encoding/csv: skip blank lines when FieldsPerRecord >= 0

Fixes #11050.

Change-Id: Ie5d16960a1f829af947d82a63fe414924cd02ff6
Reviewed-on: https://go-review.googlesource.com/10666Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/csv/reader.go

@@ -228,16 +228,15 @@ func (r *Reader) parseRecord() (fields []string, err error) {
 	}
 	r.r.UnreadRune()
 
-	// If FieldsPerRecord is greater then 0 we can assume the final
-	// length of fields to be equal to FieldsPerRecord.
-	if r.FieldsPerRecord > 0 {
-		fields = make([]string, 0, r.FieldsPerRecord)
-	}
-
 	// At this point we have at least one field.
 	for {
 		haveField, delim, err := r.parseField()
 		if haveField {
+			// If FieldsPerRecord is greater then 0 we can assume the final
+			// length of fields to be equal to FieldsPerRecord.
+			if r.FieldsPerRecord > 0 && fields == nil {
+				fields = make([]string, 0, r.FieldsPerRecord)
+			}
 			fields = append(fields, r.field.String())
 		}
 		if delim == '\n' || err == io.EOF {

src/encoding/csv/reader_test.go

@@ -86,6 +86,15 @@ field"`,
 			{"d", "e", "f"},
 		},
 	},
+	{
+		Name:               "BlankLineFieldCount",
+		Input:              "a,b,c\n\nd,e,f\n\n",
+		UseFieldsPerRecord: true,
+		Output: [][]string{
+			{"a", "b", "c"},
+			{"d", "e", "f"},
+		},
+	},
 	{
 		Name:             "TrimSpace",
 		Input:            " a,  b,   c\n",