nntp: append Archived-At and List-Archive headers

For readers using NNTP, we should do our best to advertise the
clonable HTTP/HTTPS URLs and the message permalink URL for
ease-of-referencing messages, since we don't want the NNTP server
and it's sequential article numbers to be relied on.

1 files changed, 25 insertions, 2 deletions

diff --git a/lib/PublicInbox/NNTP.pm b/lib/PublicInbox/NNTP.pm
index a4cf25e2..3e0faaf9 100644
--- a/lib/PublicInbox/NNTP.pm
+++ b/lib/PublicInbox/NNTP.pm
@@ -15,6 +15,7 @@ use Email::MIME;
 use Data::Dumper qw(Dumper);
 use POSIX qw(strftime);
 use Time::HiRes qw(clock_gettime CLOCK_MONOTONIC);
+use URI::Escape qw(uri_escape_utf8);
 use constant {
         r501 => '501 command syntax error',
         r221 => '221 Header follows',
@@ -426,6 +427,29 @@ sub cmd_quit ($) {
         undef;
 }
 
+sub header_append ($$$) {
+        my ($hdr, $k, $v) = @_;
+        my @v = $hdr->header($k);
+        foreach (@v) {
+                return if $v eq $_;
+        }
+        $hdr->header_set($k, @v, $v);
+}
+
+sub set_nntp_headers {
+        my ($hdr, $ng, $n, $mid) = @_;
+
+        # clobber some
+        $hdr->header_set('Newsgroups', $ng->{name});
+        $hdr->header_set('Xref', xref($ng, $n));
+        header_append($hdr, 'List-Post', "<mailto:$ng->{address}>");
+        if (my $url = $ng->{url}) {
+                $mid = uri_escape_utf8($mid);
+                header_append($hdr, 'Archived-At', "<$url$mid/>");
+                header_append($hdr, 'List-Archive', "<$url>");
+        }
+}
+
 sub art_lookup ($$$) {
         my ($self, $art, $set_headers) = @_;
         my $ng = $self->{ng};
@@ -468,8 +492,7 @@ found:
         return $err unless $s;
         my $lines;
         if ($set_headers) {
-                $s->header_set('Newsgroups', $ng->{name});
-                $s->header_set('Xref', xref($ng, $n));
+                set_nntp_headers($s->header_obj, $ng, $n, $mid);
                 $lines = $s->body =~ tr!\n!\n!;
 
                 # must be last