bug#35507: Gnus mojibakifies UTF-8 text/x-patch attachments from Thunderbird

[Paul Eggert]

Package: emacs,gnus
Version: 27

When I send email from Thunderbird with a patch attachment, Thunderbird
puts something like the following into the email:

  --------------AA6C74B60F40E0D600CCD03A
  Content-Type: text/x-patch;
   name="0001-Fix-decode-time-encode-time-roundtrip-on-macOS.patch"
  Content-Transfer-Encoding: 8bit
  Content-Disposition: attachment;
   filename*0="0001-Fix-decode-time-encode-time-roundtrip-on-macOS.patch"

  From 325f51c84d9ad4d9776784bd324b347ffe4fe51b Mon Sep 17 00:00:00 2001
  From: Paul Eggert <eggert@cs.ucla.edu>
  Date: Tue, 30 Apr 2019 10:45:48 -0700
  Subject: [PATCH] Fix decode-time/encode-time roundtrip on macOS
  MIME-Version: 1.0
  Content-Type: text/plain; charset=UTF-8
  Content-Transfer-Encoding: 8bit

  * src/timefns.c (Fencode_time): Ignore DST flag when the zone is
  ...

The attachment has a text/* media type but it has no charset parameter.
The patch itself (output by git format-patch) says its charset is UTF-8.
Unfortunately, Gnus doesn't recognize the patch as UTF-8 and so
mishandles the non-ASCII characters in the attachment. To reproduce the
problem, read this email with Gnus; the full attachment is attached to
this email in the Thunderbird way.

Although Internet RFC 2046 section 4.1.2 says the default charset for
text/* media types is US-ASCII, Internet RFC 6557 section 3 amends this
to say that registered text/* media types should require a charset
specification (or should say it's not needed because the payload has
that info, which obviously doesn't apply here). It later says that if
there is a strong reason to have a charset default, the default should
be UTF-8.

Unfortunately Gnus apparently doesn't default to UTF-8 for such
attachments, which means that sending a text/x-patch attachment from
Thunderbird to Gnus messes up if the attachment contains non-ASCII
characters. This has been causing problems on the Emacs mailing list for
years and it bit a correspondent of mine again today; see
<https://debbugs.gnu.org/cgi/bugreport.cgi?bug=35502#35>.

I have filed a Thunderbird bug report for this, as Thunderbird should
specify a charset; see
<https://bugzilla.mozilla.org/show_bug.cgi?id=1167982>. However, Gnus
should be a polite citizen and handle these attachments nicely rather
than converting the non-ASCII UTF-8 characters to mojibake.

0001-Fix-decode-time-encode-time-roundtrip-on-macOS.patch
Description: Text Data