searchidx: do not modify Xapian DB while iterating

Iterating through a list of documents while modifying them does
not seem to be supported in Xapian and it can trigger
DatabaseCorruptError exceptions.  This only worked with past
datasets out of dumb luck.  With the work-in-progress "v2"
public-inbox layout, this problem might become more visible
as the "thread skeleton" is partitioned out to a separate,
smaller Xapian database.

I've reproduced the problem on both Debian 8.x and 9.x with
Xapian 1.2.19 (chert backend) and 1.4.3 (glass backend)
respectively.

diff --git a/lib/PublicInbox/SearchIdx.pm b/lib/PublicInbox/SearchIdx.pm
index 3259413789a1b4e3da5099fdf0e6a5146f209e5d..f4238fe8d5380aa1c2eecaaab0d01209b3418650 100644 (file)
--- a/lib/PublicInbox/SearchIdx.pm
+++ b/lib/PublicInbox/SearchIdx.pm
@@ -740,15 +740,22 @@ sub create_ghost {
 sub merge_threads {
        my ($self, $winner_tid, $loser_tid) = @_;
        return if $winner_tid == $loser_tid;
-       my ($head, $tail) = $self->find_doc_ids('G' . $loser_tid);
        my $db = $self->{xdb};
 
-       for (; $head != $tail; $head->inc) {
-               my $docid = $head->get_docid;
-               my $doc = $db->get_document($docid);
-               $doc->remove_term('G' . $loser_tid);
-               $doc->add_term('G' . $winner_tid);
-               $db->replace_document($docid, $doc);
+       my $batch_size = 1000; # don't let @ids grow too large to avoid OOM
+       while (1) {
+               my ($head, $tail) = $self->find_doc_ids('G' . $loser_tid);
+               return if $head == $tail;
+               my @ids;
+               for (; $head != $tail && @ids < $batch_size; $head->inc) {
+                       push @ids, $head->get_docid;
+               }
+               foreach my $docid (@ids) {
+                       my $doc = $db->get_document($docid);
+                       $doc->remove_term('G' . $loser_tid);
+                       $doc->add_term('G' . $winner_tid);
+                       $db->replace_document($docid, $doc);
+               }
        }
 }