encoding/csv: avoid mangling invalid UTF-8 in Writer

In the situation where a quoted field is necessary, avoid processing
each UTF-8 rune one-by-one, which causes mangling of invalid sequences
into utf8.RuneError, causing a loss of information.
Instead, search only for the escaped characters, handle those specially
and copy everything else in between verbatim.

This symmetrically matches the behavior of Reader.

Fixes #24298

Change-Id: I9276f64891084ce8487678f663fad711b4095dbb
Reviewed-on: https://go-review.googlesource.com/99297
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/csv/writer.go

@@ -57,13 +57,27 @@ func (w *Writer) Write(record []string) error {
 			}
 			continue
 		}
+
 		if err := w.w.WriteByte('"'); err != nil {
 			return err
 		}
+		for len(field) > 0 {
+			// Search for special characters.
+			i := strings.IndexAny(field, "\"\r\n")
+			if i < 0 {
+				i = len(field)
+			}
 
-		for _, r1 := range field {
+			// Copy verbatim everything before the special character.
+			if _, err := w.w.WriteString(field[:i]); err != nil {
+				return err
+			}
+			field = field[i:]
+
+			// Encode the special character.
+			if len(field) > 0 {
 				var err error
-			switch r1 {
+				switch field[0] {
 				case '"':
 					_, err = w.w.WriteString(`""`)
 				case '\r':
@@ -76,14 +90,13 @@ func (w *Writer) Write(record []string) error {
 					} else {
 						err = w.w.WriteByte('\n')
 					}
-			default:
-				_, err = w.w.WriteRune(r1)
 				}
+				field = field[1:]
 				if err != nil {
 					return err
 				}
 			}
-
+		}
 		if err := w.w.WriteByte('"'); err != nil {
 			return err
 		}

src/encoding/csv/writer_test.go

@@ -39,6 +39,8 @@ var writeTests = []struct {
 	{Input: [][]string{{"a", "a", ""}}, Output: "a,a,\n"},
 	{Input: [][]string{{"a", "a", "a"}}, Output: "a,a,a\n"},
 	{Input: [][]string{{`\.`}}, Output: "\"\\.\"\n"},
+	{Input: [][]string{{"x09\x41\xb4\x1c", "aktau"}}, Output: "x09\x41\xb4\x1c,aktau\n"},
+	{Input: [][]string{{",x09\x41\xb4\x1c", "aktau"}}, Output: "\",x09\x41\xb4\x1c\",aktau\n"},
 }
 
 func TestWrite(t *testing.T) {