bug#15329: saveplace restores dired positions to random places

[Juri Linkov <juri@jurta.org>]

> Remembering dired positions is a good feature, but
> unfortunately in its current implementation is useless.
>
> The problem is that often the position of point in Dired
> is restored to random places because in Dired buffers
> it depends on many irrelevant ever-changing factors
> such as the total size of all files in the directory
> and available space (thus the line with this information
> changes its length), sizes of each file above the restored
> file in the Dired buffer also accounts to the incorrect position
> of point, also added/deleted files in Dired change the restored
> position to random places, different sorting order, etc.
>
> An obvious solution to this problem is to save the
> current file name in Dired instead of its point.
>
> It seems saving information other than point doesn't contradict
> the purpose of saveplace.el that automatically saves places where
> visiting them later automatically moves point to the saved position.
> Also elements of `save-place-alist' are (FILENAME . POSITION),
> not more limited (FILENAME . POINT).  So saving a string to
> POSITION would be consistent with the design of saveplace.el.

I realized this is not backward-compatible change, i.e.
older Emacs versions won't be able to read saved places
in the new format.  Unfortunately, Dired filename positions
can't be added to the current format that uses an alist
with cons pair cells.  I mean (FILENAME . POINT)
can't be changed to (FILENAME POINT DIRED-FILENAME-POSITION)
because older Emacs versions will fail to read them.

The only solution that I see is to save two alists to the places file:

((FILENAME1 . POINT1)
 (FILENAME2 . POINT2)
 ...)
((FILENAME1 DIRED-FILENAME-POSITION1)
 (FILENAME2 DIRED-FILENAME-POSITION2)
 ...)

Then the current code:

  (with-demoted-errors
    (car (read-from-string
      (buffer-substring (point-min) (point-max)))))

will read the first alist without problems,
and additional code in newer versions like

  (with-demoted-errors
    (cadr (read-from-string
      (buffer-substring (point-min) (point-max)))))

will read the second alist with more information
about the context of saved places (like bookmarks).