when select-enable-clipboard is t? Doesn't that essentially make + the default register? My intention in asking is not to dismiss this bug report entirely -- the "register is empty" error is clearly not the correct behavior -- just trying to understand the severity/impact of the problem.

[dvzubarev]

I'm confused as to why you would bother with specifying the + register at all when select-enable-clipboard is t?

p/P gets buggy sometimes and does not paste from clipboard. I don't know how to reproduce it and even whether it has something to do with evil. C-r + works every time. It's kind of muscle memory for me.

Doesn't that essentially make + the default register?

not sure what letter is associated with default register, but from quick testing it seems I can use C-r 1 instead of C-r +.