atom: implement message threading per RFC 4685

This will allows certain feed readers to render a message thread
as described in <https://www.jwz.org/doc/threading.html>.

Feed readers with knowledge of of RFC 4685 are unknown to us at
this time, but perhaps this will encourage future implementations.

Existing feed readers I've tested (newsbeuter, feed2imap) seem
to ignore these tags gracefully without degradation.

1 files changed, 26 insertions, 14 deletions

diff --git a/lib/PublicInbox/WwwAtomStream.pm b/lib/PublicInbox/WwwAtomStream.pm
index 5720384c..a6817b31 100644
--- a/lib/PublicInbox/WwwAtomStream.pm
+++ b/lib/PublicInbox/WwwAtomStream.pm
@@ -9,10 +9,10 @@ use warnings;
 # FIXME: locale-independence:
 use POSIX qw(strftime);
 use Date::Parse qw(strptime);
-
+use Digest::SHA qw(sha1_hex);
 use PublicInbox::Address;
 use PublicInbox::Hval qw(ascii_html);
-use PublicInbox::MID qw/mid_clean mid2path mid_escape/;
+use PublicInbox::MID qw/mid_clean mid_escape/;
 
 # called by PSGI server after getline:
 sub close {}
@@ -72,7 +72,8 @@ sub atom_header {
         my $mtime = (stat($ibx->{mainrepo}))[9] || time;
 
         qq(<?xml version="1.0" encoding="us-ascii"?>\n) .
-        qq{<feed\nxmlns="http://www.w3.org/2005/Atom">} .
+        qq(<feed\nxmlns="http://www.w3.org/2005/Atom"\n) .
+        qq(xmlns:thr="http://purl.org/syndication/thread/1.0">) .
         qq{$title} .
         qq(<link\nrel="alternate"\ntype="text/html") .
                 qq(\nhref="$base_url"/>) .
@@ -81,22 +82,33 @@ sub atom_header {
         feed_updated(gmtime($mtime));
 }
 
+sub mid2uuid ($) {
+        my ($mid) = @_;
+        utf8::encode($mid); # really screwed up In-Reply-To fields exist
+        $mid = sha1_hex($mid);
+        my $h = '[a-f0-9]';
+        my (@uuid5) = ($mid =~ m!\A($h{8})($h{4})($h{4})($h{4})($h{12})!o);
+        'urn:uuid:' . join('-', @uuid5);
+}
+
 # returns undef or string
 sub feed_entry {
         my ($self, $mime) = @_;
         my $ctx = $self->{ctx};
         my $hdr = $mime->header_obj;
         my $mid = mid_clean($hdr->header_raw('Message-ID'));
-
-        my $uuid = mid2path($mid);
-        $uuid =~ tr!/!!d;
-        my $h = '[a-f0-9]';
-        my (@uuid5) = ($uuid =~ m!\A($h{8})($h{4})($h{4})($h{4})($h{12})!o);
-        $uuid = 'urn:uuid:' . join('-', @uuid5);
-
-        $mid = PublicInbox::Hval->new_msgid($mid);
-        my $href = $ctx->{feed_base_url} . $mid->{href}. '/';
-
+        my $irt = PublicInbox::View::in_reply_to($hdr);
+        my $uuid = mid2uuid($mid);
+        my $base = $ctx->{feed_base_url};
+        if (defined $irt) {
+                my $irt_uuid = mid2uuid($irt);
+                $irt = mid_escape($irt);
+                $irt = qq(<thr:in-reply-to\nref="$irt_uuid"\n).
+                        qq(href="$base$irt/"/>);
+        } else {
+                $irt = '';
+        }
+        my $href = $base . mid_escape($mid) . '/';
         my $date = $hdr->header('Date');
         my @t = eval { strptime($date) } if defined $date;
         @t = gmtime(time) unless scalar @t;
@@ -124,7 +136,7 @@ sub feed_entry {
                 PublicInbox::View::multipart_text_as_html($mime, $href) .
                 '</pre>' .
                 qq!</div></content><link\nhref="$href"/>!.
-                "<id>$uuid</id></entry>";
+                "<id>$uuid</id>$irt</entry>";
 }
 
 sub feed_updated {