view: account for missing In-Reply-To header

Some mail clients do not generate In-Reply-To headers,
but do generate a proper References header.

This matches the behavior of Mail::Thread as well
as our SearchIdx code to link threads in the Xapian DB.

1 files changed, 18 insertions, 4 deletions

diff --git a/lib/PublicInbox/View.pm b/lib/PublicInbox/View.pm
index a3df319e..d2131245 100644
--- a/lib/PublicInbox/View.pm
+++ b/lib/PublicInbox/View.pm
@@ -46,6 +46,19 @@ sub feed_entry {
         PRE_WRAP . multipart_text_as_html($mime, $full_pfx) . '</pre>';
 }
 
+sub in_reply_to {
+        my ($header_obj) = @_;
+        my $irt = $header_obj->header('In-Reply-To');
+
+        return mid_clean($irt) if (defined $irt);
+
+        my $refs = $header_obj->header('References');
+        if ($refs && $refs =~ /<([^>]+)>\s*\z/s) {
+                return $1;
+        }
+        undef;
+}
+
 # this is already inside a <pre>
 sub index_entry {
         my ($fh, $mime, $level, $state) = @_;
@@ -74,7 +87,8 @@ sub index_entry {
         my $root_anchor = $state->{root_anchor};
         my $path = $root_anchor ? '../../' : '';
         my $href = $mid->as_href;
-        my $irt = $header_obj->header('In-Reply-To');
+        my $irt = in_reply_to($header_obj);
+
         my ($anchor_idx, $anchor);
         if (defined $irt) {
                 $anchor_idx = anchor_for($irt);
@@ -463,7 +477,7 @@ sub _parent_headers_nosrch {
         my ($header_obj) = @_;
         my $rv = '';
 
-        my $irt = $header_obj->header('In-Reply-To');
+        my $irt = in_reply_to($header_obj);
         if (defined $irt) {
                 my $v = PublicInbox::Hval->new_msgid($irt);
                 my $html = $v->as_html;
@@ -476,7 +490,7 @@ sub _parent_headers_nosrch {
         if ($refs) {
                 # avoid redundant URLs wasting bandwidth
                 my %seen;
-                $seen{mid_clean($irt)} = 1 if defined $irt;
+                $seen{$irt} = 1 if defined $irt;
                 my @refs;
                 my @raw_refs = ($refs =~ /<([^>]+)>/g);
                 foreach my $ref (@raw_refs) {
@@ -526,7 +540,7 @@ sub html_footer {
         my $idx = $standalone ? " <a\nhref=\"$upfx\">index</a>" : '';
         if ($idx && $srch) {
                 my $next = thread_inline(\$idx, $ctx, $mime, $full_pfx);
-                $irt = $mime->header('In-Reply-To');
+                $irt = in_reply_to($mime->header_obj);
                 if (defined $irt) {
                         $irt = PublicInbox::Hval->new_msgid($irt);
                         $irt = $irt->as_href;