Re: mm-with-unibyte-current-buffer is bad for Emacs 23

[Katsumi Yamaoka <yamaoka@jpl.org>]

>>>>> In <v9r75nhaa5.fsf@marauder.physik.uni-ulm.de> Reiner Steib wrote:

> On Tue, Feb 28 2006, Katsumi Yamaoka wrote:

>> The macro `mm-with-unibyte-current-buffer' is used here and
>> there in Gnus.  In Emacs 23, I realized there is a possibility
>> that it breaks non-ASCII text.

> Unless you can fix it, please put a note into the code like...

> ;; FIXME: ... Emacs 23 (unicode)

If anything, we should not use '(set-buffer-multibyte t)' in
unibyte buffers which aren't empty if there's no special purpose.
For instance, the following code might work in a certain limited
condition (it is for decoding encoded text and displaying it).

  (set-buffer-multibyte t)
  (decode-coding-region (point-min) (point-max) 'CODING-SYSTEM)

But we need to change the order as follows for Emacs 23.

  (decode-coding-region (point-min) (point-max) 'CODING-SYSTEM)
  (set-buffer-multibyte t)

However, it might become not working when the Emacs 23 spec will
change in the future.  I got a suggestion from Kenichi Handa
yesterday.  The best way to do such a thing is to manipulate
data outside the buffer.  For instance:

  (insert
   (prog1
       (decode-coding-string (buffer-string) 'CODING-SYSTEM)
     (erase-buffer)
     (set-buffer-multibyte t)))

I don't know very much how the macro is used in Gnus modules,
and I don't have a capacity to examine all of them either.  So,
I've added the following note to the docstring.

#v+
NOTE: Use this macro with caution in multibyte buffers (it is not
worth using this macro in unibyte buffers of course).  Use of
`(set-buffer-multibyte t)', which is run finally, is generally
harmful since it is likely to modify existing data in the buffer.
For instance, it converts "\300\255" into "\255" in Emacs 23.
#v-