fix: implement readMetadata common function for re-use (#12353)

Previous PR #12351 added functions to read from the reader
stream to reduce memory usage, use the same technique in
few other places where we are not interested in reading the
data part.

cmd/xl-storage-format-v2.go

@@ -1420,7 +1420,7 @@ func readXLMetaNoData(r io.Reader, size int64) ([]byte, error) {
 			err = readMore(size)
 			return buf, err
 		case 1, 2:
-			sz, tmp, err := ReadBytesHeader(tmp)
+			sz, tmp, err := msgp.ReadBytesHeader(tmp)
 			if err != nil {
 				return nil, err
 			}
@@ -1459,51 +1459,3 @@ func readXLMetaNoData(r io.Reader, size int64) ([]byte, error) {
 		return nil, errors.New("unknown major metadata version")
 	}
 }
-
-// ReadBytesHeader reads the 'bin' header size
-// off of 'b' and returns the size and remaining bytes.
-// Possible errors:
-// - ErrShortBytes (too few bytes)
-// - TypeError{} (not a bin object)
-// TODO: Replace when https://github.com/tinylib/msgp/pull/289 is merged.
-func ReadBytesHeader(b []byte) (sz uint32, o []byte, err error) {
-	if len(b) < 1 {
-		return 0, nil, msgp.ErrShortBytes
-	}
-	var big = binary.BigEndian
-
-	const (
-		mbin8  uint8 = 0xc4
-		mbin16 uint8 = 0xc5
-		mbin32 uint8 = 0xc6
-	)
-	switch b[0] {
-	case mbin8:
-		if len(b) < 2 {
-			err = msgp.ErrShortBytes
-			return
-		}
-		sz = uint32(b[1])
-		o = b[2:]
-		return
-	case mbin16:
-		if len(b) < 3 {
-			err = msgp.ErrShortBytes
-			return
-		}
-		sz = uint32(big.Uint16(b[1:]))
-		o = b[3:]
-		return
-	case mbin32:
-		if len(b) < 5 {
-			err = msgp.ErrShortBytes
-			return
-		}
-		sz = big.Uint32(b[1:])
-		o = b[5:]
-		return
-	default:
-		err = msgp.TypeError{Method: msgp.BinType, Encoded: msgp.NextType(b)}
-		return
-	}
-}