bytes: re-slice buffer to its previous length after call to grow()

Fixes #25435

The added test fails without the re-slice and passes with it.

Change-Id: I5ebc2a737285eb116ecc5938d8bf49050652830f
GitHub-Last-Rev: 454ddad7df8d56a1d0e05a999ed8277c5516ce01
GitHub-Pull-Request: golang/go#25436
Reviewed-on: https://go-review.googlesource.com/113495Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/buffer.go

@@ -202,6 +202,7 @@ func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
 	b.lastRead = opInvalid
 	for {
 		i := b.grow(MinRead)
+		b.buf = b.buf[:i]
 		m, e := r.Read(b.buf[i:cap(b.buf)])
 		if m < 0 {
 			panic(errNegativeRead)

src/bytes/buffer_test.go

@@ -269,6 +269,39 @@ func TestReadFrom(t *testing.T) {
 	}
 }
 
+type panicReader struct{ panic bool }
+
+func (r panicReader) Read(p []byte) (int, error) {
+	if r.panic {
+		panic(nil)
+	}
+	return 0, io.EOF
+}
+
+// Make sure that an empty Buffer remains empty when
+// it is "grown" before a Read that panics
+func TestReadFromPanicReader(t *testing.T) {
+
+	// First verify non-panic behaviour
+	var buf Buffer
+	i, err := buf.ReadFrom(panicReader{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if i != 0 {
+		t.Fatalf("unexpected return from bytes.ReadFrom (1): got: %d, want %d", i, 0)
+	}
+	check(t, "TestReadFromPanicReader (1)", &buf, "")
+
+	// Confirm that when Reader panics, the emtpy buffer remains empty
+	var buf2 Buffer
+	defer func() {
+		recover()
+		check(t, "TestReadFromPanicReader (2)", &buf2, "")
+	}()
+	buf2.ReadFrom(panicReader{panic: true})
+}
+
 func TestReadFromNegativeReader(t *testing.T) {
 	var b Buffer
 	defer func() {