[RFC] www: mbox: reuse Xapian::Enquire object

[RFC] www: mbox: reuse Xapian::Enquire object

[Eric Wong]

I'm not sure if this helps from a correctness perspective[1].
I'm hoping it can work better with MVCC in avoiding
duplicate/skipped documents in the face of concurrent
modifications.  It doesn't noticeably save CPU cycles by
avoiding parser and Enquire object setup overhead, either.

[1] also posted on xapian-discuss about it <20230817212826.M47893@dcvr>

This is a tangent from the ongoing -cindex --associate work
acceptably...
---
 lib/PublicInbox/Isearch.pm | 2 ++
 lib/PublicInbox/Mbox.pm    | 7 +++----
 lib/PublicInbox/Search.pm  | 8 +++++++-
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/lib/PublicInbox/Isearch.pm b/lib/PublicInbox/Isearch.pm
index 5cbc36fd..38c80770 100644
--- a/lib/PublicInbox/Isearch.pm
+++ b/lib/PublicInbox/Isearch.pm
@@ -119,6 +119,8 @@ sub mset_to_smsg {
 	wantarray ? ($mset->get_matches_estimated, \@msgs) : \@msgs;
 }
 
+sub run_enquire { $_[0]->{es}->run_enquire($_[1], $_[2]) }
+
 sub has_threadid { 1 }
 
 sub help { $_[0]->{es}->help }
diff --git a/lib/PublicInbox/Mbox.pm b/lib/PublicInbox/Mbox.pm
index bf61bb0e..258bb3e6 100644
--- a/lib/PublicInbox/Mbox.pm
+++ b/lib/PublicInbox/Mbox.pm
@@ -187,7 +187,7 @@ sub results_cb {
 		}
 		# refill result set, deprioritize since there's many results
 		my $srch = $ctx->{ibx}->isrch or return $ctx->gone('search');
-		my $mset = $srch->mset($ctx->{query}, $ctx->{qopts});
+		my $mset = $srch->run_enquire($ctx->{enquire}, $ctx->{qopts});
 		my $size = $mset->size or return;
 		$ctx->{qopts}->{offset} += $size;
 		$ctx->{ids} = $srch->mset_to_artnums($mset, $ctx->{qopts});
@@ -210,7 +210,7 @@ sub results_thread_cb {
 
 		# refill result set, deprioritize since there's many results
 		my $srch = $ctx->{ibx}->isrch or return $ctx->gone('search');
-		my $mset = $srch->mset($ctx->{query}, $ctx->{qopts});
+		my $mset = $srch->run_enquire($ctx->{enquire}, $ctx->{qopts});
 		my $size = $mset->size or return;
 		$ctx->{qopts}->{offset} += $size;
 		$ctx->{ids} = $srch->mset_to_artnums($mset, $ctx->{qopts});
@@ -240,11 +240,10 @@ sub mbox_all {
 	}
 	$qopts->{threads} = 1 if $q->{t};
 	$srch->query_approxidate($ctx->{ibx}->git, $q_string);
-	my $mset = $srch->mset($q_string, $qopts);
+	(my $mset, $ctx->{enquire}) = $srch->mset($q_string, $qopts);
 	$qopts->{offset} = $mset->size or
 			return [404, [qw(Content-Type text/plain)],
 				["No results found\n"]];
-	$ctx->{query} = $q_string;
 	$ctx->{ids} = $srch->mset_to_artnums($mset, $qopts);
 	require PublicInbox::MboxGz;
 	my $fn;
diff --git a/lib/PublicInbox/Search.pm b/lib/PublicInbox/Search.pm
index b2de3450..5d19ed9f 100644
--- a/lib/PublicInbox/Search.pm
+++ b/lib/PublicInbox/Search.pm
@@ -414,6 +414,11 @@ sub do_enquire { # shared with CodeSearch
 	# on `-tt' ({threads} > 1) which sets the Flagged|Important keyword
 	(($opt->{threads} // 0) == 1 && has_threadid($self)) and
 		$enq->set_collapse_key(THREADID);
+	run_enquire($self, $enq, $opt);
+}
+
+sub run_enquire {
+	my ($self, $enq, $opt) = @_;
 	retry_reopen($self, \&enquire_once, $enq,
 			$opt->{offset} || 0, $opt->{limit} || 50);
 }
@@ -450,7 +455,8 @@ sub has_threadid ($) {
 
 sub enquire_once { # retry_reopen callback
 	my (undef, $enq, $offset, $limit) = @_;
-	$enq->get_mset($offset, $limit);
+	my $mset = $enq->get_mset($offset, $limit);
+	wantarray ? ($mset, $enq) : $mset;
 }
 
 sub mset_to_smsg {