wgengine/magicsock: add single element IPPort->endpoint cache in receive path

name           old time/op  new time/op  delta
ReceiveFrom-4  21.8µs ± 2%  20.9µs ± 2%  -4.27%  (p=0.000 n=10+10)

Updates #414

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -161,6 +161,11 @@ type Conn struct {
 	// whether a DERP channel read should be done.
 	derpRecvCountLast int64 // owned by ReceiveIPv4
 
+	// ippEndpoint4 and ippEndpoint6 are owned by ReceiveIPv4 and
+	// ReceiveIPv6, respectively, to cache an IPPort->endpoint for
+	// hot flows.
+	ippEndpoint4, ippEndpoint6 ippEndpointCache
+
 	// ============================================================
 	mu     sync.Mutex // guards all following fields; see userspaceEngine lock ordering rules
 	muCond *sync.Cond
@@ -1483,7 +1488,7 @@ func (c *Conn) ReceiveIPv6(b []byte) (int, conn.Endpoint, error) {
 		if err != nil {
 			return 0, nil, err
 		}
-		if ep, ok := c.receiveIP(b[:n], pAddr.(*net.UDPAddr)); ok {
+		if ep, ok := c.receiveIP(b[:n], pAddr.(*net.UDPAddr), &c.ippEndpoint6); ok {
 			return n, ep, nil
 		}
 	}
@@ -1520,14 +1525,14 @@ func (c *Conn) ReceiveIPv4(b []byte) (n int, ep conn.Endpoint, err error) {
 			}
 			return 0, nil, err
 		}
-		if ep, ok := c.receiveIP(b[:n], pAddr.(*net.UDPAddr)); ok {
+		if ep, ok := c.receiveIP(b[:n], pAddr.(*net.UDPAddr), &c.ippEndpoint4); ok {
 			return n, ep, nil
 		}
 	}
 }
 
 // receiveIP is the shared bits of ReceiveIPv4 and ReceiveIPv6.
-func (c *Conn) receiveIP(b []byte, ua *net.UDPAddr) (ep conn.Endpoint, ok bool) {
+func (c *Conn) receiveIP(b []byte, ua *net.UDPAddr, cache *ippEndpointCache) (ep conn.Endpoint, ok bool) {
 	ipp, ok := netaddr.FromStdAddr(ua.IP, ua.Port, ua.Zone)
 	if !ok {
 		return
@@ -1539,9 +1544,18 @@ func (c *Conn) receiveIP(b []byte, ua *net.UDPAddr) (ep conn.Endpoint, ok bool)
 	if c.handleDiscoMessage(b, ipp) {
 		return
 	}
-	ep = c.findEndpoint(ipp, ua, b)
+	if cache.ipp == ipp && cache.de != nil && cache.gen == cache.de.numStopAndReset() {
-	if ep == nil {
+		ep = cache.de
-		return
+	} else {
+		ep = c.findEndpoint(ipp, ua, b)
+		if ep == nil {
+			return
+		}
+		if de, ok := ep.(*discoEndpoint); ok {
+			cache.ipp = ipp
+			cache.de = de
+			cache.gen = de.numStopAndReset()
+		}
 	}
 	c.noteRecvActivityFromEndpoint(ep)
 	return ep, true
@@ -2781,7 +2795,8 @@ func udpAddrDebugString(ua net.UDPAddr) string {
 // advertise a DiscoKey and participate in active discovery.
 type discoEndpoint struct {
 	// atomically accessed; declared first for alignment reasons
-	lastRecvUnixAtomic int64
+	lastRecvUnixAtomic    int64
+	numStopAndResetAtomic int64
 
 	// These fields are initialized once and never modified.
 	c                  *Conn
@@ -3420,6 +3435,7 @@ func (de *discoEndpoint) populatePeerStatus(ps *ipnstate.PeerStatus) {
 // It's called when a discovery endpoint is no longer present in the NetworkMap,
 // or when magicsock is transition from running to stopped state (via SetPrivateKey(zero))
 func (de *discoEndpoint) stopAndReset() {
+	atomic.AddInt64(&de.numStopAndResetAtomic, 1)
 	de.mu.Lock()
 	defer de.mu.Unlock()
 
@@ -3447,6 +3463,10 @@ func (de *discoEndpoint) stopAndReset() {
 	de.pendingCLIPings = nil
 }
 
+func (de *discoEndpoint) numStopAndReset() int64 {
+	return atomic.LoadInt64(&de.numStopAndResetAtomic)
+}
+
 // derpStr replaces DERP IPs in s with "derp-".
 func derpStr(s string) string { return strings.ReplaceAll(s, "127.3.3.40:", "derp-") }
 
@@ -3468,3 +3488,11 @@ func (c *Conn) WhoIs(ip netaddr.IP) (n *tailcfg.Node, u tailcfg.UserProfile, ok
 	}
 	return nil, u, false
 }
+
+// ippEndpointCache is a mutex-free single-element cache, mapping from
+// a single netaddr.IPPort to a single endpoint.
+type ippEndpointCache struct {
+	ipp netaddr.IPPort
+	gen int64
+	de  *discoEndpoint
+}