Atomic file replacement in Emacs

Atomic file replacement in Emacs

[Zhu Zihao]

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It's a very common case to persist data structure using `pp` or `prin1`.
However, the core function of saving a buffer, `write-region` is not
atomic. If your data structure is very very large, and something
happened(e.g. you sent SIGKILL to Emacs, or the power of your computer
was cut) during saving, it will probably result in a broken file.

File system should provide some atomic operation to deal with these
conditions. There's a "trick" to do this job -- write to a temp file in
same directory, then use libc's rename to override origin file. Because
rename is atomic in same mount point. Our file can be replaced
atomically.

I'd like to suggest a implementation draft here. If you think this is good, I
can help make a patch to add it to files.el

~~~

(defmacro with-atomic-file-replacement (file &rest body)
  "Open FILE in a temp buffer and evaluate BODY in it, replace the FILE with the buffer content atomically"
  (declare (debug t) (indent 1))
  `(call-with-atomic-file-replacement ,file (lambda () ,@body)))

(defun call-with-atomic-file-replacement (file func)
  ;; The atomic of rename in libc was guaranteed by POSIX
  ;; standard, it will fail if we rename file between two mount point. So create
  ;; temp file in the same dir  of FILE.
  (let* ((full (expand-file-name file))
         (dir (file-name-directory full))
         (tmpfile (make-temp-file dir)))
    (with-temp-buffer
      (setq buffer-file-name tmpfile)
      (insert-file-contents-literally full)
      (funcall func)
      (basic-save-buffer)
      (rename-file tmpfile full t))))



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Re: Atomic file replacement in Emacs

[Andreas Schwab]

There is already file-precious-flag.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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"And now for something completely different."

Re: Atomic file replacement in Emacs

[Stefan Monnier]

> There is already file-precious-flag.

This is used by `save-buffer` but not by `write-region`, so it only
works if you use a proper "file buffer".


        Stefan

Re: Atomic file replacement in Emacs

[Andreas Schwab]

On Okt 21 2020, Stefan Monnier wrote:

>> There is already file-precious-flag.
>
> This is used by `save-buffer` but not by `write-region`, so it only
> works if you use a proper "file buffer".

It is used by basic-save-buffer.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

Re: Atomic file replacement in Emacs

[Stefan Monnier]

>>> There is already file-precious-flag.
>> This is used by `save-buffer` but not by `write-region`, so it only
>> works if you use a proper "file buffer".
> It is used by basic-save-buffer.

Duh!  Indeed, I didn't pay enough attention to the code, I assumed it
was wrapping `write-region`.


        Stefan

Re: Atomic file replacement in Emacs

[Zhu Zihao]

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Andreas Schwab writes:

> There is already file-precious-flag.
>
> Andreas.


Nice! Thank you for remind me this good feature.

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Re: Atomic file replacement in Emacs

[Zhu Zihao]

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This looks good, but there's still a question.

`file-precious-flag` is handled by `basic-save-buffer`. This is a large
function designed for daily save usage, it will touch recentf and vc.
But for a cache/persistent file, we don't want to make big noise, we
just want to save it silently. We may need something like

(write-region nil nil "XXXX" nil nil)

Can we simplify this case?


Andreas Schwab writes:

> There is already file-precious-flag.
>
> Andreas.


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Zihao

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