[RFC] Change property drawer syntax

[Nicolas Goaziou <mail@nicolasgoaziou.fr>]

Hello,

As discussed previously, I would like to modify property drawers syntax.
The change is simple: they must be located right after a headline and
its planning line, if any.  Therefore the following cases are valid

  * Headline
    :PROPERTIES:
    :KEY: value
    :END:

  * Headline
    SCHEDULED: <2014-10-14 mar.>
    :PROPERTIES:
    :KEY: value
    :END:

but, in the following case, the scheduled keyword will not be recognized

  * Headline
    :PROPERTIES:
    :KEY: value
    :END:
    SCHEDULED: <2014-10-14 mar.>

When not empty, they also have to contain only node properties.
Moreover, node properties' keys can only contain non-whitespace
characters and cannot end with a plus sign (which is used for
accumulation). Value can contain anything but a newline character.

Thus, the following property drawer is invalid

  * Headline
    :PROPERTIES:
    :KEY: value
    Some text.
    :END:

Any invalid property drawer becomes de facto a regular drawer, with
"PROPERTIES" as its name.

Besides defining exactly the syntax of property drawers, it should also
make the property API faster in some cases. Indeed, there's no need to
search through entire (possibly huge) sections in order to find
properties attached to a headline.

However, it will break some Org documents. In particular, TODO-states
changes are usually logged before any drawer, including properties
drawers. The following function repairs them.

  (defun org-repair-property-drawers ()
    "Fix properties drawers in current buffer.
  Ignore non Org buffers."
    (when (eq major-mode 'org-mode)
      (org-with-wide-buffer
       (goto-char (point-min))
       (let ((case-fold-search t)
             (inline-re (and (featurep 'org-inlinetask)
                             (concat (org-inlinetask-outline-regexp)
                                     "END[ \t]*$"))))
         (org-map-entries
          (lambda ()
            (unless (and inline-re (org-looking-at-p inline-re))
              (save-excursion
                (let ((end (save-excursion (outline-next-heading) (point))))
                  (forward-line)
                  (when (org-looking-at-p org-planning-line-re) (forward-line))
                  (when (and (< (point) end)
                             (not (org-looking-at-p org-property-drawer-re))
                             (save-excursion
                               (re-search-forward org-property-drawer-re end t)))
                    (insert (delete-and-extract-region
                             (match-beginning 0)
                             (min (1+ (match-end 0)) end)))
                    (unless (bolp) (insert "\n"))))))))))))

Internally, changes are somewhat invasive, as they include a rewrite of
almost all the property API. They also alter clocking, tags, todo
keywords, logging, initialization, hopefully in an invisible manner.

I pushed a new branch, "top-properties" in the repository for code
review and testing. It includes unit tests, documentation and an
ORG-NEWS entry.

Feedback welcome.


Regards,

-- 
Nicolas Goaziou                                                0x80A93738