// Copyright 2019 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// +build linux

package flipcall

import (
	"fmt"
	"runtime"
	"sync/atomic"

	"golang.org/x/sys/unix"
	"gvisor.dev/gvisor/pkg/abi/linux"
)

func (ep *Endpoint) futexSetPeerActive() error {
	if atomic.CompareAndSwapUint32(ep.connState(), ep.activeState, ep.inactiveState) {
		return nil
	}
	switch cs := atomic.LoadUint32(ep.connState()); cs {
	case csShutdown:
		return ShutdownError{}
	default:
		return fmt.Errorf("unexpected connection state before FUTEX_WAKE: %v", cs)
	}
}

func (ep *Endpoint) futexWakePeer() error {
	if err := ep.futexWakeConnState(1); err != nil {
		return fmt.Errorf("failed to FUTEX_WAKE peer Endpoint: %v", err)
	}
	return nil
}

func (ep *Endpoint) futexWaitUntilActive() error {
	for {
		switch cs := atomic.LoadUint32(ep.connState()); cs {
		case ep.activeState:
			return nil
		case ep.inactiveState:
			if ep.isShutdownLocally() {
				return ShutdownError{}
			}
			if err := ep.futexWaitConnState(ep.inactiveState); err != nil {
				return fmt.Errorf("failed to FUTEX_WAIT for peer Endpoint: %v", err)
			}
			continue
		case csShutdown:
			return ShutdownError{}
		default:
			return fmt.Errorf("unexpected connection state before FUTEX_WAIT: %v", cs)
		}
	}
}

func (ep *Endpoint) futexWakeConnState(numThreads int32) error {
	if _, _, e := unix.RawSyscall(unix.SYS_FUTEX, ep.packet, linux.FUTEX_WAKE, uintptr(numThreads)); e != 0 {
		return e
	}
	return nil
}

func (ep *Endpoint) futexWaitConnState(curState uint32) error {
	_, _, e := unix.Syscall6(unix.SYS_FUTEX, ep.packet, linux.FUTEX_WAIT, uintptr(curState), 0, 0, 0)
	if e != 0 && e != unix.EAGAIN && e != unix.EINTR {
		return e
	}
	return nil
}

func yieldThread() {
	unix.Syscall(unix.SYS_SCHED_YIELD, 0, 0, 0)
	// The thread we're trying to yield to may be waiting for a Go runtime P.
	// runtime.Gosched() will hand off ours if necessary.
	runtime.Gosched()
}