ident: implement remote-part validation

This is mostly equivalent to email without IP addresses.

Signed-off-by: Yonah <contrib@gensokyo.uk>

ident/ident_test.go

@@ -88,6 +88,42 @@ func TestErrors(t *testing.T) {
 			Data: make([]byte, 1<<10),
 			Want: ident.EncodedSizeSystem,
 		}, "got 1024 bytes for a 32-byte identifier"},
+
+		{"InvalidLabelLDH start", &ident.InvalidLabelError{
+			Data:   []byte{'-'},
+			Label:  0xbad,
+			Index:  0,
+			Reason: ident.InvalidLabelLDH,
+		}, "label 2989 starts with '-'"},
+
+		{"InvalidLabelLDH end", &ident.InvalidLabelError{
+			Data:   []byte{0, '-'},
+			Label:  0xbad,
+			Index:  1,
+			Reason: ident.InvalidLabelLDH,
+		}, "label 2989 ends with '-'"},
+
+		{"InvalidLabelLDH", &ident.InvalidLabelError{
+			Data:   []byte{0},
+			Label:  0xbad,
+			Index:  0,
+			Reason: ident.InvalidLabelLDH,
+		}, `label 2989 contains invalid byte '\x00' at index 0`},
+
+		{"InvalidLabelShort", &ident.InvalidLabelError{
+			Label:  0xf00d,
+			Reason: ident.InvalidLabelShort,
+		}, "label 61453 is empty"},
+
+		{"InvalidLabelLong", &ident.InvalidLabelError{
+			Label:  0xf00d,
+			Reason: ident.InvalidLabelLong,
+		}, "label 61453 is longer than 63 bytes"},
+
+		{"InvalidLabelError", &ident.InvalidLabelError{
+			Label:  0xcafe,
+			Reason: 0xbadf00d,
+		}, "invalid label 51966 at byte 0"},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {