Implement legacy semantics for string escaping behavior

For doubly escaped strings, the v1 behavior was to apply
escaping for EscapeForHTML and EscapeForJS on the first pass,
rather than on the second pass. This does lead to unnecessarily
longer outputs since escaping on the first pass means that
the newly included '\\' characters have to be escaped again
in the second pass. Doubly escaped strings are already an
esoteric feature of v1, so we should preserve the semantic.

Also, when encoding the raw output of MarshalJSON,
the v1 behavior was to preserve any pre-existing escape sequences,
while still escaping any characters that might need escaping
per EscapeForHTML and EscapeForJS. Replicate this behavior.

arshal_default.go

@@ -207,11 +207,11 @@ func makeStringArshaler(t reflect.Type) *arshaler {
 		}
 
 		if mo.Flags.Get(jsonflags.StringifyBoolsAndStrings) {
-			b, err := jsontext.AppendQuote(nil, s) // only fails for invalid UTF-8
-			q, _ := jsontext.AppendQuote(nil, b)   // cannot fail since b is valid UTF-8
-			if err != nil && !xe.Flags.Get(jsonflags.AllowInvalidUTF8) {
-				return newMarshalErrorBefore(enc, t, err)
+			b, err := jsonwire.AppendQuote(nil, s, &mo.Flags)
+			if err != nil {
+				return newMarshalErrorBefore(enc, t, &jsontext.SyntacticError{Err: err})
 			}
+			q, _ := jsontext.AppendQuote(nil, b) // cannot fail since b is valid UTF-8
 			return enc.WriteValue(q)
 		}
 		return enc.WriteToken(jsontext.String(s))

internal/jsonflags/flags.go

@@ -51,6 +51,7 @@ const (
 		EscapeForHTML |
 		EscapeForJS |
 		EscapeInvalidUTF8 |
+		PreserveRawStrings |
 		Deterministic |
 		FormatNilMapAsNull |
 		FormatNilSliceAsNull |
@@ -84,7 +85,7 @@ const (
 	AllowInvalidUTF8    // encode or decode
 	WithinArshalCall    // encode or decode; for internal use by json.Marshal and json.Unmarshal
 	OmitTopLevelNewline // encode only; for internal use by json.Marshal and json.MarshalWrite
-	PreserveRawStrings  // encode only; for internal use by jsontext.Value.Canonicalize
+	PreserveRawStrings  // encode only; exposed in v1 and also used by jsontext.Value.Canonicalize
 	CanonicalizeNumbers // encode only; for internal use by jsontext.Value.Canonicalize
 	EscapeForHTML       // encode only
 	EscapeForJS         // encode only

internal/jsonwire/encode.go

@@ -154,18 +154,49 @@ func ReformatString(dst, src []byte, flags *jsonflags.Flags) ([]byte, int, error
 	if err != nil {
 		return dst, n, err
 	}
-	isCanonical := !flags.Get(jsonflags.EscapeForHTML | jsonflags.EscapeForJS | jsonflags.EscapeInvalidUTF8)
-	if flags.Get(jsonflags.PreserveRawStrings) || (isCanonical && valFlags.IsCanonical()) {
+
+	// If the output requires no special escapes, and the input
+	// is already in canonical form or should be preserved verbatim,
+	// then directly copy the input to the output.
+	if !flags.Get(jsonflags.EscapeForHTML|jsonflags.EscapeForJS) &&
+		(valFlags.IsCanonical() || flags.Get(jsonflags.PreserveRawStrings)) {
 		dst = append(dst, src[:n]...) // copy the string verbatim
 		return dst, n, nil
 	}
 
-	// TODO: Implement a direct, raw-to-raw reformat for strings.
-	// If the escapeRune option would have resulted in no changes to the output,
-	// it would be faster to simply append src to dst without going through
-	// an intermediary representation in a separate buffer.
+	// If the input should be preserved verbatim, we still need to
+	// respect the EscapeForHTML and EscapeForJS options.
+	// Note that EscapeInvalidUTF8 is not respected.
+	// This logic ensures that pre-escaped sequences remained escaped.
+	if flags.Get(jsonflags.PreserveRawStrings) {
+		var i, lastAppendIndex int
+		for i < n {
+			if c := src[i]; c < utf8.RuneSelf {
+				if (c == '<' || c == '>' || c == '&') && flags.Get(jsonflags.EscapeForHTML) {
+					dst = append(dst, src[lastAppendIndex:i]...)
+					dst = appendEscapedASCII(dst, c)
+					lastAppendIndex = i + 1
+				}
+				i++
+			} else {
+				r, rn := utf8.DecodeRuneInString(string(truncateMaxUTF8(src[i:])))
+				if (r == '\u2028' || r == '\u2029') && flags.Get(jsonflags.EscapeForJS) {
+					dst = append(dst, src[lastAppendIndex:i]...)
+					dst = appendEscapedUnicode(dst, r)
+					lastAppendIndex = i + rn
+				}
+				i += rn
+			}
+		}
+		return append(dst, src[lastAppendIndex:n]...), n, nil
+	}
+
+	// The input contains characters that might need escaping,
+	// unnecessary escape sequences, or invalid UTF-8.
+	// Perform a round-trip unquote and quote to properly reformat
+	// these sequences according the current flags.
 	b, _ := AppendUnquote(nil, src[:n])
-	dst, _ = AppendQuote(dst, string(b), flags)
+	dst, _ = AppendQuote(dst, b, flags)
 	return dst, n, nil
 }
 

jsontext/value.go

@@ -162,6 +162,8 @@ func (v *Value) reformat(canonical, multiline bool, prefix, indent string) error
 		eo.Flags.Set(jsonflags.AllowInvalidUTF8 | 1)
 		eo.Flags.Set(jsonflags.AllowDuplicateNames | 1)
 		eo.Flags.Set(jsonflags.PreserveRawStrings | 1)
+		eo.Flags.Set(jsonflags.EscapeForHTML | 0) // ensure strings are preserved
+		eo.Flags.Set(jsonflags.EscapeForJS | 0)   // ensure strings are preserved
 		if multiline {
 			eo.Flags.Set(jsonflags.Multiline | 1)
 			eo.Flags.Set(jsonflags.SpaceAfterColon | 1)

v1/failing.txt

@@ -21,9 +21,6 @@ TestNilMarshal
 TestNilMarshal/#08
 TestNilMarshal/#11
 TestNilMarshalerTextMapKey
-TestEncoderSetEscapeHTML
-TestEncoderSetEscapeHTML/stringOption
-TestRawMessage
 TestStringOption
 TestStringOption/Unmarshal/Null/v1
 TestStringOption/Unmarshal/Deep/v1

v1/options.go

@@ -39,6 +39,7 @@ type Options = jsonopts.Options
 //   - [IgnoreStructErrors]
 //   - [MatchCaseSensitiveDelimiter]
 //   - [OmitEmptyWithLegacyDefinition]
+//   - [PreserveRawStrings]
 //   - [RejectFloatOverflow]
 //   - [ReportLegacyErrorValues]
 //   - [StringifyWithLegacySemantics]
@@ -168,6 +169,22 @@ func OmitEmptyWithLegacyDefinition(v bool) Options {
 	}
 }
 
+// PreserveRawStrings specifies that raw JSON string values passed to
+// [jsontext.Encoder.WriteValue] and [jsontext.Encoder.WriteToken]
+// preserve their original encoding.
+// However, characters that still need escaping according to
+// [jsontext.EscapeForHTML] and [jsontext.EscapeForJS] are escaped.
+//
+// This only affects encoding and is ignored when decoding.
+// The v1 default is true.
+func PreserveRawStrings(v bool) Options {
+	if v {
+		return jsonflags.PreserveRawStrings | 1
+	} else {
+		return jsonflags.PreserveRawStrings | 0
+	}
+}
+
 // RejectFloatOverflow specifies that unmarshaling a JSON number that
 // exceeds the maximum representation of a Go float32 or float64
 // results in an error, rather than succeeding with the floating-point values