How to warn before editing or saving a buffer whose underlying file no longer exists?

Question

(NB: In this post, I will use expressions like "a buffer's underlying file" to mean, in the Emacs' documentation's terminology, "the file a buffer is visiting", i.e. the value of (buffer-file-name SOME-BUFFER) for the buffer SOME-BUFFER.)

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Sometimes it happens that a buffer's underlying file gets modified on disk by some other process.

If one tries to modify such a buffer, Emacs warns of the on-disk change, and requests confirmation; e.g. (in the minibuffer):

one_really_busy_file changed on disk; really edit the buffer? (y, n, r or C-h) 

Furthermore, if after responding y to such a confirmation request, one modifies such a buffer, and then attempts to save the modified buffer, then Emacs once again issues a warning and a confirmation request; e.g.:

one_really_busy_file has changed since visited or saved.  Save anyway? (y or n)

To be sure, Emacs does not regard all modifications to a buffer's underlying file as being equally worthy of all this fuss. Changes to the underlying file's permissions, for example, do not elicit any of this.

Strangely enough, according to out-of-the-box Emacs, a buffer's underlying file's disappearance also fails to warrant such a response!

To be more specific, Emacs remains perfectly quiet if one modifies a buffer whose underlying file has disappeared, and—even more surprising—, it remains equally silent when one then saves (C-x C-s) the modified buffer (thereby re-creating the underlying file).

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Q: How can I get Emacs to issue a warning and a confirmation request also when one attempts to modify a buffer whose underlying file has disappeared?

Answer 1

You can do this:

(add-hook 'before-save-hook 'foo)

(defun foo ()
  (when (and (buffer-file-name)  (not (file-exists-p (buffer-file-name))))
    (unless (y-or-n-p "No such file.  Create it?")
      (error "OK, not saved"))))

However, this could be regarded as a feature: Emacs creates a file for a buffer you ask it to save.

But if you don't like this behavior you might consider filing a bug report (enhancement request): M-x report-emacs-bug.

Answer 2

You can use the write-file-functions hook to do this. It's what's called an "abnormal" hook, which means its return value is taken into account, unlike before-save-hook. In this case, if it returns t, the `save-buffer' command returns without saving the buffer. E.g.

(add-hook 'write-file-functions #'foo)

(defun foo ()
  (when (and (buffer-file-name) (not (file-exists-p (buffer-file-name))))
    (not (y-or-n-p "No such file.  Create it?"))))