// Package linkwatch tells when the machine's network interfaces change:
// one appears, goes, or comes back (ppp0 when PPPoE reconnects). It
// listens to the kernel's rtnetlink link messages and calls back once
// they have been quiet for a moment, since one change brings several.
package linkwatch

import (
	"context"
	"encoding/binary"
	"errors"
	"fmt"
	"os"
	"time"

	"golang.org/x/sys/unix"
)

// Settle is how long the link messages must be quiet before the callback.
const Settle = 2 * time.Second

// Watch calls changed after interfaces changed, until ctx ends. It returns
// an error only if it can't listen at all.
func Watch(ctx context.Context, changed func()) error {
	fd, err := unix.Socket(unix.AF_NETLINK, unix.SOCK_RAW|unix.SOCK_CLOEXEC|unix.SOCK_NONBLOCK, unix.NETLINK_ROUTE)
	if err != nil {
		return fmt.Errorf("opening a netlink socket: %w", err)
	}
	if err := unix.Bind(fd, &unix.SockaddrNetlink{Family: unix.AF_NETLINK, Groups: unix.RTMGRP_LINK}); err != nil {
		unix.Close(fd)
		return fmt.Errorf("listening to link changes: %w", err)
	}
	// A non-blocking descriptor goes through Go's poller, so closing the
	// file ends a Read in progress.
	f := os.NewFile(uintptr(fd), "rtnetlink")
	go func() {
		<-ctx.Done()
		f.Close()
	}()
	events := make(chan struct{}, 1)
	go func() {
		buf := make([]byte, 64<<10)
		for {
			n, err := f.Read(buf)
			switch {
			case err != nil && ctx.Err() != nil:
				close(events)
				return
			case errors.Is(err, unix.ENOBUFS):
				// Messages were lost: something changed anyway.
			case err != nil:
				time.Sleep(time.Second)
				continue
			case !links(buf[:n]):
				continue
			}
			select {
			case events <- struct{}{}:
			default:
			}
		}
	}()
	var timer <-chan time.Time
	for {
		select {
		case _, ok := <-events:
			if !ok {
				return nil
			}
			timer = time.After(Settle)
		case <-timer:
			timer = nil
			changed()
		}
	}
}

// links reports whether a netlink datagram holds a link being added,
// changed or deleted.
func links(b []byte) bool {
	for len(b) >= unix.NLMSG_HDRLEN {
		size := int(binary.NativeEndian.Uint32(b[0:4]))
		typ := binary.NativeEndian.Uint16(b[4:6])
		if size < unix.NLMSG_HDRLEN || size > len(b) {
			return false
		}
		if typ == unix.RTM_NEWLINK || typ == unix.RTM_DELLINK {
			return true
		}
		size = (size + unix.NLMSG_ALIGNTO - 1) &^ (unix.NLMSG_ALIGNTO - 1)
		if size >= len(b) {
			return false
		}
		b = b[size:]
	}
	return false
}