Detect changed files by content

Question

When working with files changed on disk, there are two warnings

... changed on disk; really edit the buffer?
... has changed since visited or saved. Save anyway?

They're useful, but get very impractical when switching branches fort and back (as I commonly do with git).

Is it possible to make Emacs determine that the content hasn't actually changed (or has been restored to a state equals to what's in its Emacs buffer)?

The fix given in the nearly-duplicate solves the first part only. As @Stefan wrote, the second is easy using (set-visited-file-modtime).

possible duplicate http://emacs.stackexchange.com/questions/21137/fix-changed-on-disk-reread-from-disk-when-file-has-not-changed — JeanPierre, Aug 25 '16 at 16:28
FWIW, I think it'd be good to have such a behavior in Emacs, so I'd welcome a patch to Emacs which does that cleanly. It might be easier to make it work well by patching Emacs than with a hack to `ask-user-about-supersession-threat`. — Stefan, Aug 28 '16 at 22:55
@Stefan A patch would be fine, but I'm not good enough in "emacsing" yet. Anyway, I'd suggest to rewrite the 50 line monster called `basic-save-buffer` first. — maaartinus, Aug 28 '16 at 23:41

score 1 · Accepted Answer · answered Aug 28 '16 at 22:59

You might be able to tweak ask-user-about-supersession-threat to just call set-visited-file-modtime when the buffer and the file have the same contents, so that Emacs refreshes its notion of "which was the state of the file when I read it".

Checking "same contents" basically involves saving the buffer into an auxiliary file, and then comparing the two files (or loading the file into a new buffer and comparing the two buffers).

score 1 · Answer 2 · edited Apr 13 '17 at 12:52

@Stefan's answer solved it, I'm just adding the details. I'm using the following modification of compare-buffer-with-file from the related question:

(save-restriction
  (widen)
  (let ((original-buffer (current-buffer))
        (original-size (point-max)))
    (and
     (with-temp-buffer
       (insert-file-contents filename)
       (zerop (compare-buffer-substrings original-buffer 1 original-size
                                        (current-buffer) 1 (point-max))))
     (progn (set-visited-file-modtime) t)))))

Detect changed files by content

2 Answers2