PG17 Compatibility - Fix crash when pg_class is used in MERGE (#7853)

This pull request addresses Issue #7846, where specific MERGE queries on
non-distributed and distributed tables can result in crashes in certain
scenarios. The issue stems from the usage of `pg_class` catalog table,
and the `FilterShardsFromPgclass` function in Citus. This function goes
through the query's jointree to hide the shards. However, in PG17,
MERGE's join quals are in a separate structure called
`mergeJoinCondition`. Therefore FilterShardsFromPgclass was not
filtering correctly in a `MERGE` command that involves `pg_class`. To
fix the issue, we handle `mergeJoinCondition` separately in PG17.

Relevant PG commit:

postgres/postgres@0294df2

**Non-Distributed Tables:**
A MERGE query involving a non-distributed table using
`pg_catalog.pg_class` as the source may execute successfully but needs
testing to ensure stability.

**Distributed Tables:**
Performing a MERGE on a distributed table using `pg_catalog.pg_class` as
the source raises an error:
`ERROR: MERGE INTO a distributed table from Postgres table is not yet
supported`
However, in some cases, this can lead to a server crash if the
unsupported operation is not properly handled.

This is the test output from the same test conducted prior to the code
changes being implemented.

```
-- Issue #7846: Test crash scenarios with MERGE on non-distributed and distributed tables
-- Step 1: Connect to a worker node to verify shard visibility
\c postgresql://postgres@localhost::worker_1_port/regression?application_name=psql
SET search_path TO pg17;
-- Step 2: Create and test a non-distributed table
CREATE TABLE non_dist_table_12345 (id INTEGER);
-- Test MERGE on the non-distributed table
MERGE INTO non_dist_table_12345 AS target_0
USING pg_catalog.pg_class AS ref_0
ON target_0.id = ref_0.relpages
WHEN NOT MATCHED THEN DO NOTHING;
SSL SYSCALL error: EOF detected
connection to server was lost
```

src/backend/distributed/worker/worker_shard_visibility.c

@@ -441,7 +441,7 @@ FilterShardsFromPgclass(Node *node, void *context)
 		/*
 		 * We process the whole rtable rather than visiting individual RangeTblEntry's
 		 * in the walker, since we need to know the varno to generate the right
-		 * fiter.
+		 * filter.
 		 */
 		int varno = 0;
 		RangeTblEntry *rangeTableEntry = NULL;
@@ -471,20 +471,39 @@ FilterShardsFromPgclass(Node *node, void *context)
 			/* make sure the expression is in the right memory context */
 			MemoryContext originalContext = MemoryContextSwitchTo(queryContext);
 
-
 			/* add relation_is_a_known_shard(oid) IS NOT TRUE to the quals of the query */
 			Node *newQual = CreateRelationIsAKnownShardFilter(varno);
-			Node *oldQuals = query->jointree->quals;
-			if (oldQuals)
+
+#if PG_VERSION_NUM >= PG_VERSION_17
+
+			/*
+			 * In PG17, MERGE queries introduce a new struct `mergeJoinCondition`.
+			 * We need to handle this condition safely.
+			 */
+			if (query->mergeJoinCondition != NULL)
 			{
-				query->jointree->quals = (Node *) makeBoolExpr(
+				/* Add the filter to mergeJoinCondition */
+				query->mergeJoinCondition = (Node *) makeBoolExpr(
 					AND_EXPR,
-					list_make2(oldQuals, newQual),
+					list_make2(query->mergeJoinCondition, newQual),
 					-1);
 			}
 			else
+#endif
 			{
-				query->jointree->quals = newQual;
+				/* Handle older versions or queries without mergeJoinCondition */
+				Node *oldQuals = query->jointree->quals;
+				if (oldQuals)
+				{
+					query->jointree->quals = (Node *) makeBoolExpr(
+						AND_EXPR,
+						list_make2(oldQuals, newQual),
+						-1);
+				}
+				else
+				{
+					query->jointree->quals = newQual;
+				}
 			}
 
 			MemoryContextSwitchTo(originalContext);

src/test/regress/expected/pg17.out

@@ -2690,6 +2690,39 @@ SELECT * FROM sensor_readings ORDER BY 1;
 (4 rows)
 
 -- End of MERGE ... WHEN NOT MATCHED BY SOURCE tests
+-- Issue #7846: Test crash scenarios with MERGE on non-distributed and distributed tables
+-- Step 1: Connect to a worker node to verify shard visibility
+\c postgresql://postgres@localhost::worker_1_port/regression?application_name=psql
+SET search_path TO pg17;
+-- Step 2: Create and test a non-distributed table
+CREATE TABLE non_dist_table_12345 (id INTEGER);
+-- Test MERGE on the non-distributed table
+MERGE INTO non_dist_table_12345 AS target_0
+USING pg_catalog.pg_class AS ref_0
+ON target_0.id = ref_0.relpages
+WHEN NOT MATCHED THEN DO NOTHING;
+-- Step 3: Switch back to the coordinator for distributed table operations
+\c postgresql://postgres@localhost::master_port/regression?application_name=psql
+SET search_path TO pg17;
+-- Step 4: Create and test a distributed table
+CREATE TABLE dist_table_67890 (id INTEGER);
+SELECT create_distributed_table('dist_table_67890', 'id');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+-- Test MERGE on the distributed table
+MERGE INTO dist_table_67890 AS target_0
+USING pg_catalog.pg_class AS ref_0
+ON target_0.id = ref_0.relpages
+WHEN NOT MATCHED THEN DO NOTHING;
+ERROR:  MERGE INTO an distributed table from Postgres table is not yet supported
+-- Step 5: Cleanup
+DROP TABLE non_dist_table_12345;
+ERROR:  table "non_dist_table_12345" does not exist
+DROP TABLE dist_table_67890 CASCADE;
+-- End of Issue #7846
 \set VERBOSITY terse
 SET client_min_messages TO WARNING;
 DROP SCHEMA pg17 CASCADE;

src/test/regress/sql/pg17.sql

@@ -1451,6 +1451,40 @@ SELECT * FROM sensor_readings ORDER BY 1;
 
 -- End of MERGE ... WHEN NOT MATCHED BY SOURCE tests
 
+-- Issue #7846: Test crash scenarios with MERGE on non-distributed and distributed tables
+-- Step 1: Connect to a worker node to verify shard visibility
+\c postgresql://postgres@localhost::worker_1_port/regression?application_name=psql
+SET search_path TO pg17;
+
+-- Step 2: Create and test a non-distributed table
+CREATE TABLE non_dist_table_12345 (id INTEGER);
+
+-- Test MERGE on the non-distributed table
+MERGE INTO non_dist_table_12345 AS target_0
+USING pg_catalog.pg_class AS ref_0
+ON target_0.id = ref_0.relpages
+WHEN NOT MATCHED THEN DO NOTHING;
+
+-- Step 3: Switch back to the coordinator for distributed table operations
+\c postgresql://postgres@localhost::master_port/regression?application_name=psql
+SET search_path TO pg17;
+
+-- Step 4: Create and test a distributed table
+CREATE TABLE dist_table_67890 (id INTEGER);
+SELECT create_distributed_table('dist_table_67890', 'id');
+
+-- Test MERGE on the distributed table
+MERGE INTO dist_table_67890 AS target_0
+USING pg_catalog.pg_class AS ref_0
+ON target_0.id = ref_0.relpages
+WHEN NOT MATCHED THEN DO NOTHING;
+
+-- Step 5: Cleanup
+DROP TABLE non_dist_table_12345;
+DROP TABLE dist_table_67890 CASCADE;
+
+-- End of Issue #7846
+
 \set VERBOSITY terse
 SET client_min_messages TO WARNING;
 DROP SCHEMA pg17 CASCADE;