Add customization point for buffer reformatting

User might want to preserve some properties of the buffer after
reformatting which can be done by customizing
`format-all-reformat-buffer-function`.

format-all.el

@@ -188,6 +188,19 @@
  :type 'boolean
  :group 'format-all)
 
+(defcustom format-all-reformat-buffer-function
+ 'format-all-fast-reformat-buffer
+ "Function to call when reformatting the buffer.
+
+The function shall take one argument which is string with reformatted
+content and it will rewrite current buffer accordingly. If nil
+`format-all-fast-reformat-buffer' is used as default."
+ :type '(radio (function-item format-all-fast-reformat-buffer)
+ (function-item format-all-point-and-markers-preserving-reformat-buffer)
+ (function-item format-all-non-destructive-reformat-buffer)
+ (function :tag "Another function" format-all-fast-reformat-buffer))
+ :group 'format-all)
+
 (defvar format-all-after-format-functions nil
  "Hook run after each time `format-all-buffer' has formatted a buffer.
 
@@ -977,6 +990,40 @@ Consult the existing formatters for examples of BODY."
  (:languages "Python")
  (:format (format-all--buffer-easy executable)))
 
+(defun format-all-fast-reformat-buffer (content)
+ "Reformat buffer but don't preserve any additional properties of the buffer."
+ (erase-buffer)
+ (insert content))
+
+(defun format-all-point-and-markers-preserving-reformat-buffer (content)
+ "Reformat buffer and try to preserve point and markers."
+ (let ((helper-file (make-temp-file "*format-all-output*")))
+ (unwind-protect
+ (progn
+ (with-temp-file helper-file
+ (insert content))
+ (insert-file-contents helper-file nil nil nil t))
+ (delete-file helper-file))))
+
+(defun format-all-non-destructive-reformat-buffer (content)
+ "Reformat buffer so that it's as non-destructive as possible.
+
+The function might be slow in pathological cases."
+ (let ((helper-buffer (generate-new-buffer "*format-all-output*")))
+ (unwind-protect
+ (progn
+ (with-current-buffer helper-buffer
+ (insert content))
+ (replace-buffer-contents helper-buffer))
+ (kill-buffer helper-buffer))))
+
+(defun format-all--invoke-reformat-buffer-function (content)
+ "Internal helper function to reformat buffer with customized function."
+ (let ((func (or format-all-reformat-buffer-function
+ #'format-all-fast-reformat-buffer)))
+ (let ((inhibit-read-only t))
+ (funcall func content))))
+
 (defun format-all--language-id-buffer ()
  "Return the language used in the current buffer, or NIL.
 
@@ -1077,9 +1124,7 @@ LANGUAGE is the language ID of the current buffer, from
  ((not (equal f-output t))
  (setq reformatted-by
  (append reformatted-by (list f-name)))
- (let ((inhibit-read-only t))
- (erase-buffer)
- (insert f-output))
+ (format-all--invoke-reformat-buffer-function f-output)
  (setq f-status :reformatted)))
  (run-hook-with-args 'format-all-after-format-functions
  f-name f-status)