lib/PublicInbox/IMAPTracker.pm | 13 ++++++++++---
 t/imap_tracker.t               | 30 +++++++++++++++++++++++++++++-

diff --git a/lib/PublicInbox/IMAPTracker.pm b/lib/PublicInbox/IMAPTracker.pm
index 0bbabe07faef6c76f1bc4bcd94c75eabbfd8d1ab..102a74ce66b8ec0c1ba52be5200ee37d38dd08de 100644
--- a/lib/PublicInbox/IMAPTracker.pm
+++ b/lib/PublicInbox/IMAPTracker.pm
@@ -2,6 +2,7 @@ # Copyright (C) 2018-2020 all contributors <meta@public-inbox.org>
 # License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
 package PublicInbox::IMAPTracker;
 use strict;
+use parent qw(PublicInbox::Lock);
 use DBI;
 use DBD::SQLite;
 use PublicInbox::Config;
@@ -48,7 +49,10 @@ 	my $sth = $self->{dbh}->prepare_cached(<<'');
 INSERT OR REPLACE INTO imap_last (url, uid_validity, uid)
 VALUES (?, ?, ?)
 
-	$sth->execute($self->{url}, $validity, $last);
+	$self->lock_acquire;
+	my $rv = $sth->execute($self->{url}, $validity, $last);
+	$self->lock_release;
+	$rv;
 }
 
 sub new {
@@ -68,8 +72,11 @@ 		require File::Path;
 		require File::Basename;
 		File::Path::mkpath(File::Basename::dirname($dbname));
 	}
-
-	bless { url => $url, dbh => dbh_new($dbname) }, $class;
+	my $self = bless { lock_path => "$dbname.lock", url => $url }, $class;
+	$self->lock_acquire;
+	$self->{dbh} = dbh_new($dbname);
+	$self->lock_release;
+	$self;
 }
 
 1;
diff --git a/t/imap_tracker.t b/t/imap_tracker.t
index 8dc04ed77a3accde000bd6700616c215da1d0f46..01e1d0b154927d6bc03765203bd9deae56002242 100644
--- a/t/imap_tracker.t
+++ b/t/imap_tracker.t
@@ -9,8 +9,8 @@ my ($tmpdir, $for_destroy) = tmpdir();
 mkdir "$tmpdir/old" or die "mkdir $tmpdir/old: $!";
 my $old = "$tmpdir/old/imap.sqlite3";
 my $cur = "$tmpdir/data/public-inbox/imap.sqlite3";
+local $ENV{XDG_DATA_HOME} = "$tmpdir/data";
 {
-	local $ENV{XDG_DATA_HOME} = "$tmpdir/data";
 	local $ENV{PI_DIR} = "$tmpdir/old";
 
 	my $tracker = PublicInbox::IMAPTracker->new;
@@ -21,6 +21,34 @@ 	link $cur, $old or die "link $cur => $old: $!";
 	unlink $cur or die "unlink $cur: $!";
 	$tracker = PublicInbox::IMAPTracker->new;
 	ok(!-f $cur, '->new does not create new file if old is present');
+}
+SKIP: {
+	my $nproc = $ENV{TEST_STRESS_NPROC};
+	skip 'TEST_STRESS_NPROC= not set', 1 unless $nproc;
+	my $nr = $ENV{TEST_STRESS_NR} // 10000;
+	diag "TEST_STRESS_NPROC=$nproc TEST_STRESS_NR=$nr";
+	require POSIX;
+	for my $n (1..$nproc) {
+		defined(my $pid = fork) or BAIL_OUT "fork: $!";
+		if ($pid == 0) {
+			my $url = "imap://example.com/INBOX.$$";
+			my $uidval = time;
+			eval {
+				my $itrk = PublicInbox::IMAPTracker->new($url);
+				for my $uid (1..$nr) {
+					$itrk->update_last($uidval, $uid);
+					my ($uv, $u) = $itrk->get_last;
+				}
+			};
+			warn "E: $n $$ - $@\n" if $@;
+			POSIX::_exit($@ ? 1 : 0);
+		}
+	}
+	while (1) {
+		my $pid = waitpid(-1, 0);
+		last if $pid < 0;
+		is($?, 0, "$pid exited");
+	}
 }
 
 done_testing;