overidx: preserve `tid' column on re-indexing

Otherwise, walking backwards through history could mean the root
message in a thread forgets its `tid' and it prevents messages
from being looked up by it.

This bug was hidden by the fact that `sid' matches were often
good enough to link threads together.

1 files changed, 9 insertions, 2 deletions

diff --git a/lib/PublicInbox/OverIdx.pm b/lib/PublicInbox/OverIdx.pm
index 62fec0da..cc9bd7d4 100644
--- a/lib/PublicInbox/OverIdx.pm
+++ b/lib/PublicInbox/OverIdx.pm
@@ -79,8 +79,15 @@ sub mid2id {
 }
 
 sub delete_by_num {
-        my ($self, $num) = @_;
+        my ($self, $num, $tid_ref) = @_;
         my $dbh = $self->{dbh};
+        if ($tid_ref) {
+                my $sth = $dbh->prepare_cached(<<'', undef, 1);
+SELECT tid FROM over WHERE num = ? LIMIT 1
+
+                $sth->execute($num);
+                $$tid_ref = $sth->fetchrow_array; # may be undef
+        }
         foreach (qw(over id2num)) {
                 $dbh->prepare_cached(<<"")->execute($num);
 DELETE FROM $_ WHERE num = ?
@@ -262,7 +269,7 @@ sub add_over {
         my $vivified = 0;
 
         $self->begin_lazy;
-        $self->delete_by_num($num);
+        $self->delete_by_num($num, \$old_tid);
         foreach my $mid (@$mids) {
                 my $v = 0;
                 each_by_mid($self, $mid, ['tid'], sub {