bug#64821: 30.0.50; write-region errs when overwriting an already open file

[Ihor Radchenko]

Consider the following reproducer:

1. emacs -Q
2. Evaluate the following in scratch buffer

(with-temp-buffer
  (insert "test")
  (write-region nil nil "/tmp/1.txt"))

3. Open /tmp/1.txt
4. Evaluate the above sexp multiple times
5. Observe

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  expand-file-name(nil)
  userlock--check-content-unchanged("/tmp/1.txt")
  userlock--ask-user-about-supersession-threat("/tmp/1.txt")
  write-region(nil nil "/tmp/1.txt")
  (progn (insert "test") (write-region nil nil "/tmp/1.txt"))
  (unwind-protect (progn (insert "test") (write-region nil nil "/tmp/1.txt")) 
(and (buffer-name temp-buffer) (kill-buffer temp-buffer)))
  (save-current-buffer (set-buffer temp-buffer) (unwind-protect (progn (insert 
"test") (write-region nil nil "/tmp/1.txt")) (and (buffer-name temp-buffer) 
(kill-buffer temp-buffer))))
  (let ((temp-buffer (generate-new-buffer " *temp*" t))) (save-current-buffer 
(set-buffer temp-buffer) (unwind-protect (progn (insert "test") (write-region 
nil nil "/tmp/1.txt")) (and (buffer-name temp-buffer) (kill-buffer 
temp-buffer)))))
  (progn (let ((temp-buffer (generate-new-buffer " *temp*" t))) 
(save-current-buffer (set-buffer temp-buffer) (unwind-protect (progn (insert 
"test") (write-region nil nil "/tmp/1.txt")) (and (buffer-name temp-buffer) 
(kill-buffer temp-buffer))))))
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  command-execute(eval-last-sexp)

Expected: No error is thrown.

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.17.8) of 2023-07-21 built on localhost
Repository revision: 7f77120683d6f150e61e11b41d75ba16ee5210a4
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
System Description: Gentoo Linux


-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
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