wgengine/magicsock: keep discoOfAddr populated, use it for findEndpoint

Update the mapping from ip:port to discokey, so when we retrieve a
packet from the network, we can find the same conn.Endpoint that we
gave to wireguard-go previously, without making it think we've
roamed. (We did, but we're not using its roaming.)

Updates #483

wgengine/magicsock/magicsock.go

@@ -97,8 +97,7 @@ type Conn struct {
 	discoPublic     tailcfg.DiscoKey // public of discoPrivate
 	nodeOfDisco     map[tailcfg.DiscoKey]*tailcfg.Node
 	discoOfNode     map[tailcfg.NodeKey]tailcfg.DiscoKey
-
+	discoOfAddr     map[netaddr.IPPort]tailcfg.DiscoKey // validated non-DERP paths only
-	endpointOfAddr  map[netaddr.IPPort]*discoEndpoint // validated non-DERP paths only
 	endpointOfDisco map[tailcfg.DiscoKey]*discoEndpoint
 	sharedDiscoKey  map[tailcfg.DiscoKey]*[32]byte // nacl/box precomputed key
 
@@ -257,7 +256,7 @@ func newConn() *Conn {
 		peerLastDerp:    make(map[key.Public]int),
 		endpointOfDisco: make(map[tailcfg.DiscoKey]*discoEndpoint),
 		sharedDiscoKey:  make(map[tailcfg.DiscoKey]*[32]byte),
-		endpointOfAddr:  make(map[netaddr.IPPort]*discoEndpoint),
+		discoOfAddr:     make(map[netaddr.IPPort]tailcfg.DiscoKey),
 	}
 	c.endpointsUpdateWaiter = sync.NewCond(&c.mu)
 	return c
@@ -1171,14 +1170,20 @@ func (c *Conn) runDerpWriter(ctx context.Context, dc *derphttp.Client, ch <-chan
 // findEndpoint maps from a UDP address to a WireGuard endpoint, for
 // ReceiveIPv4/ReceiveIPv6.
 // The provided addr and ipp must match.
+//
+// TODO(bradfitz): add a fast path that returns nil here for normal
+// wireguard-go transport packets; IIRC wireguard-go only uses this
+// Endpoint for the relatively rare non-data packets.
 func (c *Conn) findEndpoint(ipp netaddr.IPPort, addr *net.UDPAddr) conn.Endpoint {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
 	// See if they have a discoEndpoint, for a set of peers
 	// both supporting active discovery.
-	if de, ok := c.endpointOfAddr[ipp]; ok {
+	if dk, ok := c.discoOfAddr[ipp]; ok {
-		return de
+		if ep, ok := c.endpointOfDisco[dk]; ok {
+			return ep
+		}
 	}
 
 	// Pre-disco: look up their AddrSet.
@@ -1496,7 +1501,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 			Src:  src,
 		})
 	case *disco.Pong:
-		go de.handlePong(dm)
+		de.handlePongConnLocked(dm, src)
 	case disco.CallMeMaybe:
 		if src.IP != derpMagicIPAddr {
 			// CallMeMaybe messages should only come via DERP.
@@ -1509,6 +1514,51 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 	return true
 }
 
+// cleanDiscoOfAddrLocked lazily checks a few entries in c.discoOfAddr
+// and deletes them if they're stale. It has no pointers in it so we
+// don't go through the effort of keeping it aggressively
+// pruned. Instead, we lazily clean it whenever it grows.
+//
+// c.mu must be held.
+//
+// If the caller already has a discoEndpoint mutex held as well, it
+// can be passed in as alreadyLocked so it won't be re-acquired.
+func (c *Conn) cleanDiscoOfAddrLocked(alreadyLocked *discoEndpoint) {
+	// If it's small enough, don't worry about it.
+	if len(c.discoOfAddr) < 16 {
+		return
+	}
+
+	const checkEntries = 5 // per one unit of growth
+
+	// Take advantage of Go's random map iteration to check & clean
+	// a few entries.
+	n := 0
+	for ipp, dk := range c.discoOfAddr {
+		n++
+		if n > checkEntries {
+			return
+		}
+		de, ok := c.endpointOfDisco[dk]
+		if !ok {
+			// This discokey isn't even known anymore. Clean.
+			delete(c.discoOfAddr, ipp)
+			continue
+		}
+		if de != alreadyLocked {
+			de.mu.Lock()
+		}
+		if _, ok := de.endpointState[ipp]; !ok {
+			// The discoEndpoint no longer knows about that endpoint.
+			// It must've changed. Clean.
+			delete(c.discoOfAddr, ipp)
+		}
+		if de != alreadyLocked {
+			de.mu.Unlock()
+		}
+	}
+}
+
 func (c *Conn) sharedDiscoKeyLocked(k tailcfg.DiscoKey) *[32]byte {
 	if v, ok := c.sharedDiscoKey[k]; ok {
 		return v
@@ -2512,6 +2562,7 @@ func udpAddrDebugString(ua net.UDPAddr) string {
 // discoEndpoint is a wireguard/conn.Endpoint for new-style peers that
 // advertise a DiscoKey and participate in active discovery.
 type discoEndpoint struct {
+	// These fields are initialized once and never modified.
 	c                  *Conn
 	publicKey          key.Public       // peer public key (for WireGuard + DERP)
 	discoKey           tailcfg.DiscoKey // for discovery mesages
@@ -2754,10 +2805,21 @@ func (de *discoEndpoint) noteConnectivityChange() {
 	de.trustBestAddrUntil = time.Time{}
 }
 
-func (de *discoEndpoint) handlePong(m *disco.Pong) {
+// handlePongConnLocked handles a Pong message (a reply to an earlier ping).
+// It should be called with the Conn.mu held.
+func (de *discoEndpoint) handlePongConnLocked(m *disco.Pong, src netaddr.IPPort) {
 	de.mu.Lock()
 	defer de.mu.Unlock()
 
+	if src.IP == derpMagicIPAddr {
+		// We might support pinging a node via DERP in the
+		// future to see if it's still there, but we don't
+		// yet. We shouldn't ever get here, but bail out early
+		// in case we do in the future. (In which case, hi!,
+		// you'll be modifying this code.)
+		return
+	}
+
 	sp, ok := de.sentPing[m.TxID]
 	if !ok {
 		// This is not a pong for a ping we sent. Ignore.
@@ -2765,6 +2827,13 @@ func (de *discoEndpoint) handlePong(m *disco.Pong) {
 	}
 	delete(de.sentPing, m.TxID)
 
+	if v, ok := de.c.discoOfAddr[src]; !ok || v != de.discoKey {
+		de.c.discoOfAddr[src] = de.discoKey
+		if !ok {
+			de.c.cleanDiscoOfAddrLocked(de)
+		}
+	}
+
 	now := time.Now()
 	delay := now.Sub(sp.at)
 	de.c.logf("magicsock: disco: got pong reply after %v", delay)