fuzz:

- for parse/write/parse check, change second parse to parse the write
  of the parsed value instead of the original input

encode:
- invalidKeyRune: treat utf8.RuneError as invalid rune
- add invalidKey, invalidKeyString funcs
  - checks len(key) == 0, invalidKeyRune, and utf8.Valid
- needsQuotedValueRune: if value contains utf8.RuneError quote the
  value. a literal k=\ufffd encodes to k=\"\\ufffd\"
- writeStringValue, writeBytesValue: quote any invalid utf8 string
- the above two changes fix the "reserialized data does not match"
  error found during fuzz testing

decode:
- reject invalidKey as parse error

jsonstring:
- remove "&& size == 1" when checking for rune decode error

fuzz testing output:
- .quoted:
  "0=\"\xbd\x00\""
- .output:
  panic: reserialized data does not match:
  "0=\"\\ufffd\\u0000\"\n"
  "0=\"�\\u0000\"\n"

decode.go

@@ -68,13 +68,19 @@ func (dec *Decoder) ScanKeyval() bool {
 	return false
 
 key:
+	const invalidKeyError = "invalid key"
+
 	start := dec.pos
 	for p, c := range line[dec.pos:] {
 		switch {
 		case c == '=':
 			dec.pos += p
 			if dec.pos > start {
 				dec.key = line[start:dec.pos]
+				if invalidKey(dec.key) {
+					dec.syntaxError(invalidKeyError)
+					return false
+				}
 			}
 			if dec.key == nil {
 				dec.unexpectedByte(c)
@@ -89,13 +95,21 @@ key:
 			dec.pos += p
 			if dec.pos > start {
 				dec.key = line[start:dec.pos]
+				if invalidKey(dec.key) {
+					dec.syntaxError(invalidKeyError)
+					return false
+				}
 			}
 			return true
 		}
 	}
 	dec.pos = len(line)
 	if dec.pos > start {
 		dec.key = line[start:dec.pos]
+		if invalidKey(dec.key) {
+			dec.syntaxError(invalidKeyError)
+			return false
+		}
 	}
 	return true
 

encode.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"reflect"
 	"strings"
+	"unicode/utf8"
 )
 
 // MarshalKeyvals returns the logfmt encoding of keyvals, a variadic sequence
@@ -165,19 +166,27 @@ func writeKey(w io.Writer, key interface{}) error {
 }
 
 func invalidKeyRune(r rune) bool {
-	return r <= ' ' || r == '=' || r == '"'
+	return r <= ' ' || r == '=' || r == '"' || r == utf8.RuneError
+}
+
+func invalidKeyString(key string) bool {
+	return len(key) == 0 || strings.IndexFunc(key, invalidKeyRune) != -1 || !utf8.ValidString(key)
+}
+
+func invalidKey(key []byte) bool {
+	return len(key) == 0 || bytes.IndexFunc(key, invalidKeyRune) != -1 || !utf8.Valid(key)
 }
 
 func writeStringKey(w io.Writer, key string) error {
-	if len(key) == 0 || strings.IndexFunc(key, invalidKeyRune) != -1 {
+	if invalidKeyString(key) {
 		return ErrInvalidKey
 	}
 	_, err := io.WriteString(w, key)
 	return err
 }
 
 func writeBytesKey(w io.Writer, key []byte) error {
-	if len(key) == 0 || bytes.IndexFunc(key, invalidKeyRune) != -1 {
+	if invalidKey(key) {
 		return ErrInvalidKey
 	}
 	_, err := w.Write(key)
@@ -223,14 +232,14 @@ func writeValue(w io.Writer, value interface{}) error {
 }
 
 func needsQuotedValueRune(r rune) bool {
-	return r <= ' ' || r == '=' || r == '"'
+	return r <= ' ' || r == '=' || r == '"' || r == utf8.RuneError
 }
 
 func writeStringValue(w io.Writer, value string, ok bool) error {
 	var err error
 	if ok && value == "null" {
 		_, err = io.WriteString(w, `"null"`)
-	} else if strings.IndexFunc(value, needsQuotedValueRune) != -1 {
+	} else if strings.IndexFunc(value, needsQuotedValueRune) != -1 || !utf8.ValidString(value) {
 		_, err = writeQuotedString(w, value)
 	} else {
 		_, err = io.WriteString(w, value)
@@ -240,7 +249,7 @@ func writeStringValue(w io.Writer, value string, ok bool) error {
 
 func writeBytesValue(w io.Writer, value []byte) error {
 	var err error
-	if bytes.IndexFunc(value, needsQuotedValueRune) >= 0 {
+	if bytes.IndexFunc(value, needsQuotedValueRune) >= 0 || !utf8.Valid(value) {
 		_, err = writeQuotedBytes(w, value)
 	} else {
 		_, err = w.Write(value)

fuzz.go

@@ -22,7 +22,7 @@ func Fuzz(data []byte) int {
 	if err = write(parsed, &w1); err != nil {
 		panic(err)
 	}
-	parsed, err = parse(data)
+	parsed, err = parse(w1.Bytes())
 	if err != nil {
 		panic(err)
 	}

jsonstring.go

@@ -71,7 +71,7 @@ func writeQuotedString(w io.Writer, s string) (int, error) {
 			continue
 		}
 		c, size := utf8.DecodeRuneInString(s[i:])
-		if c == utf8.RuneError && size == 1 {
+		if c == utf8.RuneError {
 			if start < i {
 				buf.WriteString(s[start:i])
 			}
@@ -129,7 +129,7 @@ func writeQuotedBytes(w io.Writer, s []byte) (int, error) {
 			continue
 		}
 		c, size := utf8.DecodeRune(s[i:])
-		if c == utf8.RuneError && size == 1 {
+		if c == utf8.RuneError {
 			if start < i {
 				buf.Write(s[start:i])
 			}
@@ -182,7 +182,7 @@ func unquoteBytes(s []byte) (t []byte, ok bool) {
 			continue
 		}
 		rr, size := utf8.DecodeRune(s[r:])
-		if rr == utf8.RuneError && size == 1 {
+		if rr == utf8.RuneError {
 			break
 		}
 		r += size