.

wcfs/internal/xbtree.py

@@ -376,42 +376,46 @@ def Restructure(ztree, newStructure):
     for i in range(len(rbucketv)-1):
         assert rbucketv[i].range.khi <= rbucketv[i+1].klo
         rbucketv[i]  .node.next_bucket = rbucketv[i+1].node
-        rbucketv[i+1].node.prev_bucket = rbucketv[i]  .node
     rbucketv[-1].node.next_bucket = None
-    rbucketv[0] .node.prev_bucket = None
 
     # associate every bucket to zbucket
     assign(zrbucketv, rbucketv)
 
 
     # set znode states according to established tnew->znode association
-
-    # bucket.embedded_in_tree will indicate that this bucket's state was
-    # embedded into containing tree node.
-    for rb in rbucketv:
-        rb.node.embedded_in_tree = False
-
     rlevelv = rlevelv_orig
-    for rnodev in reversed(rlevelv):
+    for rnodev in rlevelv:
         for rn in rnodev:
             node = rn.node
             assert isinstance(node, (Tree,Bucket))
 
             # https://github.com/zopefoundation/BTrees/blob/4.5.0-1-gc8bf24e/BTrees/BTreeTemplate.c#L1087
             if isinstance(node, Tree):
+                # special case for empty tree and tree with just one bucket without oid.
+                #
+                # we cannot special case e.g. T/T/B because on loading ZODB
+                # wants to have !NULL T.firstbucket, and so if T/B is
+                # represented as empty or as T with embedded B, for top-level T
+                # there won't be a way to link to firstbucket and
+                # Tree.__setstate__ will raise "TypeError: No firstbucket in
+                # non-empty BTree".
                 zstate = unset = object()
                 if len(node.keyv) == 0:
-                    child = node.children[0]
-                    if isinstance(child, Bucket):
-                        # empty bucket noone links to
-                        if len(child.keyv) == 0 and child.prev_bucket is None:
-                            zstate = None   # -> empty tree
-                            child.embedded_in_tree = True
-
-                        # tree with single bucket noone links to
-                        elif child.Z._p_oid is None and child.prev_bucket is None:
-                            zstate = ((child.Z.__getstate__(),),)   # tree with bucket without oid
-                            child.embedded_in_tree = True
+                    if len(rlevelv) == 2: # only 2 levels
+                        assert len(rlevelv[0]) == 1     # top-one has empty tree
+                        assert rlevelv[0][0].node is node
+                        child = node.children[0]        # bottom-on has 1 bucket
+                        assert isinstance(child, Bucket)
+                        assert len(rlevelv[1]) == 1
+                        assert rlevelv[1][0].node is child
+
+                        # empty bucket -> empty tree
+                        if len(child.keyv) == 0:
+                            zstate = None
+
+                        # tree with single bucket withou oid -> tree with embedded bucket
+                        elif child.Z._p_oid is None:
+                            zstate = ((child.Z.__getstate__(),),)
 
                 if zstate is unset:
                     # normal tree node
@@ -423,15 +427,10 @@ def Restructure(ztree, newStructure):
                     zstate = (zstate,)
 
                     # firstbucket
-                    firstbucket = node.firstbucket()
-                    print('firstbucket: %s  (embedded: %s)' % (firstbucket, firstbucket.embedded_in_tree))
-                    if not firstbucket.embedded_in_tree:
-                        zstate += (firstbucket.Z,)
+                    zstate += (node.firstbucket().Z,)
 
             # https://github.com/zopefoundation/BTrees/blob/4.5.0-1-gc8bf24e/BTrees/BucketTemplate.c#L1195
             if isinstance(node, Bucket):
-                # ignore .embedded_in_tree - wrt tree it won't make a
-                # difference if we change an object not reachable through it.
                 zstate = ()
                 for k in node.keyv:
                     zstate += (k, kv.pop(k))            # (k1, v1, k2, v2, ..., kN, vN)