Fix canonical json utf-16 surrogate pair detection logic (#423)

Author: devonh

json.go

@@ -329,7 +329,7 @@ func compactUnicodeEscape(input, output []byte, index int) ([]byte, int) {
 		// Otherwise the character only needs escaping if it is a QUOTE '"' or BACKSLASH '\\'.
 		output = append(output, '\\', byte(c))
 	} else if utf16.IsSurrogate(c) {
-		if input[index] != '\\' && input[index+1] != 'u' {
+		if input[index] != '\\' || input[index+1] != 'u' {
 			return output, index
 		}
 		index += 2 // skip the \u"

json_test.go

@@ -125,6 +125,26 @@ func TestCompactUnicodeEscapeWithUTF16Surrogate(t *testing.T) {
 	}
 }
 
+func TestCompactUnicodeEscapeWithBadUTF16Surrogate(t *testing.T) {
+	input := []byte(`\ud83d\zdc08`)
+	output, n := compactUnicodeEscape(input[2:], nil, 0)
+	if n != 4 {
+		t.Fatalf("should have consumed 4 bytes but consumed %d bytes", n)
+	}
+	if string(output) != "" {
+		t.Fatalf("expected output to be empty")
+	}
+
+	input = []byte(`\ud83d udc08`)
+	output, n = compactUnicodeEscape(input[2:], nil, 0)
+	if n != 4 {
+		t.Fatalf("should have consumed 4 bytes but consumed %d bytes", n)
+	}
+	if string(output) != "" {
+		t.Fatalf("expected output to be empty")
+	}
+}
+
 func testReadHex(t *testing.T, input string, want rune) {
 	got := readHexDigits([]byte(input))
 	if want != got {