bug#17343: 24.2; Exponential growth of files using raw-mode

[Jeremy Barbay]

Hi.

Following the short recipe below shows how a user saving files in "raw
mode" could end up with files doubling their size each time saved, if
following emacs' suggestion to save it in raw mode:

* Recipe:

  1. Save the following line in a file "testAccentsMinimal.txt"

  N����

  2. Repeatedly, 

     0) measure the size of the file (wc -c testAccentsMinimal.txt); 
     1) open emacs loading the file (emacs -q testAccentsMinimal.txt);
     2) insert and delete a character in it (manually);
     3) save it selecting the suggested raw encoding (manually);
     4) quit emacs (or force the reload of the file).

* Result:

  This should give something akin to the following, where one can see
  the size of the file growing exponentially with the number of savings.

  >wc -c testAccentsMinimal.txt ; emacs -q testAccentsMinimal.txt
  11 testAccentsMinimal.txt
  >wc -c testAccentsMinimal.txt ; emacs -q testAccentsMinimal.txt
  19 testAccentsMinimal.txt
  >wc -c testAccentsMinimal.txt ; emacs -q testAccentsMinimal.txt
  35 testAccentsMinimal.txt
  >wc -c testAccentsMinimal.txt ; emacs -q testAccentsMinimal.txt
  67 testAccentsMinimal.txt
  >wc -c testAccentsMinimal.txt ; emacs -q testAccentsMinimal.txt
  131 testAccentsMinimal.txt
  >wc -c testAccentsMinimal.txt ; emacs -q testAccentsMinimal.txt
  259 testAccentsMinimal.txt
  >wc -c testAccentsMinimal.txt ; emacs -q testAccentsMinimal.txt
  515 testAccentsMinimal.txt
  >wc -c testAccentsMinimal.txt ; emacs -q testAccentsMinimal.txt
  1027 testAccentsMinimal.txt
  >wc -c testAccentsMinimal.txt ; emacs -Q testAccentsMinimal.txt
  2051 testAccentsMinimal.txt

* (Tentative) Explanation:

  - Even though the file is saved in "raw" mode, it is read in another
    mode which prefix the "special" characters with a unicode code.
  - Due to symbols from incompatible encodings, emacs is confused about
    which encoding to use for saving and asks the user about it.

* Why it matters:

  - The faulty sequence above occured naturally from copy pasting from
    various webpages (containing accented characters) into the same
    document, and was identified when some files grew too large.  -
    Files (e.g. of notes) end up doubling in size at each edition, until
    they fill the memory and/or hard-drive, slow down the system and
    make Emacs complain about the size of the file.

* (Potential) Solutions:

  - when saving a file with conflicting encodings, instead of merely
    suggesting the raw encoding, add an option to "clean" the file
    instead of merely save it in raw mode, for instance by projecting
    the file to an encoding by deleting all symbols which are
    incompatible with it.

I think that I signaled this bug 1 year ago in Emacs 23 and was answered
at the time that this would be solved by the next version (24), but it
occured to me recently that this undesirable behavior was still there :(

I hope it helps.  
-- 
Jeremy                             (http://www.dcc.uchile.cl/~jbarbay)


In GNU Emacs 24.2.1 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2013-02-27 on raven
Windowing system distributor `The X.Org Foundation', version 11.0.11300000
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: en_US.UTF-8
  value of $LC_NUMERIC: en_US.UTF-8
  value of $LC_TIME: en_US.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Text

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo> 
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo> 
<menu-bar> <help-menu> <send-emacs-bug-report>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Loading vc-git...done
Scanning for dabbrevs...done
dabbrev-expand: No dynamic expansion for `Expo' found

Load-path shadows:
None found.

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