// Copyright 2016 The Netstack Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package stack

import (
	"strings"
	"sync"
	"sync/atomic"

	"github.com/google/netstack/ilist"
	"github.com/google/netstack/tcpip"
	"github.com/google/netstack/tcpip/buffer"
	"github.com/google/netstack/tcpip/header"
)

// NIC represents a "network interface card" to which the networking stack is
// attached.
type NIC struct {
	stack  *Stack
	id     tcpip.NICID
	linkEP LinkEndpoint

	demux *transportDemuxer

	mu          sync.RWMutex
	promiscuous bool
	primary     map[tcpip.NetworkProtocolNumber]*ilist.List
	endpoints   map[NetworkEndpointID]*referencedNetworkEndpoint
	subnets     []tcpip.Subnet
}

// PrimaryEndpointBehavior specifies how a new address should behave as a primary endpoint.
type PrimaryEndpointBehavior int

const (
	// CanBePrimaryEndpoint indicates the endpoint can be used as a primary
	// endpoint for new connections with no local address. This is the
	// default when calling NIC.AddAddress.
	CanBePrimaryEndpoint PrimaryEndpointBehavior = iota
	// FirstPrimaryEndpoint indicates the endpoint should be the first
	// primary endpoint considered. If there are multiple endpoints with
	// this behavior, the most recently-added one will be first.
	FirstPrimaryEndpoint
	// NeverPrimaryEndpoint indicates the endpoint should never be a
	// primary endpoint.
	NeverPrimaryEndpoint
)

func newNIC(stack *Stack, id tcpip.NICID, ep LinkEndpoint) *NIC {
	return &NIC{
		stack:     stack,
		id:        id,
		linkEP:    ep,
		demux:     newTransportDemuxer(stack),
		primary:   make(map[tcpip.NetworkProtocolNumber]*ilist.List),
		endpoints: make(map[NetworkEndpointID]*referencedNetworkEndpoint),
	}
}

// attachLinkEndpoint attaches the NIC to the endpoint, which will enable it
// to start delivering packets.
func (n *NIC) attachLinkEndpoint() {
	n.linkEP.Attach(n)
}

// setPromiscuousMode enables or disables promiscuous mode.
func (n *NIC) setPromiscuousMode(enable bool) {
	n.mu.Lock()
	n.promiscuous = enable
	n.mu.Unlock()
}

// Get the primary network endpoint, if there is one; otherwise pick an arbitrary endpoint from the NIC's endpoints.
func (n *NIC) getMainNICAddress(protocol tcpip.NetworkProtocolNumber) (tcpip.Address, tcpip.Subnet) {
	n.mu.RLock()
	defer n.mu.RUnlock()

	var address tcpip.Address
	var subnet tcpip.Subnet

	// Check for a primary endpoint.
	var r *referencedNetworkEndpoint
	list := n.primary[protocol]
	if list != nil {
		for e := list.Front(); e != nil; e = e.Next() {
			ref := e.(*referencedNetworkEndpoint)
			if ref.holdsInsertRef && ref.tryIncRef() {
				r = ref
				break
			}
		}

	}

	// If no primary endpoints then check for other endpoints.
	if r == nil {
		for _, ref := range n.endpoints {
			if ref != nil && ref.holdsInsertRef && ref.tryIncRef() {
				r = ref
				break
			}
		}
	}

	if r != nil {
		address = r.ep.ID().LocalAddress
		r.decRef()
	}

	// Find the least-constrained matching subnet for the address, if one exists, and return it
	if address != "" {
		for _, s := range n.subnets {
			if s.Contains(address) && !subnet.Contains(s.ID()) {
				subnet = s
			}
		}
	}

	return address, subnet
}

// primaryEndpoint returns the primary endpoint of n for the given network
// protocol.
func (n *NIC) primaryEndpoint(protocol tcpip.NetworkProtocolNumber) *referencedNetworkEndpoint {
	n.mu.RLock()
	defer n.mu.RUnlock()

	list := n.primary[protocol]
	if list == nil {
		return nil
	}

	for e := list.Front(); e != nil; e = e.Next() {
		r := e.(*referencedNetworkEndpoint)
		if r.tryIncRef() {
			return r
		}
	}

	return nil
}

// findEndpoint finds the endpoint, if any, with the given address.
func (n *NIC) findEndpoint(address tcpip.Address) *referencedNetworkEndpoint {
	n.mu.RLock()
	defer n.mu.RUnlock()

	ref := n.endpoints[NetworkEndpointID{address}]
	if ref == nil || !ref.tryIncRef() {
		return nil
	}

	return ref
}

func (n *NIC) addAddressLocked(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address, peb PrimaryEndpointBehavior, replace bool) (*referencedNetworkEndpoint, *tcpip.Error) {
	netProto, ok := n.stack.networkProtocols[protocol]
	if !ok {
		return nil, tcpip.ErrUnknownProtocol
	}

	// Create the new network endpoint.
	ep, err := netProto.NewEndpoint(n.id, addr, n.stack, n, n.linkEP)
	if err != nil {
		return nil, err
	}

	id := *ep.ID()
	if ref, ok := n.endpoints[id]; ok {
		if !replace {
			return nil, tcpip.ErrDuplicateAddress
		}

		n.removeEndpointLocked(ref)
	}

	ref := &referencedNetworkEndpoint{
		refs:           1,
		ep:             ep,
		nic:            n,
		protocol:       protocol,
		holdsInsertRef: true,
	}
	if linkRes := n.stack.linkAddrResolvers[protocol]; linkRes != nil {
		ref.linkRes = linkRes
		ref.linkCache = n.stack
		ref.linkEP = n.linkEP
	}

	n.endpoints[id] = ref

	l, ok := n.primary[protocol]
	if !ok {
		l = &ilist.List{}
		n.primary[protocol] = l
	}

	switch peb {
	case CanBePrimaryEndpoint:
		l.PushBack(ref)
	case FirstPrimaryEndpoint:
		l.PushFront(ref)
	}

	return ref, nil
}

// AddAddress adds a new address to n, so that it starts accepting packets
// targeted at the given address (and network protocol).
func (n *NIC) AddAddress(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address) *tcpip.Error {
	return n.AddAddressWithOptions(protocol, addr, CanBePrimaryEndpoint)
}

// AddAddressWithOptions is the same as AddAddress, but allows you to specify whether they new endpoint can be primary or not.
func (n *NIC) AddAddressWithOptions(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address, peb PrimaryEndpointBehavior) *tcpip.Error {
	// Add the endpoint.
	n.mu.Lock()
	_, err := n.addAddressLocked(protocol, addr, peb, false)
	n.mu.Unlock()

	return err
}

// AddSubnet adds a new subnet to n, so that it starts accepting packets
// targeted at the given address and network protocol.
func (n *NIC) AddSubnet(protocol tcpip.NetworkProtocolNumber, subnet tcpip.Subnet) {
	n.mu.Lock()
	n.subnets = append(n.subnets, subnet)
	n.mu.Unlock()
}

// RemoveSubnet removes the given subnet from n.
func (n *NIC) RemoveSubnet(subnet tcpip.Subnet) {
	n.mu.Lock()

	var filtered []tcpip.Subnet
	for _, sub := range n.subnets {
		if sub != subnet {
			filtered = append(filtered, sub)
		}
	}

	n.subnets = filtered
	n.mu.Unlock()
	return
}

func (n *NIC) ContainsSubnet(subnet tcpip.Subnet) bool {
	subnets := n.Subnets()

	for _, s := range subnets {
		if s == subnet {
			return true
		}
	}
	return false
}

// Subnets returns the Subnets associated with this NIC.
func (n *NIC) Subnets() []tcpip.Subnet {
	n.mu.RLock()
	defer n.mu.RUnlock()
	sns := make([]tcpip.Subnet, 0, len(n.subnets)+len(n.endpoints))
	for nid := range n.endpoints {
		sn, err := tcpip.NewSubnet(nid.LocalAddress, tcpip.AddressMask(strings.Repeat("\xff", len(nid.LocalAddress))))
		if err != nil {
			// This should never happen as the mask has been carefully crafted to
			// match the address.
			panic("Invalid endpoint subnet: " + err.Error())
		}
		sns = append(sns, sn)
	}
	return append(sns, n.subnets...)
}

func (n *NIC) removeEndpointLocked(r *referencedNetworkEndpoint) {
	id := *r.ep.ID()

	// Nothing to do if the reference has already been replaced with a
	// different one.
	if n.endpoints[id] != r {
		return
	}

	if r.holdsInsertRef {
		panic("Reference count dropped to zero before being removed")
	}

	delete(n.endpoints, id)
	wasInList := r.Next() != nil || r.Prev() != nil || r == n.primary[r.protocol].Front()
	if wasInList {
		n.primary[r.protocol].Remove(r)
	}

	r.ep.Close()
}

func (n *NIC) removeEndpoint(r *referencedNetworkEndpoint) {
	n.mu.Lock()
	n.removeEndpointLocked(r)
	n.mu.Unlock()
}

// RemoveAddress removes an address from n.
func (n *NIC) RemoveAddress(addr tcpip.Address) *tcpip.Error {
	n.mu.Lock()
	r := n.endpoints[NetworkEndpointID{addr}]
	if r == nil || !r.holdsInsertRef {
		n.mu.Unlock()
		return tcpip.ErrBadLocalAddress
	}

	r.holdsInsertRef = false
	n.mu.Unlock()

	r.decRef()

	return nil
}

// DeliverNetworkPacket finds the appropriate network protocol endpoint and
// hands the packet over for further processing. This function is called when
// the NIC receives a packet from the physical interface.
// Note that the ownership of the slice backing vv is retained by the caller.
// This rule applies only to the slice itself, not to the items of the slice;
// the ownership of the items is not retained by the caller.
func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, _, remoteLinkAddr tcpip.LinkAddress, protocol tcpip.NetworkProtocolNumber, vv *buffer.VectorisedView) {
	netProto, ok := n.stack.networkProtocols[protocol]
	if !ok {
		atomic.AddUint64(&n.stack.stats.IP.PacketsDiscarded, 1)
		atomic.AddUint64(&n.stack.stats.UnknownProtocolRcvdPackets, 1)
		return
	}

	if netProto.Number() == header.IPv4ProtocolNumber || netProto.Number() == header.IPv6ProtocolNumber {
		atomic.AddUint64(&n.stack.stats.IP.PacketsReceived, 1)
	}

	if len(vv.First()) < netProto.MinimumPacketSize() {
		atomic.AddUint64(&n.stack.stats.IP.PacketsDiscarded, 1)
		atomic.AddUint64(&n.stack.stats.MalformedRcvdPackets, 1)
		return
	}

	src, dst := netProto.ParseAddresses(vv.First())
	id := NetworkEndpointID{dst}

	n.mu.RLock()
	ref := n.endpoints[id]
	if ref != nil && !ref.tryIncRef() {
		ref = nil
	}
	promiscuous := n.promiscuous
	subnets := n.subnets
	n.mu.RUnlock()

	if ref == nil {
		// Check if the packet is for a subnet this NIC cares about.
		if !promiscuous {
			for _, sn := range subnets {
				if sn.Contains(dst) {
					promiscuous = true
					break
				}
			}
		}
		if promiscuous {
			// Try again with the lock in exclusive mode. If we still can't
			// get the endpoint, create a new "temporary" one. It will only
			// exist while there's a route through it.
			n.mu.Lock()
			ref = n.endpoints[id]
			if ref == nil || !ref.tryIncRef() {
				ref, _ = n.addAddressLocked(protocol, dst, CanBePrimaryEndpoint, true)
				if ref != nil {
					ref.holdsInsertRef = false
				}
			}
			n.mu.Unlock()
		}
	}

	if ref == nil {
		// This NIC doesn't care the packet. Find a NIC that cares about the packet and
		// forward it to the NIC.
		// TODO: Should forward the packet even if 'promiscuous' is enabled?
		if n.stack.Forwarding() {
			r, err := n.stack.FindRoute(0, "", dst, protocol)
			if err != nil {
				// Can't find a NIC.
				atomic.AddUint64(&n.stack.stats.IP.InvalidAddressesReceived, 1)
				return
			}
			defer r.Release()

			// Found a NIC.
			n2 := r.ref.nic
			n2.mu.RLock()
			ref := n2.endpoints[id]
			if ref != nil && !ref.tryIncRef() {
				ref = nil
			}
			n2.mu.RUnlock()

			r.LocalLinkAddress = n2.linkEP.LinkAddress()
			r.RemoteLinkAddress = remoteLinkAddr

			if ref == nil {
				// n2 doesn't have a destination endpoint.
				// Send the packet out of n2.
				if ep, ok := n2.linkEP.(BufferWritingLinkEndpoint); ok {
					ep.WriteBuffer(&r, vv, protocol)
				}
			} else {
				ref.ep.HandlePacket(&r, vv)
				ref.decRef()
			}
			return
		} else {
			atomic.AddUint64(&n.stack.stats.IP.InvalidAddressesReceived, 1)
			return
		}
	}

	r := makeRoute(protocol, dst, src, ref)
	r.LocalLinkAddress = linkEP.LinkAddress()
	r.RemoteLinkAddress = remoteLinkAddr
	ref.ep.HandlePacket(&r, vv)
	ref.decRef()
}

// DeliverTransportPacket delivers the packets to the appropriate transport
// protocol endpoint.
func (n *NIC) DeliverTransportPacket(r *Route, protocol tcpip.TransportProtocolNumber, vv *buffer.VectorisedView) {
	state, ok := n.stack.transportProtocols[protocol]
	if !ok {
		atomic.AddUint64(&n.stack.stats.UnknownProtocolRcvdPackets, 1)
		return
	}

	transProto := state.proto
	if len(vv.First()) < transProto.MinimumPacketSize() {
		atomic.AddUint64(&n.stack.stats.MalformedRcvdPackets, 1)
		return
	}

	srcPort, dstPort, err := transProto.ParsePorts(vv.First())
	if err != nil {
		atomic.AddUint64(&n.stack.stats.MalformedRcvdPackets, 1)
		return
	}

	id := TransportEndpointID{dstPort, r.LocalAddress, srcPort, r.RemoteAddress}
	if n.demux.deliverPacket(r, protocol, vv, id) {
		return
	}
	if n.stack.demux.deliverPacket(r, protocol, vv, id) {
		return
	}

	// Try to deliver to per-stack default handler.
	if state.defaultHandler != nil {
		if state.defaultHandler(r, id, vv) {
			return
		}
	}

	// We could not find an appropriate destination for this packet, so
	// deliver it to the global handler.
	if !transProto.HandleUnknownDestinationPacket(r, id, vv) {
		atomic.AddUint64(&n.stack.stats.MalformedRcvdPackets, 1)
	}
}

// ID returns the identifier of n.
func (n *NIC) ID() tcpip.NICID {
	return n.id
}

type referencedNetworkEndpoint struct {
	ilist.Entry
	refs      int32
	ep        NetworkEndpoint
	nic       *NIC
	protocol  tcpip.NetworkProtocolNumber
	linkRes   LinkAddressResolver
	linkCache LinkAddressCache
	linkEP    LinkEndpoint

	// holdsInsertRef is protected by the NIC's mutex. It indicates whether
	// the reference count is biased by 1 due to the insertion of the
	// endpoint. It is reset to false when RemoveAddress is called on the
	// NIC.
	holdsInsertRef bool
}

// decRef decrements the ref count and cleans up the endpoint once it reaches
// zero.
func (r *referencedNetworkEndpoint) decRef() {
	if atomic.AddInt32(&r.refs, -1) == 0 {
		r.nic.removeEndpoint(r)
	}
}

// incRef increments the ref count. It must only be called when the caller is
// known to be holding a reference to the endpoint, otherwise tryIncRef should
// be used.
func (r *referencedNetworkEndpoint) incRef() {
	atomic.AddInt32(&r.refs, 1)
}

// tryIncRef attempts to increment the ref count from n to n+1, but only if n is
// not zero. That is, it will increment the count if the endpoint is still
// alive, and do nothing if it has already been clean up.
func (r *referencedNetworkEndpoint) tryIncRef() bool {
	for {
		v := atomic.LoadInt32(&r.refs)
		if v == 0 {
			return false
		}

		if atomic.CompareAndSwapInt32(&r.refs, v, v+1) {
			return true
		}
	}
}