Use relation_close() more consistently in contrib/

All the code paths updated here have been using index_close() to
close a relation that was opened with relation_open(), in pgstattuple
and pageinspect.  index_close() does the same thing as relation_close(),
so there is no harm, but being inconsistent could lead to issues if the
internals of these close() functions begin to introduce some specific
logic in the future.

In passing, this commit adds some comments explaining why we are using
relation_open() instead of index_open() in a few places, which is due to
the fact that partitioned indexes are not allowed in these functions.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Discussion: https://postgr.es/m/aUKamYGiDKO6byp5@ip-10-97-1-34.eu-west-3.compute.internal

contrib/pageinspect/hashfuncs.c

@@ -415,6 +415,10 @@ hash_bitmap_info(PG_FUNCTION_ARGS)
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
 				 errmsg("must be superuser to use raw page functions")));
 
+	/*
+	 * This uses relation_open() and not index_open().  The latter allows
+	 * partitioned indexes, and these are forbidden here.
+	 */
 	indexRel = relation_open(indexRelid, AccessShareLock);
 
 	if (!IS_INDEX(indexRel) || !IS_HASH(indexRel))
@@ -486,7 +490,7 @@ hash_bitmap_info(PG_FUNCTION_ARGS)
 	bit = ISSET(freep, bitmapbit) != 0;
 
 	_hash_relbuf(indexRel, mapbuf);
-	index_close(indexRel, AccessShareLock);
+	relation_close(indexRel, AccessShareLock);
 
 	/* Build a tuple descriptor for our result type */
 	if (get_call_result_type(fcinfo, NULL, &tupleDesc) != TYPEFUNC_COMPOSITE)

contrib/pgstattuple/pgstatindex.c

@@ -514,6 +514,10 @@ pgstatginindex_internal(Oid relid, FunctionCallInfo fcinfo)
 	bool		nulls[3] = {false, false, false};
 	Datum		result;
 
+	/*
+	 * This uses relation_open() and not index_open().  The latter allows
+	 * partitioned indexes, and these are forbidden here.
+	 */
 	rel = relation_open(relid, AccessShareLock);
 
 	if (!IS_INDEX(rel) || !IS_GIN(rel))
@@ -597,6 +601,10 @@ pgstathashindex(PG_FUNCTION_ARGS)
 	float8		free_percent;
 	uint64		total_space;
 
+	/*
+	 * This uses relation_open() and not index_open().  The latter allows
+	 * partitioned indexes, and these are forbidden here.
+	 */
 	rel = relation_open(relid, AccessShareLock);
 
 	if (!IS_INDEX(rel) || !IS_HASH(rel))
@@ -691,7 +699,7 @@ pgstathashindex(PG_FUNCTION_ARGS)
 	}
 
 	/* Done accessing the index */
-	index_close(rel, AccessShareLock);
+	relation_close(rel, AccessShareLock);
 
 	/* Count unused pages as free space. */
 	stats.free_space += (uint64) stats.unused_pages * stats.space_per_page;