Only allow typed tables to hang off composite types, not e.g. tables.

This also ensures that we take a relation lock on the composite type when
creating a typed table, which is necessary to prevent the composite type
and the typed table from getting out of step in the face of concurrent
DDL.

Noah Misch, with some changes.

src/backend/parser/parse_utilcmd.c

@@ -825,6 +825,7 @@ transformOfType(CreateStmtContext *cxt, TypeName *ofTypename)
 	TupleDesc	tupdesc;
 	int			i;
 	Oid			ofTypeId;
+	bool		typeOk = false;
 
 	AssertArg(ofTypename);
 
@@ -833,7 +834,21 @@ transformOfType(CreateStmtContext *cxt, TypeName *ofTypename)
 	ofTypeId = HeapTupleGetOid(tuple);
 	ofTypename->typeOid = ofTypeId;		/* cached for later */
 
-	if (typ->typtype != TYPTYPE_COMPOSITE)
+	if (typ->typtype == TYPTYPE_COMPOSITE)
+	{
+		Relation	typeRelation;
+
+		Assert(OidIsValid(typ->typrelid));
+		typeRelation = relation_open(typ->typrelid, AccessShareLock);
+		typeOk = (typeRelation->rd_rel->relkind == RELKIND_COMPOSITE_TYPE);
+		/*
+		 * Close the parent rel, but keep our AccessShareLock on it until xact
+		 * commit.	That will prevent someone else from deleting or ALTERing
+		 * the type before the typed table creation commits.
+		 */
+		relation_close(typeRelation, NoLock);
+	}
+	if (!typeOk)
 		ereport(ERROR,
 				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
 				 errmsg("type %s is not a composite type",

src/test/regress/expected/typed_table.out

@@ -91,6 +91,8 @@ DETAIL:  drop cascades to table persons
 drop cascades to function get_all_persons()
 drop cascades to table persons2
 drop cascades to table persons3
+CREATE TABLE persons5 OF stuff; -- only CREATE TYPE AS types may be used
+ERROR:  type stuff is not a composite type
 DROP TABLE stuff;
 -- implicit casting
 CREATE TYPE person_type AS (id int, name text);

src/test/regress/sql/typed_table.sql

@@ -46,6 +46,8 @@ CREATE TABLE persons4 OF person_type (
 DROP TYPE person_type RESTRICT;
 DROP TYPE person_type CASCADE;
 
+CREATE TABLE persons5 OF stuff; -- only CREATE TYPE AS types may be used
+
 DROP TABLE stuff;