encoding/csv: disallow quote for use as Comma

'"' has special semantic meaning that conflicts with using it as Comma.

Change-Id: Ife25ba43ca25dba2ea184c1bb7579a230d376059
Reviewed-on: https://go-review.googlesource.com/99696
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/csv/reader.go

@@ -91,7 +91,7 @@ var (
 var errInvalidDelim = errors.New("csv: invalid field or comment delimiter")
 
 func validDelim(r rune) bool {
-	return r != 0 && r != '\r' && r != '\n' && utf8.ValidRune(r) && r != utf8.RuneError
+	return r != 0 && r != '"' && r != '\r' && r != '\n' && utf8.ValidRune(r) && r != utf8.RuneError
 }
 
 // A Reader reads records from a CSV-encoded file.

src/encoding/csv/reader_test.go

@@ -359,6 +359,10 @@ x,,,
 		Error: errInvalidDelim,
 	}, {
 		Name:  "BadComma3",
+		Comma: '"',
+		Error: errInvalidDelim,
+	}, {
+		Name:  "BadComma4",
 		Comma: utf8.RuneError,
 		Error: errInvalidDelim,
 	}, {

src/encoding/csv/writer_test.go

@@ -13,7 +13,9 @@ import (
 var writeTests = []struct {
 	Input   [][]string
 	Output  string
+	Error   error
 	UseCRLF bool
+	Comma   rune
 }{
 	{Input: [][]string{{"abc"}}, Output: "abc\n"},
 	{Input: [][]string{{"abc"}}, Output: "abc\r\n", UseCRLF: true},
@@ -41,6 +43,9 @@ var writeTests = []struct {
 	{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"},
+	{Input: [][]string{{"a", "a", ""}}, Output: "a|a|\n", Comma: '|'},
+	{Input: [][]string{{",", ",", ""}}, Output: ",|,|\n", Comma: '|'},
+	{Input: [][]string{{"foo"}}, Comma: '"', Error: errInvalidDelim},
 }
 
 func TestWrite(t *testing.T) {
@@ -48,9 +53,12 @@ func TestWrite(t *testing.T) {
 		b := &bytes.Buffer{}
 		f := NewWriter(b)
 		f.UseCRLF = tt.UseCRLF
+		if tt.Comma != 0 {
+			f.Comma = tt.Comma
+		}
 		err := f.WriteAll(tt.Input)
-		if err != nil {
-			t.Errorf("Unexpected error: %s\n", err)
+		if err != tt.Error {
+			t.Errorf("Unexpected error:\ngot  %v\nwant %v", err, tt.Error)
 		}
 		out := b.String()
 		if out != tt.Output {