enable the DF bit on macOS (#3946)

* set DF bit on macOS

* simplify macOS version check

* link to chromium source for macOS 11 check

sys_conn_df_darwin.go

@@ -0,0 +1,72 @@
+//go:build darwin
+
+package quic
+
+import (
+	"errors"
+	"strconv"
+	"strings"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/quic-go/quic-go/internal/utils"
+)
+
+func setDF(rawConn syscall.RawConn) (bool, error) {
+	// Setting DF bit is only supported from macOS11
+	// https://github.com/chromium/chromium/blob/117.0.5881.2/net/socket/udp_socket_posix.cc#L555
+	if supportsDF, err := isAtLeastMacOS11(); !supportsDF || err != nil {
+		return false, err
+	}
+
+	// Enabling IP_DONTFRAG will force the kernel to return "sendto: message too long"
+	// and the datagram will not be fragmented
+	var errDFIPv4, errDFIPv6 error
+	if err := rawConn.Control(func(fd uintptr) {
+		errDFIPv4 = unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_DONTFRAG, 1)
+		errDFIPv6 = unix.SetsockoptInt(int(fd), unix.IPPROTO_IPV6, unix.IPV6_DONTFRAG, 1)
+	}); err != nil {
+		return false, err
+	}
+	switch {
+	case errDFIPv4 == nil && errDFIPv6 == nil:
+		utils.DefaultLogger.Debugf("Setting DF for IPv4 and IPv6.")
+	case errDFIPv4 == nil && errDFIPv6 != nil:
+		utils.DefaultLogger.Debugf("Setting DF for IPv4.")
+	case errDFIPv4 != nil && errDFIPv6 == nil:
+		utils.DefaultLogger.Debugf("Setting DF for IPv6.")
+		// On macOS, the syscall for setting DF bit for IPv4 fails on dual-stack listeners.
+		// Treat the connection as not having DF enabled, even though the DF bit will be set
+		// when used for IPv6.
+		// See https://github.com/quic-go/quic-go/issues/3793 for details.
+		return false, nil
+	case errDFIPv4 != nil && errDFIPv6 != nil:
+		return false, errors.New("setting DF failed for both IPv4 and IPv6")
+	}
+	return true, nil
+}
+
+func isMsgSizeErr(err error) bool {
+	return errors.Is(err, unix.EMSGSIZE)
+}
+
+func isAtLeastMacOS11() (bool, error) {
+	uname := &unix.Utsname{}
+	err := unix.Uname(uname)
+	if err != nil {
+		return false, err
+	}
+
+	release := string(uname.Release[:])
+	if idx := strings.Index(release, "."); idx != -1 {
+		version, err := strconv.Atoi(release[:idx])
+		if err != nil {
+			return false, err
+		}
+		// Darwin version 20 is macOS version 11
+		// https://en.wikipedia.org/wiki/Darwin_(operating_system)#Darwin_20_onwards
+		return version >= 20, nil
+	}
+	return false, nil
+}