encoding/csv: always allow trailing commas

Original CL by rsc (11916045):

The motivation for disallowing them was RFC 4180 saying
"The last field in the record must not be followed by a comma."
I believe this is an admonition to CSV generators, not readers.
When reading, anything followed by a comma is not the last field.

Fixes #5892.

R=golang-dev, rsc, r
CC=golang-dev
https://golang.org/cl/12294043

src/pkg/encoding/csv/reader.go

@@ -72,7 +72,7 @@ func (e *ParseError) Error() string {
 
 // These are the errors that can be returned in ParseError.Error
 var (
-	ErrTrailingComma = errors.New("extra delimiter at end of line")
+	ErrTrailingComma = errors.New("extra delimiter at end of line") // no longer used
 	ErrBareQuote     = errors.New("bare \" in non-quoted-field")
 	ErrQuote         = errors.New("extraneous \" in field")
 	ErrFieldCount    = errors.New("wrong number of fields in line")
@@ -98,16 +98,14 @@ var (
 // If LazyQuotes is true, a quote may appear in an unquoted field and a
 // non-doubled quote may appear in a quoted field.
 //
-// If TrailingComma is true, the last field may be an unquoted empty field.
-//
 // If TrimLeadingSpace is true, leading white space in a field is ignored.
 type Reader struct {
-	Comma            rune // Field delimiter (set to ',' by NewReader)
-	Comment          rune // Comment character for start of line
-	FieldsPerRecord  int  // Number of expected fields per record
-	LazyQuotes       bool // Allow lazy quotes
-	TrailingComma    bool // Allow trailing comma
-	TrimLeadingSpace bool // Trim leading space
+	Comma            rune // field delimiter (set to ',' by NewReader)
+	Comment          rune // comment character for start of line
+	FieldsPerRecord  int  // number of expected fields per record
+	LazyQuotes       bool // allow lazy quotes
+	TrailingComma    bool // ignored; here for backwards compatibility
+	TrimLeadingSpace bool // trim leading space
 	line             int
 	column           int
 	r                *bufio.Reader
@@ -257,23 +255,15 @@ func (r *Reader) parseField() (haveField bool, delim rune, err error) {
 	r.field.Reset()
 
 	r1, err := r.readRune()
-	if err != nil {
-		// If we have EOF and are not at the start of a line
-		// then we return the empty field.  We have already
-		// checked for trailing commas if needed.
-		if err == io.EOF && r.column != 0 {
-			return true, 0, err
-		}
-		return false, 0, err
+	for err == nil && r.TrimLeadingSpace && r1 != '\n' && unicode.IsSpace(r1) {
+		r1, err = r.readRune()
 	}
 
-	if r.TrimLeadingSpace {
-		for r1 != '\n' && unicode.IsSpace(r1) {
-			r1, err = r.readRune()
-			if err != nil {
-				return false, 0, err
-			}
-		}
+	if err == io.EOF && r.column != 0 {
+		return true, 0, err
+	}
+	if err != nil {
+		return false, 0, err
 	}
 
 	switch r1 {
@@ -349,25 +339,5 @@ func (r *Reader) parseField() (haveField bool, delim rune, err error) {
 		return false, 0, err
 	}
 
-	if !r.TrailingComma {
-		// We don't allow trailing commas.  See if we
-		// are at the end of the line (being mindful
-		// of trimming spaces).
-		c := r.column
-		r1, err = r.readRune()
-		if r.TrimLeadingSpace {
-			for r1 != '\n' && unicode.IsSpace(r1) {
-				r1, err = r.readRune()
-				if err != nil {
-					break
-				}
-			}
-		}
-		if err == io.EOF || r1 == '\n' {
-			r.column = c // report the comma
-			return false, 0, r.error(ErrTrailingComma)
-		}
-		r.unreadRune()
-	}
 	return true, r1, nil
 }

src/pkg/encoding/csv/reader_test.go

@@ -171,32 +171,32 @@ field"`,
 		Output: [][]string{{"a", "b", "c"}, {"d", "e"}},
 	},
 	{
-		Name:  "BadTrailingCommaEOF",
-		Input: "a,b,c,",
-		Error: "extra delimiter at end of line", Line: 1, Column: 5,
+		Name:   "TrailingCommaEOF",
+		Input:  "a,b,c,",
+		Output: [][]string{{"a", "b", "c", ""}},
 	},
 	{
-		Name:  "BadTrailingCommaEOL",
-		Input: "a,b,c,\n",
-		Error: "extra delimiter at end of line", Line: 1, Column: 5,
+		Name:   "TrailingCommaEOL",
+		Input:  "a,b,c,\n",
+		Output: [][]string{{"a", "b", "c", ""}},
 	},
 	{
-		Name:             "BadTrailingCommaSpaceEOF",
+		Name:             "TrailingCommaSpaceEOF",
 		TrimLeadingSpace: true,
 		Input:            "a,b,c, ",
-		Error:            "extra delimiter at end of line", Line: 1, Column: 5,
+		Output:           [][]string{{"a", "b", "c", ""}},
 	},
 	{
-		Name:             "BadTrailingCommaSpaceEOL",
+		Name:             "TrailingCommaSpaceEOL",
 		TrimLeadingSpace: true,
 		Input:            "a,b,c, \n",
-		Error:            "extra delimiter at end of line", Line: 1, Column: 5,
+		Output:           [][]string{{"a", "b", "c", ""}},
 	},
 	{
-		Name:             "BadTrailingCommaLine3",
+		Name:             "TrailingCommaLine3",
 		TrimLeadingSpace: true,
 		Input:            "a,b,c\nd,e,f\ng,hi,",
-		Error:            "extra delimiter at end of line", Line: 3, Column: 4,
+		Output:           [][]string{{"a", "b", "c"}, {"d", "e", "f"}, {"g", "hi", ""}},
 	},
 	{
 		Name:   "NotTrailingComma3",
@@ -231,7 +231,7 @@ x,,,
 		},
 	},
 	{
-		Name:             "Issue 2366",
+		Name:             "TrailingCommaIneffective1",
 		TrailingComma:    true,
 		TrimLeadingSpace: true,
 		Input:            "a,b,\nc,d,e",
@@ -241,11 +241,14 @@ x,,,
 		},
 	},
 	{
-		Name:             "Issue 2366a",
+		Name:             "TrailingCommaIneffective2",
 		TrailingComma:    false,
 		TrimLeadingSpace: true,
 		Input:            "a,b,\nc,d,e",
-		Error:            "extra delimiter at end of line",
+		Output: [][]string{
+			{"a", "b", ""},
+			{"c", "d", "e"},
+		},
 	},
 }