Test only: reproduce the standby-confirmation lost wakeup.

diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -4141,7 +4141,16 @@
 	 * the receipt of the LSN.
 	 */
 	if (wait_event == WAIT_EVENT_WAIT_FOR_STANDBY_CONFIRMATION)
+	{
+		/* Widen the window before joining the waitlist. */
+		if (access("walsnd_race_standby", F_OK) == 0)
+		{
+			elog(LOG, "walsnd_race: window open");
+			pg_usleep(3000000L);
+			elog(LOG, "walsnd_race: window closed");
+		}
 		ConditionVariablePrepareToSleep(&WalSndCtl->wal_confirm_rcv_cv);
+	}
 	else if (MyWalSnd->kind == REPLICATION_KIND_PHYSICAL)
 	{
 		ConditionVariablePrepareToSleep(&WalSndCtl->wal_flush_cv);
diff --git a/src/test/recovery/meson.build b/src/test/recovery/meson.build
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -69,6 +69,7 @@
       't/058_shutdown_crash_restart.pl',
       't/059_remote_apply_status_interval.pl',
       't/060_multixact_crashed_updater.pl',
+      't/061_walsender_standby_wakeup.pl',
     ],
   },
 }
diff --git a/src/test/recovery/t/061_walsender_standby_wakeup.pl b/src/test/recovery/t/061_walsender_standby_wakeup.pl
new file mode 100644
--- /dev/null
+++ b/src/test/recovery/t/061_walsender_standby_wakeup.pl
@@ -0,0 +1,111 @@
+
+# Test only: a logical walsender loses the standby-confirmation wakeup.
+#
+# With the matching walsender.c delay, the walsender checks the standby slot,
+# then sleeps 3 s before joining wal_confirm_rcv_cv.  The standby confirms in
+# that gap, so the broadcast is lost and the walsender sleeps until its 10 s
+# timeout.  The assertions pass when the wakeup is lost.  An unrelated wakeup
+# can mask the race.
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+my $primary = PostgreSQL::Test::Cluster->new('primary');
+$primary->init(allows_streaming => 'logical');
+$primary->append_conf(
+	'postgresql.conf', qq(
+autovacuum = off
+wal_sender_timeout = 0
+));
+$primary->start;
+
+$primary->safe_psql(
+	'postgres', qq(
+	CREATE TABLE tab_int (a int);
+	SELECT pg_create_physical_replication_slot('sb1_slot');
+	SELECT pg_create_logical_replication_slot('test_slot', 'test_decoding', false, false, true);
+));
+$primary->append_conf('postgresql.conf',
+	"synchronized_standby_slots = 'sb1_slot'");
+$primary->reload;
+
+my $backup_name = 'backup';
+$primary->backup($backup_name);
+my $standby1 = PostgreSQL::Test::Cluster->new('standby1');
+$standby1->init_from_backup($primary, $backup_name, has_streaming => 1);
+$standby1->append_conf('postgresql.conf', "primary_slot_name = 'sb1_slot'");
+$standby1->start;
+$primary->wait_for_replay_catchup($standby1);
+
+# Stream the failover slot.  No status messages, so the client never wakes the
+# walsender.
+my ($stdout, $stderr);
+my $recv = IPC::Run::start(
+	[
+		'pg_recvlogical',
+		'--dbname' => $primary->connstr('postgres') . ' application_name=test_slot',
+		'--slot' => 'test_slot',
+		'--start',
+		'--file' => $primary->basedir . '/test_slot.out',
+		'--status-interval' => '0',
+		'--fsync-interval' => '0',
+	],
+	'>' => \$stdout,
+	'2>' => \$stderr);
+
+$primary->safe_psql('postgres', "INSERT INTO tab_int VALUES (1);");
+$primary->wait_for_catchup('test_slot', 'sent', $primary->lsn('insert'));
+
+# Enable the delay and pause the walreceiver, so the next commit waits for
+# the standby.
+my $marker = $primary->data_dir . '/walsnd_race_standby';
+append_to_file($marker, '');
+my $receiverpid =
+  $standby1->safe_psql('postgres', "SELECT pid FROM pg_stat_wal_receiver");
+like($receiverpid, qr/^[0-9]+$/, "have walreceiver pid $receiverpid");
+kill 'STOP', $receiverpid;
+
+my $offset = -s $primary->logfile;
+my $before = $primary->lsn('insert');
+$primary->safe_psql('postgres', "INSERT INTO tab_int VALUES (2);");
+$offset = $primary->wait_for_log(qr/walsnd_race: window open/, $offset);
+unlink $marker;
+
+# Resume the walreceiver.  The standby confirms while the walsender is not yet
+# on the waitlist.
+kill 'CONT', $receiverpid;
+$primary->wait_for_log(qr/walsnd_race: window closed/, $offset);
+sleep(1);
+
+# The standby has confirmed, yet the walsender hasn't sent the commit and is
+# still waiting for confirmation.
+is( $primary->safe_psql(
+		'postgres', qq(
+	SELECT (SELECT restart_lsn FROM pg_replication_slots
+			WHERE slot_name = 'sb1_slot') > '$before',
+		   r.sent_lsn <= '$before', a.wait_event
+	FROM pg_stat_replication r JOIN pg_stat_activity a USING (pid)
+	WHERE r.application_name = 'test_slot')),
+	't|t|WaitForStandbyConfirmation',
+	'walsender waits although the standby has confirmed');
+
+# The next confirmation wakes it.
+$primary->safe_psql('postgres', "INSERT INTO tab_int VALUES (3);");
+$primary->wait_for_catchup('test_slot', 'sent', $primary->lsn('insert'));
+pass('walsender resumes after the next confirmation');
+
+$recv->signal('TERM');
+$recv->finish();
+
+done_testing();