netip: parse partial IPv4

Add support for parsing partial IPv4 IPs (such as "10.20").
See https://man7.org/linux/man-pages/man3/inet_addr.3.html

Fixes golang#36822

src/net/lookup_test.go

@@ -481,7 +481,6 @@ var revAddrTests = []struct {
 	{"1::", "0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.0.ip6.arpa.", ""},
 	{"1234:567::89a:bcde", "e.d.c.b.a.9.8.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.7.6.5.0.4.3.2.1.ip6.arpa.", ""},
 	{"1234:567:fefe:bcbc:adad:9e4a:89a:bcde", "e.d.c.b.a.9.8.0.a.4.e.9.d.a.d.a.c.b.c.b.e.f.e.f.7.6.5.0.4.3.2.1.ip6.arpa.", ""},
-	{"1.2.3", "", "unrecognized address"},
 	{"1.2.3.4.5", "", "unrecognized address"},
 	{"1234:567:bcbca::89a:bcde", "", "unrecognized address"},
 	{"1234:567::bcbc:adad::89a:bcde", "", "unrecognized address"},

src/net/netip/netip.go

@@ -125,6 +125,13 @@ func ParseAddr(s string) (Addr, error) {
 			return Addr{}, parseAddrError{in: s, msg: "missing IPv6 address"}
 		}
 	}
+
+	// Single number IP
+	addr, err := parseIPv4(s)
+	if err == nil {
+		return addr, nil
+	}
+
 	return Addr{}, parseAddrError{in: s, msg: "unable to parse IP"}
 }
 
@@ -154,19 +161,20 @@ func (err parseAddrError) Error() string {
 
 // parseIPv4 parses s as an IPv4 address (in form "192.168.0.1").
 func parseIPv4(s string) (ip Addr, err error) {
-	var fields [4]uint8
-	var val, pos int
+	if len(s) == 0 {
+		return Addr{}, parseAddrError{in: s, msg: "unable to parse IP"}
+	}
+
+	var fields [4]uint
+	var val, pos uint
 	var digLen int // number of digits in current octet
 	for i := 0; i < len(s); i++ {
 		if s[i] >= '0' && s[i] <= '9' {
 			if digLen == 1 && val == 0 {
 				return Addr{}, parseAddrError{in: s, msg: "IPv4 field has octet with leading zero"}
 			}
-			val = val*10 + int(s[i]) - '0'
+			val = val*10 + uint(s[i]) - '0'
 			digLen++
-			if val > 255 {
-				return Addr{}, parseAddrError{in: s, msg: "IPv4 field has value >255"}
-			}
 		} else if s[i] == '.' {
 			// .1.2.3
 			// 1.2.3.
@@ -178,19 +186,52 @@ func parseIPv4(s string) (ip Addr, err error) {
 			if pos == 3 {
 				return Addr{}, parseAddrError{in: s, msg: "IPv4 address too long"}
 			}
-			fields[pos] = uint8(val)
+			fields[pos] = val
 			pos++
 			val = 0
 			digLen = 0
 		} else {
 			return Addr{}, parseAddrError{in: s, msg: "unexpected character", at: s[i:]}
 		}
 	}
-	if pos < 3 {
-		return Addr{}, parseAddrError{in: s, msg: "IPv4 address too short"}
+	fields[pos] = val
+
+	switch pos {
+	// a.b.c.d  Each of the four numeric parts specifies a byte of the address
+	case 3:
+		// NOP
+	// a.b.c Parts a and b specify the first two bytes of the binary address.
+	// Part c is interpreted as a 16-bit value that defines the rightmost two bytes of the binary address
+	case 2:
+		c := fields[2]
+		fields[3] = c & 0xFF
+		fields[2] = (c >> 8)
+	// a.b  Part a specifies the first byte of the binary address.
+	// Part b is interpreted as a 24-bit value that defines the rightmost three bytes of the binary address.
+	case 1:
+		b := fields[1]
+		fields[3] = b & 0xFF
+		fields[2] = (b >> 8) & 0xFF
+		fields[1] = b >> 16
+	// a  The value a is interpreted as a 32-bit value that is stored directly into the binary address
+	// without any byte rearrangement.
+	case 0:
+		a := fields[0]
+		fields[3] = a & 0xFF
+		fields[2] = (a >> 8) & 0xFF
+		fields[1] = (a >> 16) & 0xFF
+		fields[0] = a >> 24
+	}
+
+	var bFields [4]uint8
+	for i, f := range fields {
+		if f > 0xFF {
+			return Addr{}, parseAddrError{in: s, msg: "IPv4 field has value >255"}
+		}
+		bFields[i] = uint8(f)
 	}
-	fields[3] = uint8(val)
-	return AddrFrom4(fields), nil
+
+	return AddrFrom4(bFields), nil
 }
 
 // parseIPv6 parses s as an IPv6 address (in form "2001:db8::68").

src/net/netip/netip_test.go

@@ -218,7 +218,7 @@ func TestParseAddr(t *testing.T) {
 		"bad",
 		// Single number. Some parsers accept this as an IPv4 address in
 		// big-endian uint32 form, but we don't.
-		"1234",
+		// "1234",
 		// IPv4 with a zone specifier
 		"1.2.3.4%eth0",
 		// IPv4 field must have at least one digit
@@ -232,15 +232,12 @@ func TestParseAddr(t *testing.T) {
 		// IPv4 in dotted hex form
 		"0xc0.0xa8.0x8c.0xff",
 		// IPv4 in class B form
-		"192.168.12345",
+		// "192.168.12345",
 		// IPv4 in class B form, with a small enough number to be
 		// parseable as a regular dotted decimal field.
-		"127.0.1",
 		// IPv4 in class A form
-		"192.1234567",
 		// IPv4 in class A form, with a small enough number to be
 		// parseable as a regular dotted decimal field.
-		"127.1",
 		// IPv4 field has value >255
 		"192.168.300.1",
 		// IPv4 with too many fields
@@ -302,6 +299,67 @@ func TestParseAddr(t *testing.T) {
 	}
 }
 
+type shortIPCase struct {
+	in  string
+	out string
+}
+
+func genShortIP(count, a, b, c, d int) shortIPCase {
+	test := shortIPCase{
+		out: fmt.Sprintf("%d.%d.%d.%d", a, b, c, d),
+	}
+
+	switch count {
+	case 3:
+		test.in = fmt.Sprintf("%d.%d.%d", a, b, (c<<8)|d)
+	case 2:
+		test.in = fmt.Sprintf("%d.%d", a, (b<<16)|(c<<8)|d)
+	case 1:
+		v := (a << 24) | (b << 16) | (c << 8) | d
+		test.in = fmt.Sprintf("%d", v)
+	}
+
+	return test
+}
+
+func TestParseIPV4Short(t *testing.T) {
+
+	var shortIPCases = []shortIPCase{
+		genShortIP(3, 10, 20, 30, 40),
+		genShortIP(2, 10, 20, 30, 40),
+		genShortIP(1, 10, 20, 30, 40),
+	}
+
+	for _, test := range shortIPCases {
+		t.Run(test.in, func(t *testing.T) {
+			ip, err := ParseAddr(test.in)
+			if err != nil {
+				t.Errorf("unexpected error: %s", err)
+				return
+			}
+
+			if got := ip.String(); got != test.out {
+				t.Errorf("ParseAddr(%q): got %s, expected %s", test.in, got, test.out)
+			}
+		})
+	}
+
+	var badIPCases = []string{
+		"10.20.65536",
+		"10.16777216",
+		"4294967296",
+	}
+
+	for _, s := range badIPCases {
+		t.Run(s, func(t *testing.T) {
+			ip, err := ParseAddr(s)
+			if err == nil {
+				t.Errorf("ParseAddr(%q): expected error, got %v", s, ip)
+			}
+		})
+	}
+}
+
 func TestAddrFromSlice(t *testing.T) {
 	tests := []struct {
 		ip       []byte