// Copyright 2017 Luke Shumaker
//
// For just ListenFds:
//   Copyright 2015 CoreOS, Inc.
//   Copyright 2015-2016 Luke Shumaker
//
// 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.

package main

import (
	"fmt"
	"net"
	"os"
	"strconv"
	"strings"

	unix "syscall"
)

// ListenFds returns a list of file descriptors passed in by the
// service manager as part of the socket-based activation logic.
//
// If unsetEnv is true, then (regardless of whether the function call
// itself succeeds or not) it will unset the environmental variables
// LISTEN_FDS and LISTEN_PID, which will cause further calls to this
// function to fail.
//
// In the case of an error, this function returns nil.
func ListenFds(unsetEnv bool) []*os.File {
	if unsetEnv {
		defer func() {
			_ = os.Unsetenv("LISTEN_PID")
			_ = os.Unsetenv("LISTEN_FDS")
			_ = os.Unsetenv("LISTEN_FDNAMES")
		}()
	}

	pid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
	if err != nil || pid != os.Getpid() {
		return nil
	}

	nfds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
	if err != nil || nfds < 1 {
		return nil
	}

	names := strings.Split(os.Getenv("LISTEN_FDNAMES"), ":")

	files := make([]*os.File, 0, nfds)
	for i := 0; i < nfds; i++ {
		fd := i + 3
		unix.CloseOnExec(fd)
		name := "unknown"
		if i < len(names) {
			name = names[i]
		}
		files = append(files, os.NewFile(uintptr(fd), name))
	}

	return files
}

func main() {
	files := ListenFds(false)
	if len(files) != 1 {
		fmt.Fprintf(os.Stderr, "expected 1 fd, got %d\n", len(files))
		os.Exit(1)
	}

	sync_sock, err := net.FileConn(files[0])
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(127)
	}

	var dat [4096]byte
	for {
		n, err := sync_sock.Read(dat[:])
		if err != nil {
			fmt.Fprintln(os.Stderr, err)
			os.Exit(127)
		}
		if n > 0 {
			os.Exit(0)
		}
	}
}