From 7b875ebf568de3e2dd72d142ff7ba803a4651118 Mon Sep 17 00:00:00 2001
From: Nikhil Benesch <nikhil.benesch@gmail.com>
Date: Thu, 23 Nov 2023 02:19:53 -0500
Subject: [PATCH] pgoutput: don't unconditionally fill in missing values with
 NULL

52e4f0cd4 introduced a bug in pgoutput in which missing values in tuples
were incorrectly filled in with NULL. The problem was the use of
CreateTupleDescCopy where CreateTupleDescCopyConstr was required, as the
former drops the constraints in the tuple description (specifically, the
default value constraint) on the floor.

The bug could result in incorrectness when a table replicated via
`REPLICA IDENTITY FULL` underwent a schema change that added a column
with a default value.
---
 src/backend/replication/pgoutput/pgoutput.c |  4 +-
 src/test/subscription/t/100_bugs.pl         | 55 +++++++++++++++++++++
 2 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/src/backend/replication/pgoutput/pgoutput.c b/src/backend/replication/pgoutput/pgoutput.c
index e8add5ee5d..f9ed1083df 100644
--- a/src/backend/replication/pgoutput/pgoutput.c
+++ b/src/backend/replication/pgoutput/pgoutput.c
@@ -1136,8 +1136,8 @@ init_tuple_slot(PGOutputData *data, Relation relation,
 	 * Create tuple table slots. Create a copy of the TupleDesc as it needs to
 	 * live as long as the cache remains.
 	 */
-	oldtupdesc = CreateTupleDescCopy(RelationGetDescr(relation));
-	newtupdesc = CreateTupleDescCopy(RelationGetDescr(relation));
+	oldtupdesc = CreateTupleDescCopyConstr(RelationGetDescr(relation));
+	newtupdesc = CreateTupleDescCopyConstr(RelationGetDescr(relation));
 
 	entry->old_slot = MakeSingleTupleTableSlot(oldtupdesc, &TTSOpsHeapTuple);
 	entry->new_slot = MakeSingleTupleTableSlot(newtupdesc, &TTSOpsHeapTuple);
diff --git a/src/test/subscription/t/100_bugs.pl b/src/test/subscription/t/100_bugs.pl
index 7e5221afff..089a4fcaf0 100644
--- a/src/test/subscription/t/100_bugs.pl
+++ b/src/test/subscription/t/100_bugs.pl
@@ -438,4 +438,59 @@ is( $node_subscriber->safe_psql(
 $node_publisher->stop('fast');
 $node_subscriber->stop('fast');
 
+# The bug was that pgoutput was incorrectly replacing missing attributes in
+# tuples with NULL. This could result in incorrect replication with
+# `REPLICA IDENTITY FULL`.
+
+$node_publisher->rotate_logfile();
+$node_publisher->start();
+
+$node_subscriber->rotate_logfile();
+$node_subscriber->start();
+
+# Set up a table with schema `(a int, b bool)` where the `b` attribute is
+# missing for one row due to the `ALTER TABLE ... ADD COLUMN ... DEFAULT`
+# fast path.
+$node_publisher->safe_psql('postgres',
+	"CREATE TABLE tab_default (a int)");
+$node_publisher->safe_psql('postgres',
+	"ALTER TABLE tab_default REPLICA IDENTITY FULL");
+$node_publisher->safe_psql('postgres',
+	"INSERT INTO tab_default VALUES (1)");
+$node_publisher->safe_psql('postgres',
+	"ALTER TABLE tab_default ADD COLUMN b bool DEFAULT false NOT NULL");
+$node_publisher->safe_psql('postgres',
+	"INSERT INTO tab_default VALUES (2, true)");
+
+# Replicate to the subscriber.
+$node_subscriber->safe_psql('postgres',
+	"CREATE TABLE tab_default (a int, b bool)");
+$node_publisher->safe_psql('postgres',
+	"CREATE PUBLICATION pub1 FOR ALL TABLES");
+$node_subscriber->safe_psql('postgres',
+	"CREATE SUBSCRIPTION sub1 CONNECTION '$publisher_connstr' PUBLICATION pub1"
+);
+$node_subscriber->wait_for_subscription_sync($node_publisher, 'sub1');
+$result = $node_subscriber->safe_psql('postgres',
+	"SELECT a, b FROM tab_default");
+is($result, qq(1|f
+2|t), 'check snapshot on subscriber');
+
+# Update all rows in the table and ensure the rows with the missing `b`
+# attribute replicate correctly.
+$node_publisher->safe_psql('postgres',
+	"UPDATE tab_default SET a = a + 1");
+$node_publisher->wait_for_catchup('sub1');
+
+# When the bug is present, the `1|f` row will not be updated to `2|f` because
+# the publisher incorrectly fills in `NULL` for `b` and publishes an update
+# for `1|NULL`, which doe snot exist in the subscriber.
+$result = $node_subscriber->safe_psql('postgres',
+	"SELECT a, b FROM tab_default");
+is($result, qq(2|f
+3|t), 'check replicated update on subscriber');
+
+$node_publisher->stop('fast');
+$node_subscriber->stop('fast');
+
 done_testing();
-- 
2.41.0