Add try_index_open(), conditional variant of index_open()

try_index_open() is able to open an index if its relkind fits, except
that it would return NULL instead of generated an error if the relation
does not exist.  This new routine will be used by an upcoming patch to
make REINDEX on partitioned relations more robust when an index in a
partition tree is dropped.

Extracted from a larger patch by the same author.

Author: Fei Changhong
Discussion: https://postgr.es/m/tencent_6A52106095ACDE55333E3AD33F304C0C3909@qq.com
Backpatch-through: 14

src/backend/access/index/indexam.c

@@ -107,6 +107,7 @@ do { \
 static IndexScanDesc index_beginscan_internal(Relation indexRelation,
 											  int nkeys, int norderbys, Snapshot snapshot,
 											  ParallelIndexScanDesc pscan, bool temp_snap);
+static inline void validate_relation_kind(Relation r);
 
 
 /* ----------------------------------------------------------------
@@ -135,12 +136,30 @@ index_open(Oid relationId, LOCKMODE lockmode)
 
 	r = relation_open(relationId, lockmode);
 
-	if (r->rd_rel->relkind != RELKIND_INDEX &&
+	validate_relation_kind(r);
-		r->rd_rel->relkind != RELKIND_PARTITIONED_INDEX)
+
-		ereport(ERROR,
+	return r;
-				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+}
-				 errmsg("\"%s\" is not an index",
+
-						RelationGetRelationName(r))));
+/* ----------------
+ *		try_index_open - open a index relation by relation OID
+ *
+ *		Same as index_open, except return NULL instead of failing
+ *		if the relation does not exist.
+ * ----------------
+ */
+Relation
+try_index_open(Oid relationId, LOCKMODE lockmode)
+{
+	Relation	r;
+
+	r = try_relation_open(relationId, lockmode);
+
+	/* leave if index does not exist */
+	if (!r)
+		return NULL;
+
+	validate_relation_kind(r);
 
 	return r;
 }
@@ -168,6 +187,24 @@ index_close(Relation relation, LOCKMODE lockmode)
 		UnlockRelationId(&relid, lockmode);
 }
 
+/* ----------------
+ *		validate_relation_kind - check the relation's kind
+ *
+ *		Make sure relkind is an index or a partitioned index.
+ * ----------------
+ */
+static inline void
+validate_relation_kind(Relation r)
+{
+	if (r->rd_rel->relkind != RELKIND_INDEX &&
+		r->rd_rel->relkind != RELKIND_PARTITIONED_INDEX)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("\"%s\" is not an index",
+						RelationGetRelationName(r))));
+}
+
+
 /* ----------------
  *		index_insert - insert an index tuple into a relation
  * ----------------

src/include/access/genam.h

@@ -139,6 +139,7 @@ typedef struct IndexOrderByDistance
 #define IndexScanIsValid(scan) PointerIsValid(scan)
 
 extern Relation index_open(Oid relationId, LOCKMODE lockmode);
+extern Relation try_index_open(Oid relationId, LOCKMODE lockmode);
 extern void index_close(Relation relation, LOCKMODE lockmode);
 
 extern bool index_insert(Relation indexRelation,