How to never auto-save large files — or, how to run a function whenever any buffer starts — or, how to abort loading a minor mode in one of its hooks

Question

I want to tell Emacs to never auto-save files bigger than ~1MB. I know how to turn off auto-save-mode when the file size is above a certain limit, but not how to make that function run automatically in every buffer. I don't want to add a mode hook for every major mode.

This was my naive attempt to use auto-save-mode-hook to solve the problem:

(add-hook 'auto-save-mode-hook
          (lambda ()
            (if (buffer-file-name)
                (if (> (file-attribute-size
                        (file-attributes
                         (buffer-file-name)))
                       1000000)
                    (auto-save-mode 0))
              (auto-save-mode 0))))

I get an error about excessively deep nesting; presumably, running (auto-save-mode 0) is executing auto-save-mode-hook again. I'm not quite sure why this would happen, given that it is turning off the mode, not turning it on, but clearly this approach is not going to work as I wrote it. Does anyone have either an alternate strategy or a fix to mine?

Answer 1

presumably, running (auto-save-mode 0) is executing auto-save-mode-hook again. I'm not quite sure why this would happen, given that it is turning off the mode, not turning it on,

C-h f auto-save-mode: "... The mode’s hook is called both when the mode is enabled and when it is disabled. ..."

So your code will run again and again.

---

You may need to check whether the mode is on before toggling it.

Just draft:

(add-hook 'auto-save-mode-hook
          #'(lambda ()
              (when (and buffer-auto-save-file-name
                         (>= buffer-saved-size 0))
                ;; OK, auto-save-mode is enabled.
                ;; Maybe you want to disable it.
                (when (Do_you_want_to_turn_off_it?)
                  (auto-save-mode -1)))))