[dev.fuzz] internal/fuzz: write a newline to the end of a corpus file

If someone manually adds/alters a corpus file to add
extra spacing or remove the final newline, the file
can still be decoded. However, this change ensures that
the fuzzing engine correctly writes the final newline.

Fixes golang/go#48130

Change-Id: Ib5556d4a6e4e0bfd9bc2edab357b7c25bedfd176
Reviewed-on: https://go-review.googlesource.com/c/go/+/349055
Trust: Katie Hockman <katie@golang.org>
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_minimize.txt

@@ -163,6 +163,9 @@ func main() {
 		os.Exit(1)
 	}
 
+	// Trim the newline at the end of the file
+	got = bytes.TrimSpace(got)
+
 	// Make sure that there were exactly 100 bytes written to the corpus entry
 	prefix := []byte("[]byte(")
 	i := bytes.Index(got, prefix)

src/internal/fuzz/encoding.go

@@ -22,21 +22,21 @@ func marshalCorpusFile(vals ...interface{}) []byte {
 	if len(vals) == 0 {
 		panic("must have at least one value to marshal")
 	}
-	b := bytes.NewBuffer([]byte(encVersion1))
+	b := bytes.NewBuffer([]byte(encVersion1 + "\n"))
 	// TODO(katiehockman): keep uint8 and int32 encoding where applicable,
 	// instead of changing to byte and rune respectively.
 	for _, val := range vals {
 		switch t := val.(type) {
 		case int, int8, int16, int64, uint, uint16, uint32, uint64, float32, float64, bool:
-			fmt.Fprintf(b, "\n%T(%v)", t, t)
+			fmt.Fprintf(b, "%T(%v)\n", t, t)
 		case string:
-			fmt.Fprintf(b, "\nstring(%q)", t)
+			fmt.Fprintf(b, "string(%q)\n", t)
 		case rune: // int32
-			fmt.Fprintf(b, "\nrune(%q)", t)
+			fmt.Fprintf(b, "rune(%q)\n", t)
 		case byte: // uint8
-			fmt.Fprintf(b, "\nbyte(%q)", t)
+			fmt.Fprintf(b, "byte(%q)\n", t)
 		case []byte: // []uint8
-			fmt.Fprintf(b, "\n[]byte(%q)", t)
+			fmt.Fprintf(b, "[]byte(%q)\n", t)
 		default:
 			panic(fmt.Sprintf("unsupported type: %T", t))
 		}

src/internal/fuzz/encoding_test.go

@@ -66,16 +66,21 @@ byte('☃')`,
 		},
 		{
 			in: `go test fuzz v1
+string("has final newline")
+`,
+			ok: true, // has final newline
+		},
+		{
+			in: `go test fuzz v1
 string("extra")
 []byte("spacing")  
     `,
-			ok: true,
+			ok: true, // extra spaces in the final newline
 		},
 		{
 			in: `go test fuzz v1
 float64(0)
-float32(0)
+float32(0)`,
-`,
 			ok: true, // will be an integer literal since there is no decimal
 		},
 		{
@@ -114,9 +119,12 @@ float32(2.5)`,
 			if err != nil {
 				t.Fatalf("marshal unexpected error: %v", err)
 			}
-			want := strings.TrimSpace(test.in)
+			if newB[len(newB)-1] != '\n' {
-			if want != string(newB) {
+				t.Error("didn't write final newline to corpus file")
-				t.Errorf("values changed after unmarshal then marshal\nbefore: %q\nafter:  %q", want, newB)
+			}
+			before, after := strings.TrimSpace(test.in), strings.TrimSpace(string(newB))
+			if before != after {
+				t.Errorf("values changed after unmarshal then marshal\nbefore: %q\nafter:  %q", before, after)
 			}
 		})
 	}