X ZBigArrray: Don't throw away data cache on e.g. .shape change

bigarray/array_zodb.py

 # -*- coding: utf-8 -*-
 # Wendelin.bigarray | ZODB-Persistent BigArray
-# Copyright (C) 2014-2015  Nexedi SA and Contributors.
+# Copyright (C) 2014-2019  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -56,7 +56,8 @@ class ZBigArray(BigArray,
         self.zfile = ZBigFile(blksize)
         self._v_fileh = None
 
-
+    # __setstate__ (re-)loads ZBigArray data from DB, e.g. after live ZBigArray
+    # object was invalidated.
     def __setstate__(self, state):
         super(ZBigArray, self).__setstate__(state)
 
@@ -68,18 +69,15 @@ class ZBigArray(BigArray,
             if not hasattr(self, '_'+k):
                 setattr(self, '_'+k, v)
 
-        # NOTE __setstate__() is done after either
-        #   - 1st time loading from DB, or
-        #   - loading from DB after invalidation.
-        #
-        # as invalidation can happen e.g. by just changing .shape in another DB
-        # connection (e.g. resizing array and appending some data), via always
-        # resetting ._v_fileh we discard all data from it.
+        # (re-)set ._v_fileh; make sure not to loose fileh and its cache if
+        # .zfile has not changed. By preserving fileh we don't throw away data
+        # cache on e.g. just .shape change.
         #
-        # IOW we discard whole cache just because some data were appended.
-        #
-        # -> TODO (optimize) do not through ._v_fileh if we can (.zfile not changed, etc)
-        self._v_fileh = None
+        # TODO test/bench that cache is preserved after .shape change (e.g. append)
+        _v_fileh = getattr(self, '_v_fileh', None)
+        if _v_fileh is not None and self.zfile is not _v_fileh.zfile:
+            _v_fileh = None
+        self._v_fileh = _v_fileh
 
 
     # open fileh lazily, so that when we open it, zfile was already associated