internal/fuzz: minimize bytes to be human readable

Try to replace every byte with one of the following printable
characters: "012789ABCXYZabcxyz !\"#$%&'()*+,.".

Fixes #48129

Change-Id: Ie58f6bbc3431d50d9f0a3f608ba63e854ac6ce79
Reviewed-on: https://go-review.googlesource.com/c/go/+/352614
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Amelia Downs <adowns@vmware.com>
Trust: Katie Hockman <katie@golang.org>
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/internal/fuzz/minimize.go

@@ -79,6 +79,25 @@ func minimizeBytes(v []byte, try func(interface{}) bool, shouldStop func() bool)
 			j = len(v)
 		}
 	}
+
+	// Then, try to make it more simplified and human-readable by trying to replace each
+	// byte with a printable character.
+	printableChars := []byte("012789ABCXYZabcxyz !\"#$%&'()*+,.")
+	for i, b := range v {
+		if shouldStop() {
+			return
+		}
+
+		for _, pc := range printableChars {
+			v[i] = pc
+			if try(v) {
+				// Successful. Move on to the next byte in v.
+				break
+			}
+			// Unsuccessful. Revert v[i] back to original value.
+			v[i] = b
+		}
+	}
 }
 
 func minimizeInteger(v uint, try func(interface{}) bool, shouldStop func() bool) {

src/internal/fuzz/minimize_test.go

@@ -13,6 +13,8 @@ import (
 	"fmt"
 	"reflect"
 	"testing"
+	"unicode"
+	"unicode/utf8"
 )
 
 func TestMinimizeInput(t *testing.T) {
@@ -54,7 +56,7 @@ func TestMinimizeInput(t *testing.T) {
 				return fmt.Errorf("bad %v", e.Values[0])
 			},
 			input:    []interface{}{[]byte{1, 2, 3, 4, 5}},
-			expected: []interface{}{[]byte{2, 3}},
+			expected: []interface{}{[]byte("00")},
 		},
 		{
 			name: "set_of_bytes",
@@ -71,6 +73,18 @@ func TestMinimizeInput(t *testing.T) {
 			input:    []interface{}{[]byte{0, 1, 2, 3, 4, 5}},
 			expected: []interface{}{[]byte{0, 4, 5}},
 		},
+		{
+			name: "non_ascii_bytes",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].([]byte)
+				if len(b) == 3 {
+					return fmt.Errorf("bad %v", e.Values[0])
+				}
+				return nil
+			},
+			input:    []interface{}{[]byte("ท")}, // ท is 3 bytes
+			expected: []interface{}{[]byte("000")},
+		},
 		{
 			name: "ones_string",
 			fn: func(e CorpusEntry) error {
@@ -89,6 +103,31 @@ func TestMinimizeInput(t *testing.T) {
 			input:    []interface{}{"001010001000000000000000000"},
 			expected: []interface{}{"111"},
 		},
+		{
+			name: "string_length",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].(string)
+				if len(b) == 5 {
+					return fmt.Errorf("bad %v", e.Values[0])
+				}
+				return nil
+			},
+			input:    []interface{}{"zzzzz"},
+			expected: []interface{}{"00000"},
+		},
+		{
+			name: "string_with_letter",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].(string)
+				r, _ := utf8.DecodeRune([]byte(b))
+				if unicode.IsLetter(r) {
+					return fmt.Errorf("bad %v", e.Values[0])
+				}
+				return nil
+			},
+			input:    []interface{}{"ZZZZZ"},
+			expected: []interface{}{"A"},
+		},
 		{
 			name: "int",
 			fn: func(e CorpusEntry) error {