REFACTOR: Opinion: TXT records are one long string (#2631)

Co-authored-by: Costas Drogos <costas.drogos@gmail.com>
Co-authored-by: imlonghao <git@imlonghao.com>
Co-authored-by: Jeffrey Cafferata <jeffrey@jcid.nl>
Co-authored-by: Vincent Hagen <blackshadev@users.noreply.github.com>

pkg/txtutil/txtcode.go

@@ -0,0 +1,155 @@
+//go:generate stringer -type=State
+
+package txtutil
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+)
+
+// ParseQuoted parses a string of RFC1035-style quoted items. The resulting
+// items are then joined into one string. This is useful for parsing TXT
+// records.
+// Examples:
+// `foo` => foo
+// `"foo"` => foo
+// `"f\"oo"` => f"oo
+// `"f\\oo"` => f\oo
+// `"foo" "bar"` => foobar
+// `"foo" bar` => foobar
+func ParseQuoted(s string) (string, error) {
+	return txtDecode(s)
+}
+
+// EncodeQuoted encodes a string into a series of quoted 255-octet chunks. That
+// is, when decoded each chunk would be 255-octets with the remainder in the
+// last chunk.
+//
+// The output looks like:
+//
+//	`""`                                      empty
+//	`"255\"octets"`                           quotes are escaped
+//	`"255\\octets"`                           backslashes are escaped
+//	`"255octets" "255octets" "remainder"`     long strings are chunked
+func EncodeQuoted(t string) string {
+	return txtEncode(ToChunks(t))
+}
+
+type State int
+
+const (
+	StateStart     State = iota // Looking for a non-space
+	StateUnquoted               // A run of unquoted text
+	StateQuoted                 // Quoted text
+	StateBackslash              // last char was backlash in a quoted string
+	StateWantSpace              // expect space after closing quote
+)
+
+func isRemaining(s string, i, r int) bool {
+	return (len(s) - 1 - i) > r
+}
+
+// txtDecode decodes TXT strings quoted/escaped as Tom interprets RFC10225.
+func txtDecode(s string) (string, error) {
+	// Parse according to RFC1035 zonefile specifications.
+	// "foo"  -> one string: `foo``
+	// "foo" "bar"  -> two strings: `foo` and `bar`
+	// quotes and backslashes are escaped using \
+
+	/*
+
+		BNF:
+			txttarget := `""`` | item | item ` ` item*
+			item := quoteditem | unquoteditem
+			quoteditem := quote innertxt quote
+			quote := `"`
+			innertxt := (escaped | printable )*
+			escaped := `\\` | `\"`
+			printable := (printable ASCII chars)
+			unquoteditem := (printable ASCII chars but not `"` nor ' ')
+
+	*/
+
+	//printer.Printf("DEBUG: txtDecode txt inboundv=%v\n", s)
+
+	b := &bytes.Buffer{}
+	state := StateStart
+	for i, c := range s {
+
+		//printer.Printf("DEBUG: state=%v rune=%v\n", state, string(c))
+
+		switch state {
+
+		case StateStart:
+			if c == ' ' {
+				// skip whitespace
+			} else if c == '"' {
+				state = StateQuoted
+			} else {
+				state = StateUnquoted
+				b.WriteRune(c)
+			}
+
+		case StateUnquoted:
+
+			if c == ' ' {
+				state = StateStart
+			} else {
+				b.WriteRune(c)
+			}
+
+		case StateQuoted:
+
+			if c == '\\' {
+				if isRemaining(s, i, 1) {
+					state = StateBackslash
+				} else {
+					return "", fmt.Errorf("txtDecode quoted string ends with backslash q(%q)", s)
+				}
+			} else if c == '"' {
+				state = StateWantSpace
+			} else {
+				b.WriteRune(c)
+			}
+
+		case StateBackslash:
+			b.WriteRune(c)
+			state = StateQuoted
+
+		case StateWantSpace:
+			if c == ' ' {
+				state = StateStart
+			} else {
+				return "", fmt.Errorf("txtDecode expected whitespace after close quote q(%q)", s)
+			}
+
+		}
+	}
+
+	r := b.String()
+	//printer.Printf("DEBUG: txtDecode txt decodedv=%v\n", r)
+	return r, nil
+}
+
+// txtEncode encodes TXT strings in RFC1035 format as interpreted by Tom.
+func txtEncode(ts []string) string {
+	//printer.Printf("DEBUG: txtEncode txt outboundv=%v\n", ts)
+	if (len(ts) == 0) || (strings.Join(ts, "") == "") {
+		return `""`
+	}
+
+	var r []string
+
+	for i := range ts {
+		tx := ts[i]
+		tx = strings.ReplaceAll(tx, `\`, `\\`)
+		tx = strings.ReplaceAll(tx, `"`, `\"`)
+		tx = `"` + tx + `"`
+		r = append(r, tx)
+	}
+	t := strings.Join(r, ` `)
+
+	//printer.Printf("DEBUG: txtEncode txt  encodedv=%v\n", t)
+	return t
+}