This is xnu-8019. See this file in:
/*
 * Copyright (c) 2014 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_LICENSE_HEADER_END@
 */

#include <sys/resource.h>
#include <errno.h>
#include <mach/port.h>

extern int __setpriority(int which, id_t who, int prio);
extern void _pthread_clear_qos_tsd(mach_port_t);

/*
 * Stub function to account for special case return code from setpriority
 * when called with PRIO_DARWIN_THREAD.
 */
int
setpriority(int which, id_t who, int prio)
{
	int rv = __setpriority(which, who, prio);
	if (which == PRIO_DARWIN_THREAD && rv == -2) {
		_pthread_clear_qos_tsd(MACH_PORT_NULL);
		rv = 0;
	}

	return rv;
}