Avoid unconditionally filling in missing values with NULL in pgoutput.

52e4f0c 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. The problem is that in such cases updates fill NULL
values in old tuples for missing columns for default values. Then on the
subscriber, we failed to find a matching tuple and missed updating the
required row.

Author: Nikhil Benesch
Reviewed-by: Hou Zhijie, Amit Kapila
Backpatch-through: 15
Discussion: http://postgr.es/m/CAPWqQZTEpZQamYsGMn6ZDRvVywwpVPiKH6OY4KSgA+NmeqFNzA@mail.gmail.com

Author: Amit Kapila

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);

src/test/subscription/t/100_bugs.pl

@@ -438,4 +438,57 @@
 $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.
+$node_publisher->safe_psql(
+	'postgres', qq(
+	CREATE TABLE tab_default (a int);
+	ALTER TABLE tab_default REPLICA IDENTITY FULL;
+	INSERT INTO tab_default VALUES (1);
+	ALTER TABLE tab_default ADD COLUMN b bool DEFAULT false NOT NULL;
+	INSERT INTO tab_default VALUES (2, true);
+	CREATE PUBLICATION pub1 FOR TABLE tab_default;
+));
+
+# Replicate to the subscriber.
+$node_subscriber->safe_psql(
+	'postgres', qq(
+	CREATE TABLE tab_default (a int, b bool);
+	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 doesn't 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();