[dev.fuzz] internal/fuzz: add mutator for int types

Assuming this works, will follow up with another CL
that mutates other types.

Change-Id: Id61acaacd56ca41e3be52e400f8f768672313bbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/308169
Trust: Katie Hockman <katie@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/testdata/script/test_fuzz_mutate_crash.txt

@@ -28,6 +28,12 @@ stdout 'testdata[/\\]corpus[/\\]FuzzWithTwoTypes[/\\]'
 stdout 'these inputs caused a crash!'
 go run check_testdata.go FuzzWithTwoTypes
 
+# Running the fuzzer should find a crashing input quickly for an integer
+! go test -run=FuzzInt -fuzz=FuzzInt -fuzztime=5s
+stdout 'testdata[/\\]corpus[/\\]FuzzInt[/\\]'
+stdout 'this input caused a crash!'
+go run check_testdata.go FuzzInt
+
 ! go test -run=FuzzWithNilPanic -fuzz=FuzzWithNilPanic -fuzztime=5s
 stdout 'testdata[/\\]corpus[/\\]FuzzWithNilPanic[/\\]'
 stdout 'runtime.Goexit'
@@ -132,6 +138,14 @@ func FuzzWithTwoTypes(f *testing.F) {
 	})
 }
 
+func FuzzInt(f *testing.F) {
+	f.Fuzz(func(t *testing.T, a int) {
+		if 200 > a && a < 250 {
+			panic("this input caused a crash!")
+		}
+	})
+}
+
 func FuzzWithBadExit(f *testing.F) {
 	f.Add([]byte("aa"))
 	f.Fuzz(func(t *testing.T, b []byte) {

src/internal/fuzz/mutator.go

@@ -7,6 +7,7 @@ package fuzz
 import (
 	"encoding/binary"
 	"fmt"
+	"math"
 	"reflect"
 	"unsafe"
 )
@@ -75,11 +76,53 @@ func (m *mutator) mutate(vals []interface{}, maxBytes int) {
 		}
 		m.mutateBytes(&v)
 		vals[i] = v
+	case int8:
+		vals[i] = int8(m.mutateInt(int64(v), math.MaxInt8))
+	case int16:
+		vals[i] = int16(m.mutateInt(int64(v), math.MaxInt16))
+	case int32:
+		vals[i] = int32(m.mutateInt(int64(v), math.MaxInt32))
+	case int64:
+		vals[i] = m.mutateInt(v, int64(maxInt))
+	case int:
+		vals[i] = int(m.mutateInt(int64(v), int64(maxInt)))
 	default:
 		panic(fmt.Sprintf("type not supported for mutating: %T", vals[i]))
 	}
 }
 
+func (m *mutator) mutateInt(v, maxValue int64) int64 {
+	numIters := 1 + m.r.exp2()
+	var max int64
+	for iter := 0; iter < numIters; iter++ {
+		switch m.rand(2) {
+		case 0:
+			// Add a random number
+			if v >= maxValue {
+				continue
+			}
+			max = 100
+			if v > 0 && maxValue-v < max {
+				// Don't let v exceed maxValue
+				max = maxValue - v
+			}
+			v += int64(m.rand(int(max)))
+		case 1:
+			// Subtract a random number
+			if v <= -maxValue {
+				continue
+			}
+			max = 100
+			if v < 0 && maxValue+v < max {
+				// Don't let v drop below -maxValue
+				max = maxValue + v
+			}
+			v -= int64(m.rand(int(max)))
+		}
+	}
+	return v
+}
+
 func (m *mutator) mutateBytes(ptrB *[]byte) {
 	b := *ptrB
 	defer func() {
@@ -267,6 +310,11 @@ var (
 	interesting32 = []int32{-2147483648, -100663046, -32769, 32768, 65535, 65536, 100663045, 2147483647}
 )
 
+const (
+	maxUint = ^uint(0)
+	maxInt  = maxUint >> 1
+)
+
 func init() {
 	for _, v := range interesting8 {
 		interesting16 = append(interesting16, int16(v))