net: store IPv4 returned from cgo resolver as 4-byte slice net.IP

net.IP states that a 16-byte slice can still be an IPv4 address.
But after netip.Addr is introduced, it requires extra care to keep
it as an IPv4 address when converting it to a netip.Addr using
netip.AddrFromSlice.

To address this issue, let's change the cgo resolver to return
4-byte net.IP for IPv4. The change will save us 12 bytes too.

Please note that the go resolver already return IPv4 as 4-byte
slice.

The test TestResolverLookupIP has been modified to cover this
behavior. So no new test is added.

src/net/cgo_unix.go

@@ -337,12 +337,3 @@ func cgoSockaddr(ip IP, zone string) (*C.struct_sockaddr, C.socklen_t) {
 	}
 	return nil, 0
 }
-
-func copyIP(x IP) IP {
-	if len(x) < 16 {
-		return x.To16()
-	}
-	y := make(IP, len(x))
-	copy(y, x)
-	return y
-}

src/net/ip.go

@@ -757,3 +757,9 @@ func ParseCIDR(s string) (IP, *IPNet, error) {
 	m := CIDRMask(n, 8*iplen)
 	return ip, &IPNet{IP: ip.Mask(m), Mask: m}, nil
 }
+
+func copyIP(x IP) IP {
+	y := make(IP, len(x))
+	copy(y, x)
+	return y
+}

src/net/lookup_test.go

@@ -10,6 +10,7 @@ import (
 	"context"
 	"fmt"
 	"internal/testenv"
+	"net/netip"
 	"reflect"
 	"runtime"
 	"sort"
@@ -1289,18 +1290,16 @@ func TestResolverLookupIP(t *testing.T) {
 						t.Fatalf("DefaultResolver.LookupIP(%q, %q): failed with unexpected error: %v", network, host, err)
 					}
 
-					var v4Addrs []IP
-					var v6Addrs []IP
+					var v4Addrs []netip.Addr
+					var v6Addrs []netip.Addr
 					for _, ip := range ips {
-						switch {
-						case ip.To4() != nil:
-							// We need to skip the test below because To16 will
-							// convent an IPv4 address to an IPv4-mapped IPv6
-							// address.
-							v4Addrs = append(v4Addrs, ip)
-						case ip.To16() != nil:
-							v6Addrs = append(v6Addrs, ip)
-						default:
+						if addr, ok := netip.AddrFromSlice(ip); ok {
+							if addr.Is4() {
+								v4Addrs = append(v4Addrs, addr)
+							} else {
+								v6Addrs = append(v6Addrs, addr)
+							}
+						} else {
 							t.Fatalf("IP=%q is neither IPv4 nor IPv6", ip)
 						}
 					}
@@ -1322,7 +1321,7 @@ func TestResolverLookupIP(t *testing.T) {
 						t.Errorf("DefaultResolver.LookupIP(%q, %q): unexpected IPv4 addresses: %v", network, host, v4Addrs)
 					}
 					if network == "ip4" && len(v6Addrs) > 0 {
-						t.Errorf("DefaultResolver.LookupIP(%q, %q): unexpected IPv6 addresses: %v", network, host, v6Addrs)
+						t.Errorf("DefaultResolver.LookupIP(%q, %q): unexpected IPv6 or IPv4-mapped IPv6 addresses: %v", network, host, v6Addrs)
 					}
 				})
 			}

src/net/lookup_windows.go

@@ -134,11 +134,11 @@ func (r *Resolver) lookupIP(ctx context.Context, network, name string) ([]IPAddr
 			switch result.Family {
 			case syscall.AF_INET:
 				a := (*syscall.RawSockaddrInet4)(addr).Addr
-				addrs = append(addrs, IPAddr{IP: IPv4(a[0], a[1], a[2], a[3])})
+				addrs = append(addrs, IPAddr{IP: copyIP(a[:])})
 			case syscall.AF_INET6:
 				a := (*syscall.RawSockaddrInet6)(addr).Addr
 				zone := zoneCache.name(int((*syscall.RawSockaddrInet6)(addr).Scope_id))
-				addrs = append(addrs, IPAddr{IP: IP{a[0], a[1], a[2], a[3], a[4], a[5], a[6], a[7], a[8], a[9], a[10], a[11], a[12], a[13], a[14], a[15]}, Zone: zone})
+				addrs = append(addrs, IPAddr{IP: copyIP(a[:]), Zone: zone})
 			default:
 				return nil, &DNSError{Err: syscall.EWINDOWS.Error(), Name: name}
 			}