> From: Jimmy Yuen Ho Wong <wyuenho@gmail.com>
> Date: Thu, 12 Jun 2025 17:58:34 +0100
>
>
> When investigating https://github.com/emacs-lsp/lsp-mode/issues/4782 and
> https://github.com/purcell/emacs-reformatter/issues/63, I've discovered
> the following bug in insert-file-contents.
>
> Reproduction:
>
> 1. echo "hello world" > ~/test.txt
> 2. emacs -q ~/test.txt
> 3. M-x eval-expression RET (add-hook 'after-change-functions (lambda (&rest _) (message "buffer-file-name: %s, current-buffer: %s" (buffer-file-name) (current-buffer))) nil t) RET
> 4. Type something into the buffer
> 5. M-x eval-expression RET (insert-file-contents (buffer-file-name) nil nil nil t) RET
> 6. Switch to the *Messages* buffer and observe the buffer file name is nil.
>
> Expectation:
>
> insert-file-contents should not set buffer-file-name to nil
I don't think it does. (buffer-file-name) returns nil when the call
is evaluated in the minibuffer, and I thin that's what you see. After
performing the above recipe the buffer-filename of the buffer that
visits test.txt remains to be "test.txt", at least in my testing. So
the only evidence that it's set to nil are the messages logged in
*Messages*, and they are about a different buffer.
When a lisp expression is evaluated in the minibuffer, the current buffer is the file buffer, not the minibuffer. You can easily verify this with M-x eval-expression RET (current-buffer) RET.
The crux of the matter is the lambda added to after-change-function. `insert-file-contents` calls `signals_after_change` (fileio.c line 5007 on master), which will run the after-change-functions. It is at this moment the buffer-file-name is nil.
In addition, the following will NOT result in insert-file-contents temporarily setting the buffer-file-name of the current buffer to nil:
(let ((coding-system-for-read 'no-conversion)
(coding-system-for-write 'no-conversion))
(insert-file-contents (buffer-file-name) nil nil nil t))
Which suggests some of the code conversion logic in the C function is erroneously setting the current buffer's file-name to nil, and then running the after-change-functions before resetting it.