[O] org-src--contents-for-write-back : preserve original major-mode, and avoid mixing tabs and spaces in org-mode buffers

[Brent Goodrick]

Hi,

I found a bug in org-mode where emacs-lisp code that is in a
already-indented source block in an org-mode buffer is improperly
indented when editing it via C-c '. Take the following contrived
example emacs-lisp source code:

 1. Here is a list item with a emacs-lisp source block:
    #+BEGIN_SRC emacs-lisp :results value
      (let ((uuid "c2327c73-6da3-4421-8bda-194783a00e8f"))
        (progn
          (let ((xxx 'yyy))
            (let ((xxx 'yyy))
              (while t
                (message "infinite loop"))))))
    #+END_SRC

After C-c ', indenting it, and C-c ' again, it renders as
follows (tabs converted to spaces for this email, since I have
`indent-tabs-mode' set to t in my emacs-lisp mode, which
is the Emacs default):

 1. Here is a list item with a emacs-lisp source block:
    #+BEGIN_SRC emacs-lisp :results value
      (let ((uuid "c2327c73-6da3-4421-8bda-194783a00e8f"))
        (progn
          (let ((xxx 'yyy))
        (let ((xxx 'yyy))
          (while t
            (message "infinite loop"))))))
    #+END_SRC

Notice how the indentation looks bad due to the mixture of tabs
and spaces.

The bug is in the `org-src--contents-for-write-back' function. It
uses a temp buffer. The temp buffer's major-mode is left to be
the default, which is fundamental-mode, which knows nothing about
how to indent lisp code properly. So in the fix below, I run the
major-mode function from the original buffer. But even with that
fix, the indentation must also use spaces in order to avoid
mixing tabs and spaces in the resulting Org buffer.  My fix,
shown below, is to initialize the major-mode, and set
`indent-tabs-mode' to nil, before it runs the `write-back'
code.

See "CHANGE" comments for the changes made:

(defun org-src--contents-for-write-back ()
  "Return buffer contents in a format appropriate for write back.
        Assume point is in the corresponding edit buffer."
  (let ((indentation (or org-src--block-indentation 0))
    (preserve-indentation org-src--preserve-indentation)
    (contents (org-with-wide-buffer (buffer-string)))
    (write-back org-src--allow-write-back))
    ;; CHANGE: Save off the original mode into orig-major-mode:
    (let ((orig-major-mode major-mode))
      (with-temp-buffer
        (insert (org-no-properties contents))
        ;; CHANGE: Switch to the original mode to obtain mode-specific indentation behavior:
        (funcall orig-major-mode)
        ;; CHANGE: Do not mix tabs and spaces during indentation:
        (setq indent-tabs-mode nil)
    (goto-char (point-min))
    (when (functionp write-back) (funcall write-back))
    (unless (or preserve-indentation (= indentation 0))
      (let ((ind (make-string indentation ?\s)))
        (goto-char (point-min))
        (while (not (eobp))
          (when (looking-at-p "[ \t]*\\S-") (insert ind))
          (forward-line))))
    (buffer-string)))))

If the above change seems correct, can someone apply it to org-mode?

Thanks,
Brent Goodrick