Remove various special checks around default roles

Default roles really should be like regular roles, for the most part.
This removes a number of checks that were trying to make default roles
extra special by not allowing them to be used as regular roles.

We still prevent users from creating roles in the "pg_" namespace or
from altering roles which exist in that namespace via ALTER ROLE, as
we can't preserve such changes, but otherwise the roles are very much
like regular roles.

Based on discussion with Robert and Tom.

src/backend/commands/alter.c

@@ -747,9 +747,6 @@ ExecAlterOwnerStmt(AlterOwnerStmt *stmt)
 {
 	Oid			newowner = get_rolespec_oid(stmt->newowner, false);
 
-	check_rolespec_name(stmt->newowner,
-						"Cannot make reserved roles owners of objects.");
-
 	switch (stmt->objectType)
 	{
 		case OBJECT_DATABASE:

src/backend/commands/foreigncmds.c

@@ -1148,10 +1148,6 @@ CreateUserMapping(CreateUserMappingStmt *stmt)
 	else
 		useId = get_rolespec_oid(stmt->user, false);
 
-	/* Additional check to protect reserved role names */
-	check_rolespec_name(stmt->user,
-						"Cannot specify reserved role as mapping user.");
-
 	/* Check that the server exists. */
 	srv = GetForeignServerByName(stmt->servername, false);
 
@@ -1252,10 +1248,6 @@ AlterUserMapping(AlterUserMappingStmt *stmt)
 	else
 		useId = get_rolespec_oid(stmt->user, false);
 
-	/* Additional check to protect reserved role names */
-	check_rolespec_name(stmt->user,
-						"Cannot alter reserved role mapping user.");
-
 	srv = GetForeignServerByName(stmt->servername, false);
 
 	umId = GetSysCacheOid2(USERMAPPINGUSERSERVER,
@@ -1345,11 +1337,6 @@ RemoveUserMapping(DropUserMappingStmt *stmt)
 	else
 	{
 		useId = get_rolespec_oid(stmt->user, stmt->missing_ok);
-
-		/* Additional check to protect reserved role names */
-		check_rolespec_name(stmt->user,
-							"Cannot remove reserved role mapping user.");
-
 		if (!OidIsValid(useId))
 		{
 			/*

src/backend/commands/policy.c

@@ -176,13 +176,8 @@ policy_role_list_to_array(List *roles, int *num_roles)
 			return role_oids;
 		}
 		else
-		{
-			/* Additional check to protect reserved role names */
-			check_rolespec_name((Node *) spec,
-							"Cannot specify reserved role as policy target");
 			role_oids[i++] =
 				ObjectIdGetDatum(get_rolespec_oid((Node *) spec, false));
-		}
 	}
 
 	return role_oids;

src/backend/commands/schemacmds.c

@@ -65,10 +65,6 @@ CreateSchemaCommand(CreateSchemaStmt *stmt, const char *queryString)
 	else
 		owner_uid = saved_uid;
 
-	/* Additional check to protect reserved role names */
-	check_rolespec_name(stmt->authrole,
-						"Cannot specify reserved role as owner.");
-
 	/* fill schema name with the user name if not specified */
 	if (!schemaName)
 	{

src/backend/commands/tablecmds.c

@@ -3566,8 +3566,6 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 												(List *) cmd->def, lockmode);
 			break;
 		case AT_ChangeOwner:	/* ALTER OWNER */
-			check_rolespec_name(cmd->newowner,
-								"Cannot specify reserved role as owner.");
 			ATExecChangeOwner(RelationGetRelid(rel),
 							  get_rolespec_oid(cmd->newowner, false),
 							  false, lockmode);

src/backend/commands/tablespace.c

@@ -256,10 +256,6 @@ CreateTableSpace(CreateTableSpaceStmt *stmt)
 	else
 		ownerId = GetUserId();
 
-	/* Additional check to protect reserved role names */
-	check_rolespec_name(stmt->owner,
-						"Cannot specify reserved role as owner.");
-
 	/* Unix-ify the offered path, and strip any trailing slashes */
 	location = pstrdup(stmt->location);
 	canonicalize_path(location);

src/backend/commands/user.c

@@ -1262,18 +1262,10 @@ GrantRole(GrantRoleStmt *stmt)
 	ListCell   *item;
 
 	if (stmt->grantor)
-	{
-		check_rolespec_name(stmt->grantor,
-							"Cannot specify reserved role as grantor.");
 		grantor = get_rolespec_oid(stmt->grantor, false);
-	}
 	else
 		grantor = GetUserId();
 
-	foreach(item, stmt->grantee_roles)
-		check_rolespec_name(lfirst(item),
-							"Cannot GRANT roles to a reserved role.");
-
 	grantee_ids = roleSpecsToIds(stmt->grantee_roles);
 
 	/* AccessShareLock is enough since we aren't modifying pg_authid */
@@ -1364,9 +1356,6 @@ ReassignOwnedObjects(ReassignOwnedStmt *stmt)
 					 errmsg("permission denied to reassign objects")));
 	}
 
-	check_rolespec_name(stmt->newrole,
-						"Cannot specify reserved role as owner.");
-
 	/* Must have privileges on the receiving side too */
 	newrole = get_rolespec_oid(stmt->newrole, false);
 

src/backend/commands/variable.c

@@ -794,10 +794,6 @@ check_session_authorization(char **newval, void **extra, GucSource source)
 		return false;
 	}
 
-	/* Do not allow setting role to a reserved role. */
-	if (strncmp(*newval, "pg_", 3) == 0)
-		return false;
-
 	/* Look up the username */
 	roleTup = SearchSysCache1(AUTHNAME, PointerGetDatum(*newval));
 	if (!HeapTupleIsValid(roleTup))
@@ -858,9 +854,6 @@ check_role(char **newval, void **extra, GucSource source)
 		roleid = InvalidOid;
 		is_superuser = false;
 	}
-	/* Do not allow setting role to a reserved role. */
-	else if (strncmp(*newval, "pg_", 3) == 0)
-		return false;
 	else
 	{
 		if (!IsTransactionState())