Use a new magic number for protocol-3 filestorage.

src/ZODB/FileStorage/FileStorage.py

@@ -78,12 +78,13 @@ from ZODB._compat import loads
 from ZODB._compat import decodebytes
 from ZODB._compat import encodebytes
 from ZODB._compat import _protocol
+from ZODB._compat import FILESTORAGE_MAGIC
 
 
 # Not all platforms have fsync
 fsync = getattr(os, "fsync", None)
 
-packed_version = b"FS21"
+packed_version = FILESTORAGE_MAGIC
 
 logger = logging.getLogger('ZODB.FileStorage')
 

src/ZODB/_compat.py

@@ -23,6 +23,7 @@ try:
     IMPORT_MAPPING = {}
     NAME_MAPPING = {}
     _protocol = 1
+    FILESTORAGE_MAGIC = b"FS21"
 except ImportError:
     # Python 3.x: can't use stdlib's pickle because
     # http://bugs.python.org/issue6784
@@ -57,6 +58,7 @@ except ImportError:
     def loads(s):
         return zodbpickle.pickle.loads(s, encoding='ASCII', errors='bytes')
     _protocol = 3
+    FILESTORAGE_MAGIC = b"FS30"
 
 
 # XXX: consistent spelling of inst_persistent_id/persistent_id?

src/ZODB/scripts/fstest.py

@@ -39,6 +39,7 @@ from __future__ import print_function
 import binascii
 import struct
 import sys
+from ZODB._compat import FILESTORAGE_MAGIC
 
 class FormatError(ValueError):
     """There is a problem with the format of the FileStorage."""
@@ -47,7 +48,7 @@ class Status:
     checkpoint = b'c'
     undone = b'u'
 
-packed_version = b'FS21'
+packed_version = FILESTORAGE_MAGIC
 
 TREC_HDR_LEN = 23
 DREC_HDR_LEN = 42