MANIFEST                       |  1 +
 lib/PublicInbox/LeiOverview.pm | 35 +++++++++++++++++++++++++++++++++++
 lib/PublicInbox/LeiXSearch.pm  |  9 +++++----
 lib/PublicInbox/Lock.pm        |  2 +-
 t/lei_overview.t               | 33 +++++++++++++++++++++++++++++++++

diff --git a/MANIFEST b/MANIFEST
index 2ca240fc90ea317f2e75935a3806dc2e4da531fb..0ebdaccc4c8d23bea760591c0e48ee5b6f209d63 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -338,6 +338,7 @@ t/kqnotify.t
 t/lei-oneshot.t
 t/lei.t
 t/lei_dedupe.t
+t/lei_overview.t
 t/lei_store.t
 t/lei_to_mail.t
 t/lei_xsearch.t
diff --git a/lib/PublicInbox/LeiOverview.pm b/lib/PublicInbox/LeiOverview.pm
index ec0921baf75c69c63102787330b143f26af04488..ef5f27c1309bd78f697adf558e654b00cf372601 100644
--- a/lib/PublicInbox/LeiOverview.pm
+++ b/lib/PublicInbox/LeiOverview.pm
@@ -6,8 +6,11 @@ # "ovv" => "Overview viewer"
 package PublicInbox::LeiOverview;
 use strict;
 use v5.10.1;
+use parent qw(PublicInbox::Lock);
 use POSIX qw(strftime);
+use Fcntl qw(F_GETFL O_APPEND);
 use File::Spec;
+use File::Temp ();
 use PublicInbox::MID qw($MID_EXTRACT);
 use PublicInbox::Address qw(pairs);
 use PublicInbox::Config;
@@ -17,6 +20,21 @@ # cf. https://en.wikipedia.org/wiki/JSON_streaming
 my $JSONL = 'ldjson|ndjson|jsonl'; # 3 names for the same thing
 
 sub _iso8601 ($) { strftime('%Y-%m-%dT%H:%M:%SZ', gmtime($_[0])) }
+
+# we open this in the parent process before ->wq_do handoff
+sub ovv_out_lk_init ($) {
+	my ($self) = @_;
+	$self->{tmp_lk_id} = "$self.$$";
+	my $tmp = File::Temp->new("lei-ovv.out.$$.lock-XXXXXX",
+					TMPDIR => 1, UNLINK => 0);
+	$self->{lock_path} = $tmp->filename;
+}
+
+sub ovv_out_lk_cancel ($) {
+	my ($self) = @_;
+	($self->{tmp_lk_id}//'') eq "$self.$$" and
+		unlink(delete($self->{lock_path}));
+}
 
 sub new {
 	my ($class, $lei) = @_;
@@ -50,8 +68,17 @@ 	if ($out eq '/dev/stdout') {
 		$isatty = -t $lei->{1};
 		$lei->start_pager if $isatty;
 		$opt->{pretty} //= $isatty;
+		if (!$isatty && -f _) {
+			my $fl = fcntl($lei->{1}, F_GETFL, 0) //
+				return $lei->fail("fcntl(stdout): $!");
+			ovv_out_lk_init($self) unless ($fl & O_APPEND);
+		} else {
+			ovv_out_lk_init($self);
+		}
 	} elsif ($json) {
 		return $lei->fail('JSON formats only output to stdout');
+	} else {
+		return $lei->fail("TODO: $out -f $fmt");
 	}
 	$self;
 }
@@ -109,6 +136,7 @@
 sub ovv_atexit_child {
 	my ($self, $lei) = @_;
 	if (my $bref = delete $lei->{ovv_buf}) {
+		my $lk = $self->lock_for_scope;
 		print { $lei->{1} } $$bref;
 	}
 }
@@ -142,7 +170,9 @@
 sub ovv_each_smsg_cb {
 	my ($self, $lei) = @_;
 	$lei->{ovv_buf} = \(my $buf = '');
+	delete(@$self{qw(lock_path tmp_lk_id)}) unless $lei->{-parallel};
 	my $json = $self->{json}->new;
+	$lei->{1}->autoflush(1);
 	if ($json) {
 		$json->utf8->canonical;
 		$json->ascii(1) if $lei->{opt}->{ascii};
@@ -164,6 +194,7 @@ 				}
 			} sort keys %$smsg);
 			$buf .= $EOR;
 			if (length($buf) > 65536) {
+				my $lk = $self->lock_for_scope;
 				print { $lei->{1} } $buf;
 				$buf = '';
 			}
@@ -175,6 +206,7 @@ 			my ($smsg, $mitem) = @_;
 			delete @$smsg{qw(tid num)};
 			$buf .= $json->encode(_unbless_smsg(@_)) . $ORS;
 			if (length($buf) > 65536) {
+				my $lk = $self->lock_for_scope;
 				print { $lei->{1} } $buf;
 				$buf = '';
 			}
@@ -185,5 +217,8 @@ 			my ($smsg, $mitem) = @_;
 		}
 	} # else { ...
 }
+
+no warnings 'once';
+*DESTROY = \&ovv_out_lk_cancel;
 
 1;
diff --git a/lib/PublicInbox/LeiXSearch.pm b/lib/PublicInbox/LeiXSearch.pm
index 80e7a7f76124d74ce36b868a709370459e5c4a4c..ee93e074b1cacca62055cff1d507399ea5c224c5 100644
--- a/lib/PublicInbox/LeiXSearch.pm
+++ b/lib/PublicInbox/LeiXSearch.pm
@@ -158,20 +158,21 @@
 sub do_query {
 	my ($self, $lei_orig, $srcs) = @_;
 	my ($lei, @io) = $lei_orig->atfork_parent_wq($self);
-
+	my $remotes = $self->{remotes} // [];
 	pipe(my ($eof_wait, $qry_done)) or die "pipe $!";
 	$io[0] = $qry_done; # don't need stdin
-	$io[1]->autoflush(1);
-	$io[2]->autoflush(1);
+
 	if ($lei->{opt}->{thread}) {
+		$lei->{-parallel} = scalar(@$remotes) + scalar(@$srcs) - 1;
 		for my $ibxish (@$srcs) {
 			$self->wq_do('query_thread_mset', \@io, $lei, $ibxish);
 		}
 	} else {
+		$lei->{-parallel} = scalar(@$remotes);
 		$self->wq_do('query_mset', \@io, $lei, $srcs);
 	}
 	# TODO
-	for my $rmt (@{$self->{remotes} // []}) {
+	for my $rmt (@$remotes) {
 		$self->wq_do('query_thread_mbox', \@io, $lei, $rmt);
 	}
 	@io = ();
diff --git a/lib/PublicInbox/Lock.pm b/lib/PublicInbox/Lock.pm
index 2c5ebf270a3aed694a9f3c68156f1c431bd69b30..bb213de448c3f6c4ca08a0f8dd7ec34b40420590 100644
--- a/lib/PublicInbox/Lock.pm
+++ b/lib/PublicInbox/Lock.pm
@@ -37,7 +37,7 @@
 # caller must use return value
 sub lock_for_scope {
 	my ($self, @single_pid) = @_;
-	$self->lock_acquire;
+	lock_acquire($self) or return; # lock_path not set
 	PublicInbox::OnDestroy->new(@single_pid, \&lock_release, $self);
 }
 
diff --git a/t/lei_overview.t b/t/lei_overview.t
new file mode 100644
index 0000000000000000000000000000000000000000..896cc01a87fd8fca8805f17e4138ec015576c8da
--- /dev/null
+++ b/t/lei_overview.t
@@ -0,0 +1,33 @@
+#!perl -w
+# Copyright (C) 2021 all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+use strict;
+use v5.10.1;
+use Test::More;
+use PublicInbox::TestCommon;
+use POSIX qw(_exit);
+require_ok 'PublicInbox::LeiOverview';
+
+my $ovv = bless {}, 'PublicInbox::LeiOverview';
+$ovv->ovv_out_lk_init;
+my $lock_path = $ovv->{lock_path};
+ok(-f $lock_path, 'lock init');
+undef $ovv;
+ok(!-f $lock_path, 'lock DESTROY');
+
+$ovv = bless {}, 'PublicInbox::LeiOverview';
+$ovv->ovv_out_lk_init;
+$lock_path = $ovv->{lock_path};
+ok(-f $lock_path, 'lock init #2');
+my $pid = fork // BAIL_OUT "fork $!";
+if ($pid == 0) {
+	undef $ovv;
+	_exit(0);
+}
+is(waitpid($pid, 0), $pid, 'child exited');
+is($?, 0, 'no error in child process');
+ok(-f $lock_path, 'lock was not destroyed by child');
+undef $ovv;
+ok(!-f $lock_path, 'lock DESTROY #2');
+
+done_testing;