//go:build linux

package agent

import (
	"errors"
	"fmt"
	"net"
	"os"
	"runtime"

	"github.com/containernetworking/plugins/pkg/ns"
	"github.com/vishvananda/netlink"
)

// SetupRequest is the netlink setup input for one pod.
type SetupRequest struct {
	ContainerID string
	Netns       string // /proc/<pid>/ns/net
	IfName      string // pod-side iface name (typically "eth0")
	HostIface   string // host-side veth name from HostIfaceName
	IP6         net.IP // /128 inside pod
	IP4         net.IP // /32 inside pod (may be nil)
}

// LinkLocalGW is the deterministic IPv6 LL gateway placed on every host
// veth. Pod default route uses this as next-hop. Avoids waiting for kernel
// LL DAD on the host side.
var linkLocalGW = net.ParseIP("fe80::1")

// v4ProxyGW is the well-known link-local IPv4 used by container CNIs as a
// next-hop for proxy-arp gateways (cilium, calico, kindnet — all use this).
var v4ProxyGW = net.IPv4(169, 254, 1, 1)

// Setup creates the veth pair, configures the host side, moves the peer
// into the pod netns, configures the pod side, and writes host routes.
// All steps are idempotent: an already-existing object that matches the
// desired state is treated as success.
func Setup(req SetupRequest) error {
	if req.HostIface == "" {
		req.HostIface = HostIfaceName(req.ContainerID)
	}
	if req.IfName == "" {
		req.IfName = "eth0"
	}

	// Create veth pair (or reuse existing).
	host, peer, err := ensureVeth(req.HostIface, req.IfName)
	if err != nil {
		return fmt.Errorf("ensure veth: %w", err)
	}

	// Host-side: addrgenmode none → up → fe80::1/64 → sysctls.
	if err := configureHostSide(host); err != nil {
		return fmt.Errorf("configure host side %s: %w", host.Attrs().Name, err)
	}

	// Move peer into pod netns + configure (only if it's still on host).
	hostNS, err := ns.GetCurrentNS()
	if err != nil {
		return fmt.Errorf("get current netns: %w", err)
	}
	defer hostNS.Close()

	if peer != nil {
		// Peer is still on the host — move it.
		podNS, err := ns.GetNS(req.Netns)
		if err != nil {
			return fmt.Errorf("open pod netns %s: %w", req.Netns, err)
		}
		defer podNS.Close()
		if err := netlink.LinkSetNsFd(peer, int(podNS.Fd())); err != nil {
			return fmt.Errorf("move peer %s into pod ns: %w", peer.Attrs().Name, err)
		}
	}

	// Configure pod-side from inside the pod netns.
	if err := configurePodSide(req); err != nil {
		return fmt.Errorf("configure pod side: %w", err)
	}

	// Host route(s): one /128 (and /32 if v4) pointing at the host veth.
	if err := setHostRoute(host.Attrs().Index, req.IP6, 128); err != nil {
		return fmt.Errorf("host route v6: %w", err)
	}
	if req.IP4 != nil {
		if err := setHostRoute(host.Attrs().Index, req.IP4, 32); err != nil {
			return fmt.Errorf("host route v4: %w", err)
		}
	}

	return nil
}

// Teardown removes the host-side veth (which also tears down the peer in
// the pod netns) and the host /128 + /32 routes. All operations are
// idempotent — missing objects are not errors.
func Teardown(containerID string, ip6, ip4 net.IP) error {
	hostName := HostIfaceName(containerID)
	host, err := netlink.LinkByName(hostName)
	if err == nil {
		// Routes are removed when the link goes away, but be explicit so
		// stale routes can't outlive the veth on a corrupt state.
		if ip6 != nil {
			_ = netlink.RouteDel(&netlink.Route{LinkIndex: host.Attrs().Index, Dst: cidrFor(ip6, 128)})
		}
		if ip4 != nil {
			_ = netlink.RouteDel(&netlink.Route{LinkIndex: host.Attrs().Index, Dst: cidrFor(ip4, 32)})
		}
		if err := netlink.LinkDel(host); err != nil && !errors.Is(err, os.ErrNotExist) {
			return fmt.Errorf("delete %s: %w", hostName, err)
		}
	} else if !linkNotFound(err) {
		return fmt.Errorf("lookup %s: %w", hostName, err)
	}
	return nil
}

// ensureVeth returns the host link (always) and the peer link (only if it's
// still on the host — nil if it has already been moved into a netns).
func ensureVeth(hostName, peerName string) (netlink.Link, netlink.Link, error) {
	if existing, err := netlink.LinkByName(hostName); err == nil {
		// Already exists; the peer may be on the host or in a netns.
		peer, _ := netlink.LinkByName(peerName) // peer name is "eth0" — usually only matches in pod ns
		_ = peer
		// Don't try to find peer on host by name (collides). Return nil peer; ensureVeth caller treats nil as "already moved".
		return existing, nil, nil
	}
	// Need to create.
	veth := &netlink.Veth{
		LinkAttrs: netlink.LinkAttrs{Name: hostName, MTU: 1500},
		PeerName:  peerName,
	}
	if err := netlink.LinkAdd(veth); err != nil {
		return nil, nil, fmt.Errorf("link add: %w", err)
	}
	host, err := netlink.LinkByName(hostName)
	if err != nil {
		return nil, nil, fmt.Errorf("lookup host after add: %w", err)
	}
	peer, err := netlink.LinkByName(peerName)
	if err != nil {
		return nil, nil, fmt.Errorf("lookup peer after add: %w", err)
	}
	return host, peer, nil
}

func configureHostSide(host netlink.Link) error {
	name := host.Attrs().Name

	// addrgenmode = none (suppress kernel LL).
	if err := netlink.LinkSetVfHardwareAddr(host, 0, nil); err != nil {
		// This SetVf isn't the right call; instead use LinkSetGroup or use sysfs directly.
		// Fallback: write to /proc/sys/net/ipv6/conf/<iface>/addr_gen_mode = 1
	}
	_ = sysctlWrite("/proc/sys/net/ipv6/conf/"+name+"/addr_gen_mode", "1")

	// Bring up.
	if err := netlink.LinkSetUp(host); err != nil {
		return fmt.Errorf("set up: %w", err)
	}

	// fe80::1/64.
	addr := &netlink.Addr{IPNet: &net.IPNet{IP: linkLocalGW, Mask: net.CIDRMask(64, 128)}}
	if err := netlink.AddrAdd(host, addr); err != nil && !errors.Is(err, os.ErrExist) {
		return fmt.Errorf("addr add fe80::1: %w", err)
	}

	// sysctls.
	for _, kv := range []struct{ k, v string }{
		{"/proc/sys/net/ipv4/conf/" + name + "/proxy_arp", "1"},
		{"/proc/sys/net/ipv4/conf/" + name + "/forwarding", "1"},
		{"/proc/sys/net/ipv6/conf/" + name + "/forwarding", "1"},
	} {
		if err := sysctlWrite(kv.k, kv.v); err != nil {
			return err
		}
	}
	return nil
}

func configurePodSide(req SetupRequest) error {
	podNS, err := ns.GetNS(req.Netns)
	if err != nil {
		return err
	}
	defer podNS.Close()

	return podNS.Do(func(ns.NetNS) error {
		runtime.LockOSThread()
		defer runtime.UnlockOSThread()

		eth0, err := netlink.LinkByName(req.IfName)
		if err != nil {
			return fmt.Errorf("lookup pod %s: %w", req.IfName, err)
		}

		_ = sysctlWrite("/proc/sys/net/ipv6/conf/"+req.IfName+"/addr_gen_mode", "1")
		if err := netlink.LinkSetUp(eth0); err != nil {
			return fmt.Errorf("set up pod %s: %w", req.IfName, err)
		}

		if req.IP6 != nil {
			a := &netlink.Addr{IPNet: &net.IPNet{IP: req.IP6, Mask: net.CIDRMask(128, 128)}}
			if err := netlink.AddrAdd(eth0, a); err != nil && !errors.Is(err, os.ErrExist) {
				return fmt.Errorf("pod ip6 add: %w", err)
			}
			// Default route via fe80::1, no scope on-link issues because LL is reachable on the link.
			if err := netlink.RouteAdd(&netlink.Route{
				LinkIndex: eth0.Attrs().Index,
				Dst:       &net.IPNet{IP: net.IPv6zero, Mask: net.CIDRMask(0, 128)},
				Gw:        linkLocalGW,
			}); err != nil && !errors.Is(err, os.ErrExist) {
				return fmt.Errorf("pod default v6 route: %w", err)
			}
		}

		if req.IP4 != nil {
			a := &netlink.Addr{IPNet: &net.IPNet{IP: req.IP4, Mask: net.CIDRMask(32, 32)}}
			if err := netlink.AddrAdd(eth0, a); err != nil && !errors.Is(err, os.ErrExist) {
				return fmt.Errorf("pod ip4 add: %w", err)
			}
			// On-link route to the proxy gateway, then default via that gateway.
			if err := netlink.RouteAdd(&netlink.Route{
				LinkIndex: eth0.Attrs().Index,
				Scope:     netlink.SCOPE_LINK,
				Dst:       &net.IPNet{IP: v4ProxyGW, Mask: net.CIDRMask(32, 32)},
			}); err != nil && !errors.Is(err, os.ErrExist) {
				return fmt.Errorf("pod onlink v4 route: %w", err)
			}
			if err := netlink.RouteAdd(&netlink.Route{
				LinkIndex: eth0.Attrs().Index,
				Dst:       &net.IPNet{IP: net.IPv4zero, Mask: net.CIDRMask(0, 32)},
				Gw:        v4ProxyGW,
			}); err != nil && !errors.Is(err, os.ErrExist) {
				return fmt.Errorf("pod default v4 route: %w", err)
			}
		}

		return nil
	})
}

func setHostRoute(linkIndex int, ip net.IP, prefix int) error {
	r := &netlink.Route{
		LinkIndex: linkIndex,
		Scope:     netlink.SCOPE_LINK,
		Dst:       cidrFor(ip, prefix),
	}
	if err := netlink.RouteReplace(r); err != nil {
		return err
	}
	return nil
}

func cidrFor(ip net.IP, prefix int) *net.IPNet {
	if ip.To4() != nil {
		return &net.IPNet{IP: ip.To4(), Mask: net.CIDRMask(prefix, 32)}
	}
	return &net.IPNet{IP: ip.To16(), Mask: net.CIDRMask(prefix, 128)}
}

func sysctlWrite(path, value string) error {
	if err := os.WriteFile(path, []byte(value), 0o644); err != nil {
		// Some sysctls don't exist for newly-created interfaces until ipv6 is
		// loaded; treat ENOENT as best-effort.
		if errors.Is(err, os.ErrNotExist) {
			return nil
		}
		return fmt.Errorf("sysctl %s=%s: %w", path, value, err)
	}
	return nil
}

func linkNotFound(err error) bool {
	if err == nil {
		return false
	}
	var lnf netlink.LinkNotFoundError
	return errors.As(err, &lnf)
}