bug#64117: 28.2; "gnus-request-set-mark: Buffer is read-only" when exiting agentized, unplugged summary buffer

[Jens Schmidt via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

This issue should be also present in Emacs master, judging by the
code.  However I have reproduced this issue only in my
"production Emacs" on 28.2 and, in particular, *not* in an "emacs
-Q" environment.  Gnus setup complete with IMAP and all is
finicky, so probably you accept this bug report without all that.

Here are the steps to reproduce the issue:

- Ensure you have an agentized nnimap group G with some unread
   articles.

- Ensure Gnus is unplugged.

- Enter group G from the *Group* buffer by pressing "RET" on it.

- Mark some unread articles in the summary buffer of G as "read"
   by positioning point on them and pressing "d".

- Exit group G from the summary buffer by pressing "q".

The last step results in an error

   gnus-request-set-mark: Buffer is read-only: #<buffer *Summary 
nnimap+<host>:INBOX*>

for me.

Expected result is that the agent writes the information on the
marks that have changed while unplugged to a separate file which
it later, when Gnus gets plugged again, replays.

The reason for the bug us most likely commit

 
https://git.savannah.gnu.org/cgit/emacs.git/commit/src?id=cb12a84f2c519a48dd87453c925e3bc36d9944db

and here the change

   * lisp/gnus/nnagent.el: Don't use a unibyte buffer.

The author has removed a macro call to `mm-with-unibyte-buffer'
in function `nnagent-request-set-mark' without replacing it by,
for example, a call to `with-temp-buffer'.  Accordingly, function
`nnagent-request-set-mark' tries to operate on the current
buffer, which happens to be the read-only summary buffer.

Adding a `with-temp-buffer' around the inserts and the call to
`write-region' fixes this issue (showing 28.2-based code):

(deffoo nnagent-request-set-mark (group action server)
   (with-temp-buffer
     (insert "(gnus-agent-synchronize-group-flags \""
	    group
	    "\" '")
     (gnus-pp action)
     (insert " \""
	    (gnus-method-to-server gnus-command-method)
	    "\"")
     (insert ")\n")
     (let ((coding-system-for-write nnheader-file-coding-system))
       (write-region (point-min) (point-max) (gnus-agent-lib-file "flags")
		    t 'silent)))
   ;; Also set the marks for the original back end that keeps marks in
   ;; the local system.
   (let ((gnus-agent nil))
     (when (and (memq (car gnus-command-method) '(nntp))
	       (gnus-check-backend-function 'request-set-mark
					    (car gnus-command-method)))
       (funcall (gnus-get-function gnus-command-method 'request-set-mark)
	       group action server)))
   nil)

I could provide a patch on emacs-29 but only without actually
testing it...