bug#74624: 29.4.50; Gnus cannot parse some filenames(UTF8) in an attachment

[Visuwesh <visuweshm@gmail.com>]

[சனி நவம்பர் 30, 2024] Eli Zaretskii wrote:

>> From: Konstantin <reich-cv@yandex.ru>
>> Date: Sat, 30 Nov 2024 18:59:25 +0300
>> 
>> >From time to time i get emails with attachments from my colleges, which they send from
>> "Roundcube" web-interface. 
>> 
>> Often, i cannot open these attachments by =RET=(gnus-article-press-button)
>> or save them =o=(gnus-mime-save-part) with correct name.
>> (interestingly =X-m=(gnus-summary-save-parts) works correctly)
>> 
>> The reason is gnus cannot parse correctly some attached filenames.
>> 
>> The example of such attachment (I took it from gnus-summary-show-raw-article)
>> 
>>  --=_d38c0abddd645077f401d42fa430d9d5
>> Content-Transfer-Encoding: base64
>> Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document;
>>  name="=?UTF-8?Q?=D0=9E=D0=B1=D0=B7=D0=BE=D1=80_2024_=28=D0=BD=D0=B0_=2Ed?=
>>  =?UTF-8?Q?ocx?="
>> Content-Disposition: attachment;
>>  filename*0*=UTF-8''%D0%9E%D0%B1%D0%B7%D0%BE%D1%80%202024%20%28%D0%BD%D0;
>>  filename*1*=%B0%20.docx;
>>  size=10
>> 
>> c2Rmc2FmYXNmCg==
>> --=_d38c0abddd645077f401d42fa430d9d5--
>> 
>> I have tried to examine the reason. As i see it,  
>> gnus-data for such attachment is formed incorrectly:
>> 
>> (#<buffer  *mm*-480444>
>>      ("application/vnd.openxmlformats-officedocument.word..."
>>      (name . "Обзор 2024 (на .docx"))
>>      base64 nil
>>      ("attachment" (size . "10")
>>      (filename . "Обзор 2024 (н\320")) nil nil nil)
>> 
>> One can see that the filename is broken.
>> It should be "Обзор 2024 (на .docx" just like the name.
>
> It looks like Gnus fails to decipher the file name when it is split in
> the middle of a UTF-8 sequence.
>
> I don't know Gnus.  If you can help me by showing where the value of
> 'gnus-data property is calculated, I might be able to find the bug and
> suggest a fix.

The decoding of the filename in the Content-Disposition header is done
in mm-dissect-buffer by calling mail-header-parse-content-disposition.
Specifically, rfc2231-parse-string.  The following patch fixes the issue
on my end:

diff --git a/lisp/mail/rfc2231.el b/lisp/mail/rfc2231.el
index 33324cafb5b..632e270a922 100644
--- a/lisp/mail/rfc2231.el
+++ b/lisp/mail/rfc2231.el
@@ -193,7 +193,7 @@ rfc2231-parse-string
 		     (push (list attribute value encoded) cparams))
 		    ;; Repetition of a part; do nothing.
 		    ((and elem
-			  (null number))
+			  (null part))
 		     )
 		    ;; Concatenate continuation parts.
 		    (t

NUMBER is the variable used during the parsing portion of the function
in the big condition-case form above the cl-loop form which the patch
modifies.  In the header below

    Content-Disposition: attachment;
      filename*0*=UTF-8''%D0%9E%D0%B1%D0%B7%D0%BE%D1%80%202024%20%28%D0%BD%D0;
      filename*1*=%B0%20.docx;
      size=10

the function first parses filename*0* and here NUMBER is 0, then
filename*1* and here NUMBER is 1.  By the time it finishes parsing size,
NUMBER is set to nil.  The loop should use the value of NUMBER pushed to
PARAMETERS as the 3rd element (referred to as `part' by the cl-loop
form) instead of whatever value NUMBER happened to be when we parsed the
last element.