net/tstun/table: add initial RoutingTable implementation

It is based on `*tempfork/device.AllowedIPs`.

Updates tailscale/corp#8020

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/tstun/table/table.go

@@ -0,0 +1,90 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package table provides a Routing Table implementation which allows
+// looking up the peer that should be used to route a given IP address.
+package table
+
+import (
+	"net/netip"
+
+	"tailscale.com/tempfork/device"
+	"tailscale.com/types/key"
+	"tailscale.com/util/mak"
+)
+
+// RoutingTableBuilder is a builder for a RoutingTable.
+// It is not safe for concurrent use.
+type RoutingTableBuilder struct {
+	// peers is a map from node public key to the peer that owns that key.
+	// It is only used to handle insertions and deletions.
+	peers map[key.NodePublic]*device.Peer
+
+	// prefixTrie is a trie of prefixes which facilitates looking up the
+	// peer that owns a given IP address.
+	prefixTrie *device.AllowedIPs
+}
+
+// Remove removes the given peer from the routing table.
+func (t *RoutingTableBuilder) Remove(peer key.NodePublic) {
+	p, ok := t.peers[peer]
+	if !ok {
+		return
+	}
+	t.prefixTrie.RemoveByPeer(p)
+	delete(t.peers, peer)
+}
+
+// InsertOrReplace inserts the given peer and prefixes into the routing table.
+func (t *RoutingTableBuilder) InsertOrReplace(peer key.NodePublic, pfxs ...netip.Prefix) {
+	p, ok := t.peers[peer]
+	if !ok {
+		p = device.NewPeer(peer)
+		mak.Set(&t.peers, peer, p)
+	} else {
+		t.prefixTrie.RemoveByPeer(p)
+	}
+	if len(pfxs) == 0 {
+		return
+	}
+	if t.prefixTrie == nil {
+		t.prefixTrie = new(device.AllowedIPs)
+	}
+	for _, pfx := range pfxs {
+		t.prefixTrie.Insert(pfx, p)
+	}
+}
+
+// Build returns a RoutingTable that can be used to look up peers.
+// Build resets the RoutingTableBuilder to its zero value.
+func (t *RoutingTableBuilder) Build() *RoutingTable {
+	pt := t.prefixTrie
+	t.prefixTrie = nil
+	t.peers = nil
+	return &RoutingTable{
+		prefixTrie: pt,
+	}
+}
+
+// RoutingTable provides a mapping from IP addresses to peers identified by
+// their public node key. It is used to find the peer that should be used to
+// route a given IP address.
+// It is immutable after creation.
+//
+// It is safe for concurrent use.
+type RoutingTable struct {
+	prefixTrie *device.AllowedIPs
+}
+
+// Lookup returns the peer that would be used to route the given IP address.
+// If no peer is found, Lookup returns the zero value.
+func (t *RoutingTable) Lookup(ip netip.Addr) (_ key.NodePublic, ok bool) {
+	if t == nil {
+		return key.NodePublic{}, false
+	}
+	p := t.prefixTrie.Lookup(ip.AsSlice())
+	if p == nil {
+		return key.NodePublic{}, false
+	}
+	return p.Key(), true
+}

tempfork/device/peer.go

@@ -7,8 +7,22 @@ package device
 
 import (
 	"container/list"
+
+	"tailscale.com/types/key"
 )
 
 type Peer struct {
 	trieEntries list.List
+
+	key key.NodePublic
+}
+
+func NewPeer(k key.NodePublic) *Peer {
+	return &Peer{
+		key: k,
+	}
+}
+
+func (p *Peer) Key() key.NodePublic {
+	return p.key
 }