ident: implement remote-part validation

This is mostly equivalent to email without IP addresses.

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

ident/remote.go

@@ -0,0 +1,153 @@
+package ident
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"strconv"
+)
+
+// ldh are bytes that may appear in a DNS label of Remote.
+var ldh = [1 << 8]bool{
+	'-': true, // special: must not be the first or last byte
+
+	'0': true, '1': true, '2': true, '3': true, '4': true, '5': true, '6': true,
+	'7': true, '8': true, '9': true,
+
+	'A': true, 'B': true, 'C': true, 'D': true, 'E': true, 'F': true, 'G': true,
+	'H': true, 'I': true, 'J': true, 'K': true, 'L': true, 'M': true, 'N': true,
+	'O': true, 'P': true, 'Q': true, 'R': true, 'S': true, 'T': true, 'U': true,
+	'V': true, 'W': true, 'X': true, 'Y': true, 'Z': true,
+
+	'a': true, 'b': true, 'c': true, 'd': true, 'e': true, 'f': true, 'g': true,
+	'h': true, 'i': true, 'j': true, 'k': true, 'l': true, 'm': true, 'n': true,
+	'o': true, 'p': true, 'q': true, 'r': true, 's': true, 't': true, 'u': true,
+	'v': true, 'w': true, 'x': true, 'y': true, 'z': true,
+}
+
+const (
+	// RemoteMax is the maximum permissible size of [Remote].
+	RemoteMax = 1<<8 - 1
+
+	// LabelSeparator separates labels in a [Remote].
+	LabelSeparator = '.'
+	// LabelMax is the maximum permissible size of a label in [Remote].
+	LabelMax = 1<<6 - 1
+)
+
+// Remote represents the remote part of [F].
+//
+// A remote must contain one or more non-empty labels, separated by
+// [LabelSeparator]. Each label must be no more than [LabelMax] bytes long, and
+// contain only bytes permitted by the "LDH rule" described in
+// [Section 2 of RFC 3696]. A remote must, in total, be no more than [RemoteMax]
+// bytes long.
+//
+// [Section 2 of RFC 3696]: https://www.rfc-editor.org/rfc/rfc3696#section-2
+type Remote struct {
+	Data [RemoteMax + 1]byte
+	Len  uint8
+}
+
+// Encode appends the underlying string to dst and returns the extended buffer.
+func (r *Remote) Encode(dst []byte) []byte {
+	return append(dst, r.Data[:r.Len]...)
+}
+
+// String returns a copy of the underlying string.
+func (r *Remote) String() string { return string(r.Data[:r.Len]) }
+
+// MarshalText returns the result of Encode.
+func (r *Remote) MarshalText() (data []byte, err error) {
+	return r.Encode(nil), nil
+}
+
+var (
+	// ErrShortRemote is returned validating an empty remote.
+	ErrShortRemote = errors.New("missing remote part")
+	// ErrLongRemote is returned validating a remote longer than [RemoteMax].
+	ErrLongRemote = errors.New("got more than 255 bytes of remote")
+)
+
+const (
+	// InvalidLabelLDH denotes an [InvalidLabelError] describing an invalid
+	// label containing bytes not permitted by the LDH rule.
+	InvalidLabelLDH = iota
+	// InvalidLabelShort denotes an [InvalidLabelError] describing an empty label.
+	InvalidLabelShort
+	// InvalidLabelLong denotes an [InvalidLabelError] describing a label
+	// exceeding its maximum length of [LabelMax].
+	InvalidLabelLong
+)
+
+// An InvalidLabelError describes a rejected DNS label part of a [Remote].
+type InvalidLabelError struct {
+	Data   []byte
+	Label  int
+	Index  int
+	Reason int
+}
+
+func (e *InvalidLabelError) Error() string {
+	s := "label " + strconv.Itoa(e.Label) + " "
+	switch e.Reason {
+	case InvalidLabelLDH:
+		if e.Data[e.Index] == '-' {
+			if e.Index == 0 {
+				s += "starts"
+			} else {
+				s += "ends"
+			}
+			s += " with '-'"
+			break
+		}
+		s += fmt.Sprintf(
+			"contains invalid byte %q at index %d",
+			e.Data[e.Index], e.Index,
+		)
+
+	case InvalidLabelShort:
+		s += "is empty"
+
+	case InvalidLabelLong:
+		s += "is longer than 63 bytes"
+
+	default:
+		return "invalid label " + strconv.Itoa(e.Label) +
+			" at byte " + strconv.Itoa(e.Index)
+	}
+	return s
+}
+
+// UnmarshalText validates and copies data to r.
+func (r *Remote) UnmarshalText(data []byte) error {
+	if len(data) == 0 {
+		return ErrShortRemote
+	}
+
+	r.Len = uint8(len(data))
+	if copy(r.Data[:], data) == len(r.Data) {
+		return ErrLongRemote
+	}
+
+	for i, label := range bytes.Split(r.Data[:r.Len], []byte{LabelSeparator}) {
+		if len(label) == 0 {
+			return &InvalidLabelError{label, i, -1, InvalidLabelShort}
+		} else if len(label) > LabelMax {
+			return &InvalidLabelError{label, i, LabelMax, InvalidLabelLong}
+		}
+
+		if label[0] == '-' {
+			return &InvalidLabelError{label, i, 0, InvalidLabelLDH}
+		} else if label[len(label)-1] == '-' {
+			return &InvalidLabelError{label, i, len(label) - 1, InvalidLabelLDH}
+		}
+		for j, b := range label {
+			if !ldh[b] {
+				return &InvalidLabelError{label, i, j, InvalidLabelLDH}
+			}
+		}
+	}
+
+	return nil
+}