wgengine/magicsock: remove final alloc from ReceiveFrom

And now that we don't have to play escape analysis and inlining games,
simplify the code.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -257,6 +257,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/util/groupmember                               from tailscale.com/ipn/ipnserver
         tailscale.com/util/lineread                                  from tailscale.com/hostinfo+
         tailscale.com/util/multierr                                  from tailscale.com/cmd/tailscaled+
+        tailscale.com/util/netconv                                   from tailscale.com/wgengine/magicsock
         tailscale.com/util/osshare                                   from tailscale.com/cmd/tailscaled+
         tailscale.com/util/pidowner                                  from tailscale.com/ipn/ipnserver
         tailscale.com/util/racebuild                                 from tailscale.com/logpolicy
@@ -382,7 +383,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         net/http/httputil                                            from tailscale.com/cmd/tailscaled+
         net/http/internal                                            from net/http+
         net/http/pprof                                               from tailscale.com/cmd/tailscaled+
-        net/netip                                                    from net
+        net/netip                                                    from net+
         net/textproto                                                from golang.org/x/net/http/httpguts+
         net/url                                                      from crypto/x509+
         os                                                           from crypto/rand+

wgengine/magicsock/magicsock.go

@@ -17,6 +17,7 @@ import (
 	"math"
 	"math/rand"
 	"net"
+	"net/netip"
 	"reflect"
 	"runtime"
 	"sort"
@@ -53,6 +54,7 @@ import (
 	"tailscale.com/types/netmap"
 	"tailscale.com/types/nettype"
 	"tailscale.com/util/clientmetric"
+	"tailscale.com/util/netconv"
 	"tailscale.com/util/uniq"
 	"tailscale.com/version"
 	"tailscale.com/wgengine/monitor"
@@ -3025,36 +3027,31 @@ func (c *RebindingUDPConn) ReadFromNetaddr(b []byte) (n int, ipp netaddr.IPPort,
 		pconn := c.currentConn()
 
 		// Optimization: Treat *net.UDPConn specially.
-		// ReadFromUDP gets partially inlined, avoiding allocating a *net.UDPAddr,
-		// as long as pAddr itself doesn't escape.
+		// This lets us avoid allocations by calling ReadFromUDPAddrPort.
 		// The non-*net.UDPConn case works, but it allocates.
-		var pAddr *net.UDPAddr
 		if udpConn, ok := pconn.(*net.UDPConn); ok {
-			n, pAddr, err = udpConn.ReadFromUDP(b)
+			var ap netip.AddrPort
+			n, ap, err = udpConn.ReadFromUDPAddrPort(b)
+			ipp = netconv.AsIPPort(ap)
 		} else {
 			var addr net.Addr
 			n, addr, err = pconn.ReadFrom(b)
-			if addr != nil {
-				pAddr, ok = addr.(*net.UDPAddr)
-				if !ok {
+			pAddr, ok := addr.(*net.UDPAddr)
+			if addr != nil && !ok {
 				return 0, netaddr.IPPort{}, fmt.Errorf("RebindingUDPConn.ReadFromNetaddr: underlying connection returned address of type %T, want *netaddr.UDPAddr", addr)
 			}
-			}
-		}
-
-		if err != nil {
-			if pconn != c.currentConn() {
-				continue
-			}
-		} else {
-			// Convert pAddr to a netaddr.IPPort.
-			// This prevents pAddr from escaping.
-			var ok bool
+			if pAddr != nil {
 				ipp, ok = netaddr.FromStdAddr(pAddr.IP, pAddr.Port, pAddr.Zone)
 				if !ok {
 					return 0, netaddr.IPPort{}, errors.New("netaddr.FromStdAddr failed")
 				}
 			}
+		}
+
+		if err != nil && pconn != c.currentConn() {
+			// The connection changed underfoot. Try again.
+			continue
+		}
 		return n, ipp, err
 	}
 }

wgengine/magicsock/magicsock_test.go

@@ -1345,14 +1345,18 @@ func TestReceiveFromAllocs(t *testing.T) {
 	}
 	// Go 1.16 and before: allow 3 allocs.
 	// Go 1.17: allow 2 allocs.
-	// Go Tailscale fork, Go 1.18+: allow 1 alloc.
+	// Go 1.17, Tailscale fork: allow 1 alloc.
+	// Go 1.18+: allow 0 allocs.
+	// Go 2.0: allow -1 allocs (projected).
 	major, ts := goMajorVersion(runtime.Version())
 	maxAllocs := 3
 	switch {
-	case major == 17:
+	case major == 17 && !ts:
 		maxAllocs = 2
-	case major >= 18, ts:
+	case major == 17 && ts:
 		maxAllocs = 1
+	case major >= 18:
+		maxAllocs = 0
 	}
 	t.Logf("allowing %d allocs for Go version %q", maxAllocs, runtime.Version())
 	roundTrip := setUpReceiveFrom(t)