add ToIP to complement ToNetAddr

The specific implementations of net.Addr are internal implementation
details of this package, and a downstream consumner shouldn't relay on
the internal use of net.IPAddr / net.TCPAddr to implement that interface
for direct extraction of IP. The consumer would instead need to go through
a round of string conversion to retreive IP safely. Adding an IP extraction
function to the interface here will allow the IP to be retreived more
efficiently.

convert.go

@@ -98,6 +98,25 @@ func FromIP(ip net.IP) (ma.Multiaddr, error) {
 	return FromIPAndZone(ip, "")
 }
 
+// ToIP converts a Multiaddr to a net.IP when possible
+func ToIP(addr ma.Multiaddr) (net.IP, error) {
+	n, err := ToNetAddr(addr)
+	if err != nil {
+		return nil, err
+	}
+
+	switch netAddr := n.(type) {
+	case *net.UDPAddr:
+		return netAddr.IP, nil
+	case *net.TCPAddr:
+		return netAddr.IP, nil
+	case *net.IPAddr:
+		return netAddr.IP, nil
+	default:
+		return nil, fmt.Errorf("non IP Multiaddr: %T", netAddr)
+	}
+}
+
 // DialArgs is a convenience function that returns network and address as
 // expected by net.Dial. See https://godoc.org/net#Dial for an overview of
 // possible return values (we do not support the unixpacket ones yet). Unix

convert_test.go

@@ -51,11 +51,20 @@ func testToNetAddr(t *testing.T, maddr, ntwk, addr string) {
 	// should convert properly
 	switch ntwk {
 	case "tcp":
-		_ = naddr.(*net.TCPAddr)
+		taddr := naddr.(*net.TCPAddr)
+		if ip, err := ToIP(m); err != nil || !taddr.IP.Equal(ip) {
+			t.Fatalf("ToIP() and ToNetAddr diverged: %s != %s", taddr, ip)
+		}
 	case "udp":
-		_ = naddr.(*net.UDPAddr)
+		uaddr := naddr.(*net.UDPAddr)
+		if ip, err := ToIP(m); err != nil || !uaddr.IP.Equal(ip) {
+			t.Fatalf("ToIP() and ToNetAddr diverged: %s != %s", uaddr, ip)
+		}
 	case "ip":
-		_ = naddr.(*net.IPAddr)
+		ipaddr := naddr.(*net.IPAddr)
+		if ip, err := ToIP(m); err != nil || !ipaddr.IP.Equal(ip) {
+			t.Fatalf("ToIP() and ToNetAddr diverged: %s != %s", ipaddr, ip)
+		}
 	}
 }