bug#63378: A single X-Debbugs-CC header must be used

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hi,

After some tests, it appears that a single X-Debbugs-CC header must be
used, otherwise the last one is the one that prevails.  This matches my
reading of the 'process' script of the GNU Debbugs instance [0], and
thus must conform to the same email rules outlined in RFC5322 for the To
or CC fields (only one such header must be used); multiple values can be
separated by a comma. [1]

--8<---------------cut here---------------start------------->8---
my %header;

for my $hdr (@headerlines) {
    $hdr = decode_rfc1522($hdr);
    $_ = $hdr;
    s/\n\s/ /g;
    &finish if m/^x-loop: (\S+)$/i && $1 eq "$gMaintainerEmail";
    my $ins = !m/^subject:/i && !m/^reply-to:/i && !m/^return-path:/i
           && !m/^From / && !m/^X-Debbugs-/i && !m/^cc:/i && !m/^to:/i;
    $fwd .= $hdr."\n" if $ins;
    # print DEBUG ">$_<\n";
    if (s/^(\S+):\s*//) {
	my $v = lc $1;
	print DEBUG ">$v=$_<\n";
        ## There may be multiple To: or Cc: headers (see bug#5996).
        if ( ($v eq 'to' || $v eq 'cc') &&
	     defined $header{$v} && length($header{$v}) ) {
            $header{$v} = $header{$v} . ', ' . $_ if length($_);
        } else {
            $header{$v} = $_;
        }
    } else {
	print DEBUG "!>$_<\n";
    }
}
$header{'message-id'} = '' if not defined $header{'message-id'};
--8<---------------cut here---------------end--------------->8---

Only 'to' or 'cc' multiple headers are coalesced into one; otherwise the
$header specific key (for a given header) is overridden to the last
value encountered at line '$header{$v} = $_;', IIUC.

Our teams.scm script should be adjusted to produce a single X-Debbugs-CC
header with comma-separated values.

[0]  https://gitlab.com/npostavs/debbugs/-/blob/gnu-reconstruction/scripts/process#L171
[1]  https://datatracker.ietf.org/doc/rfc5322/, 3.6 Field Definitions

-- 
Thanks,
Maxim