Block ALTER TABLE .. DROP NOT NULL on columns in replica identity index

Replica identities that depend directly on an index rely on a set of
properties, one of them being that all the columns defined in this index
have to be marked as NOT NULL.  There was a hole in the logic with ALTER
TABLE DROP NOT NULL, where it was possible to remove the NOT NULL
property of a column part of an index used as replica identity, so block
it to avoid problems with logical decoding down the road.

The same check was already done columns part of a primary key, so the
fix is straight-forward.

Author: Haiying Tang, Hou Zhijie
Reviewed-by: Dilip Kumar, Michael Paquier
Discussion: https://postgr.es/m/OS0PR01MB6113338C102BEE8B2FFC5BD9FB619@OS0PR01MB6113.jpnprd01.prod.outlook.com
Backpatch-through: 10

src/backend/commands/tablecmds.c

@@ -7020,7 +7020,8 @@ ATExecDropNotNull(Relation rel, const char *colName, LOCKMODE lockmode)
 						colName, RelationGetRelationName(rel))));
 
 	/*
-	 * Check that the attribute is not in a primary key
+	 * Check that the attribute is not in a primary key or in an index used as
+	 * a replica identity.
 	 *
 	 * Note: we'll throw error even if the pkey index is not valid.
 	 */
@@ -7040,20 +7041,32 @@ ATExecDropNotNull(Relation rel, const char *colName, LOCKMODE lockmode)
 			elog(ERROR, "cache lookup failed for index %u", indexoid);
 		indexStruct = (Form_pg_index) GETSTRUCT(indexTuple);
 
-		/* If the index is not a primary key, skip the check */
-		if (indexStruct->indisprimary)
+		/*
+		 * If the index is not a primary key or an index used as replica
+		 * identity, skip the check.
+		 */
+		if (indexStruct->indisprimary || indexStruct->indisreplident)
 		{
 			/*
-			 * Loop over each attribute in the primary key and see if it
-			 * matches the to-be-altered attribute
+			 * Loop over each attribute in the primary key or the index used
+			 * as replica identity and see if it matches the to-be-altered
+			 * attribute.
 			 */
 			for (i = 0; i < indexStruct->indnkeyatts; i++)
 			{
 				if (indexStruct->indkey.values[i] == attnum)
-					ereport(ERROR,
-							(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
-							 errmsg("column \"%s\" is in a primary key",
-									colName)));
+				{
+					if (indexStruct->indisprimary)
+						ereport(ERROR,
+								(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
+								 errmsg("column \"%s\" is in a primary key",
+										colName)));
+					else
+						ereport(ERROR,
+								(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
+								 errmsg("column \"%s\" is in index used as replica identity",
+										colName)));
+				}
 			}
 		}
 

src/test/regress/expected/replica_identity.out

@@ -223,6 +223,10 @@ ALTER TABLE test_replica_identity3 ALTER COLUMN id TYPE bigint;
 Indexes:
     "test_replica_identity3_id_key" UNIQUE, btree (id) REPLICA IDENTITY
 
+-- ALTER TABLE DROP NOT NULL is not allowed for columns part of an index
+-- used as replica identity.
+ALTER TABLE test_replica_identity3 ALTER COLUMN id DROP NOT NULL;
+ERROR:  column "id" is in index used as replica identity
 DROP TABLE test_replica_identity;
 DROP TABLE test_replica_identity2;
 DROP TABLE test_replica_identity3;

src/test/regress/sql/replica_identity.sql

@@ -94,6 +94,10 @@ ALTER TABLE test_replica_identity3 REPLICA IDENTITY USING INDEX test_replica_ide
 ALTER TABLE test_replica_identity3 ALTER COLUMN id TYPE bigint;
 \d test_replica_identity3
 
+-- ALTER TABLE DROP NOT NULL is not allowed for columns part of an index
+-- used as replica identity.
+ALTER TABLE test_replica_identity3 ALTER COLUMN id DROP NOT NULL;
+
 DROP TABLE test_replica_identity;
 DROP TABLE test_replica_identity2;
 DROP TABLE test_replica_identity3;