encoding/csv: add a Fuzz function

Adds a sample Fuzz test function to package encoding/csv based on
https://github.com/dvyukov/go-fuzz-corpus/blob/master/csv/main.go

Updates #19109
Updates #31309

Change-Id: Ieb0cb6caa1df72dbb7e29df4bdeed0bfa91187d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/174302Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/csv/fuzz.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build gofuzz
+
+package csv
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+)
+
+func Fuzz(data []byte) int {
+	score := 0
+	buf := new(bytes.Buffer)
+
+	for _, tt := range []Reader{
+		Reader{},
+		Reader{Comma: ';'},
+		Reader{Comma: '\t'},
+		Reader{LazyQuotes: true},
+		Reader{TrimLeadingSpace: true},
+		Reader{Comment: '#'},
+		Reader{Comment: ';'},
+	} {
+		r := NewReader(bytes.NewReader(data))
+		r.Comma = tt.Comma
+		r.Comment = tt.Comment
+		r.LazyQuotes = tt.LazyQuotes
+		r.TrimLeadingSpace = tt.TrimLeadingSpace
+
+		records, err := r.ReadAll()
+		if err != nil {
+			continue
+		}
+		score = 1
+
+		buf.Reset()
+		w := NewWriter(buf)
+		w.Comma = tt.Comma
+		err = w.WriteAll(records)
+		if err != nil {
+			fmt.Printf("writer  = %#v\n", w)
+			fmt.Printf("records = %v\n", records)
+			panic(err)
+		}
+
+		r = NewReader(buf)
+		r.Comma = tt.Comma
+		r.Comment = tt.Comment
+		r.LazyQuotes = tt.LazyQuotes
+		r.TrimLeadingSpace = tt.TrimLeadingSpace
+		result, err := r.ReadAll()
+		if err != nil {
+			fmt.Printf("reader  = %#v\n", r)
+			fmt.Printf("records = %v\n", records)
+			panic(err)
+		}
+
+		if !reflect.DeepEqual(records, result) {
+			fmt.Println("records = \n", records)
+			fmt.Println("result  = \n", records)
+			panic("not equal")
+		}
+	}
+
+	return score
+}