[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>

1 files changed, 14 insertions, 6 deletions

diff --git a/src/internal/fuzz/encoding_test.go b/src/internal/fuzz/encoding_test.go
index 314f82a995..b429d429c6 100644
--- a/src/internal/fuzz/encoding_test.go
+++ b/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 want != string(newB) {
-				t.Errorf("values changed after unmarshal then marshal\nbefore: %q\nafter:  %q", want, newB)
+			if newB[len(newB)-1] != '\n' {
+				t.Error("didn't write final newline to corpus file")
+			}
+			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)
 			}
 		})
 	}