From 2036cd523b6f531f4db494e97914de8a49a579e6 Mon Sep 17 00:00:00 2001
From: Sunil S <sunil.s@broadcom.com>
Date: Mon, 7 Jul 2025 11:29:02 +0530
Subject: [PATCH v10 2/3] Test WAL receiver early start upon reaching
 consistency

This test ensures that when a standby reaches consistency,
the WAL receiver starts immediately and begins streaming using the latest valid
 WAL segment already available on disk.
This behavior minimizes delay and avoids waiting for WAL file once all the
locally available WAL file is restored and helps in providing `HIGH AVAILABILITY`
incase of Primary crash/failure.

More it helps quicker recovery when `recovery_min_apply_delay` in large and saves
Primary from running out of space.

Co-authors: Sunil S<sunilfeb26@gmail.com>, Soumyadeep Chakraborty <soumyadeep2007@gmail.com>, Ashwin Agrawal, Asim Praveen, Wu Hao, Konstantin Knizhnik
Discussion: https://www.postgresql.org/message-id/flat/CANXE4Tc3FNvZ_xAimempJWv_RH9pCvsZH7Yq93o1VuNLjUT-mQ%40mail.gmail.com
---
 src/test/recovery/t/049_walreciver_start.pl | 96 +++++++++++++++++++++
 1 file changed, 96 insertions(+)
 create mode 100644 src/test/recovery/t/049_walreciver_start.pl

diff --git a/src/test/recovery/t/049_walreciver_start.pl b/src/test/recovery/t/049_walreciver_start.pl
new file mode 100644
index 00000000000..da31c470867
--- /dev/null
+++ b/src/test/recovery/t/049_walreciver_start.pl
@@ -0,0 +1,96 @@
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# Checks for wal_receiver_start_at = 'consistency'
+use strict;
+use warnings;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use File::Copy;
+use Test::More tests => 2;
+
+# Initialize primary node and start it.
+my $node_primary = PostgreSQL::Test::Cluster->new('test');
+$node_primary->init(allows_streaming => 1);
+$node_primary->start;
+
+# Initial workload.
+$node_primary->safe_psql(
+	'postgres', qq {
+CREATE TABLE test_walreceiver_start(i int);
+SELECT pg_switch_wal();
+});
+
+# Take backup.
+my $backup_name = 'my_backup';
+$node_primary->backup($backup_name);
+
+# Run a post-backup workload, whose WAL we will manually copy over to the
+# standby before it starts.
+my $wal_file_to_copy = $node_primary->safe_psql('postgres',
+	"SELECT pg_walfile_name(pg_current_wal_lsn());");
+$node_primary->safe_psql(
+	'postgres', qq {
+INSERT INTO test_walreceiver_start VALUES(1);
+SELECT pg_switch_wal();
+});
+
+# Initialize standby node from the backup and copy over the post-backup WAL.
+my $node_standby = PostgreSQL::Test::Cluster->new('standby');
+$node_standby->init_from_backup($node_primary, $backup_name,
+	has_streaming => 1);
+copy($node_primary->data_dir . '/pg_wal/' . $wal_file_to_copy,
+	$node_standby->data_dir . '/pg_wal')
+  or die "Copy failed: $!";
+
+# Set up a long delay to prevent the standby from replaying past the first
+# commit outside the backup.
+$node_standby->append_conf('postgresql.conf',
+	"recovery_min_apply_delay = '2h'");
+# Set up the walreceiver to start as soon as consistency is reached.
+$node_standby->append_conf('postgresql.conf',
+	"wal_receiver_start_at = 'consistency'");
+
+$node_standby->start();
+
+# The standby should have reached consistency and should be blocked waiting for
+# recovery_min_apply_delay.
+$node_standby->poll_query_until(
+	'postgres', qq{
+SELECT wait_event = 'RecoveryApplyDelay' FROM pg_stat_activity
+WHERE backend_type='startup';
+}) or die "Timed out checking if startup is in recovery_min_apply_delay";
+
+# The walreceiver should have started, streaming from the end of valid locally
+# available WAL, i.e from the WAL file that was copied over.
+$node_standby->poll_query_until('postgres',
+	"SELECT COUNT(1) = 1 FROM pg_stat_wal_receiver;")
+  or die "Timed out while waiting for streaming to start";
+my $receive_start_lsn = $node_standby->safe_psql('postgres',
+	'SELECT receive_start_lsn FROM pg_stat_wal_receiver');
+is( $node_primary->safe_psql(
+		'postgres', "SELECT pg_walfile_name('$receive_start_lsn');"),
+	$wal_file_to_copy,
+	"walreceiver started from end of valid locally available WAL");
+
+# Now run a workload which should get streamed over.
+$node_primary->safe_psql(
+	'postgres', qq {
+SELECT pg_switch_wal();
+INSERT INTO test_walreceiver_start VALUES(2);
+});
+
+# The walreceiver should be caught up, including all WAL generated post backup.
+$node_primary->wait_for_catchup('standby', 'flush');
+
+# Now clear the delay so that the standby can replay the received WAL.
+$node_standby->safe_psql('postgres',
+	'ALTER SYSTEM SET recovery_min_apply_delay TO 0;');
+$node_standby->reload;
+
+# Now the replay should catch up.
+$node_primary->wait_for_catchup('standby', 'replay');
+is( $node_standby->safe_psql(
+		'postgres', 'SELECT count(*) FROM test_walreceiver_start;'),
+	2,
+	"querying test_walreceiver_start now should return 2 rows");
-- 
2.50.1

