Avoid a 32 kB file allocation on every bitBucketFilePutter.Put

io.Copy usually allocates a 32kB buffer, and due to the large
number of files processed by tar-split, this shows up in Go profiles
as a very large alloc_space total.

It doesn't seem to actually be a measurable problem in any way,
but we can allocate the buffer only once per tar-split creation,
at no additional cost to existing allocations, so let's do so,
and remove the distraction.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

tar/storage/getter.go

@@ -92,11 +92,12 @@ func NewDiscardFilePutter() FilePutter {
 }
 
 type bitBucketFilePutter struct {
+	buffer [32 * 1024]byte // 32 kB is the buffer size currently used by io.Copy, as of August 2021.
 }
 
 func (bbfp *bitBucketFilePutter) Put(name string, r io.Reader) (int64, []byte, error) {
 	c := crc64.New(CRCTable)
-	i, err := io.Copy(c, r)
+	i, err := io.CopyBuffer(c, r, bbfp.buffer[:])
 	return i, c.Sum(nil), err
 }