net/netip: fix formatting of IPv4-in-6 address with zone

Weird, but don't drop the zone when stringifying.

Fixes #50111

Change-Id: I5fbccdfedcdc77a77ee6bafc8d82b8ec8ec7220c
Reviewed-on: https://go-review.googlesource.com/c/go/+/371094
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Matt Layher <[email protected]>
Trust: Matt Layher <[email protected]>
Trust: Ian Lance Taylor <[email protected]>

Author: bradfitz

src/net/netip/netip.go

@@ -769,7 +769,11 @@ func (ip Addr) String() string {
 	default:
 		if ip.Is4In6() {
 			// TODO(bradfitz): this could alloc less.
-			return "::ffff:" + ip.Unmap().String()
+			if z := ip.Zone(); z != "" {
+				return "::ffff:" + ip.Unmap().String() + "%" + z
+			} else {
+				return "::ffff:" + ip.Unmap().String()
+			}
 		}
 		return ip.string6()
 	}
@@ -787,7 +791,12 @@ func (ip Addr) AppendTo(b []byte) []byte {
 	default:
 		if ip.Is4In6() {
 			b = append(b, "::ffff:"...)
-			return ip.Unmap().appendTo4(b)
+			b = ip.Unmap().appendTo4(b)
+			if z := ip.Zone(); z != "" {
+				b = append(b, '%')
+				b = append(b, z...)
+			}
+			return b
 		}
 		return ip.appendTo6(b)
 	}
@@ -947,10 +956,16 @@ func (ip Addr) MarshalText() ([]byte, error) {
 		b := make([]byte, 0, max)
 		if ip.Is4In6() {
 			b = append(b, "::ffff:"...)
-			return ip.Unmap().appendTo4(b), nil
+			b = ip.Unmap().appendTo4(b)
+			if z := ip.Zone(); z != "" {
+				b = append(b, '%')
+				b = append(b, z...)
+			}
+			return b, nil
 		}
 		return ip.appendTo6(b), nil
 	}
+
 }
 
 // UnmarshalText implements the encoding.TextUnmarshaler interface.

src/net/netip/netip_test.go

@@ -114,6 +114,12 @@ func TestParseAddr(t *testing.T) {
 			ip:  MkAddr(Mk128(0x0001000200000000, 0x0000ffffc0a88cff), intern.Get("eth1")),
 			str: "1:2::ffff:c0a8:8cff%eth1",
 		},
+		// 4-in-6 with zone
+		{
+			in:  "::ffff:192.168.140.255%eth1",
+			ip:  MkAddr(Mk128(0, 0x0000ffffc0a88cff), intern.Get("eth1")),
+			str: "::ffff:192.168.140.255%eth1",
+		},
 		// IPv6 with capital letters.
 		{
 			in:  "FD9E:1A04:F01D::1",