.

wcfs/internal/xbtree.py

@@ -385,8 +385,14 @@ def Restructure(ztree, newStructure):
 
 
     # 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 rlevelv:
+    for rnodev in reversed(rlevelv):
         for rn in rnodev:
             node = rn.node
             assert isinstance(node, (Tree,Bucket))
@@ -400,10 +406,12 @@ def Restructure(ztree, newStructure):
                         # 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 zstate is unset:
                     # normal tree node
@@ -415,10 +423,15 @@ def Restructure(ztree, newStructure):
                     zstate = (zstate,)
 
                     # firstbucket
-                    zstate += (node.firstbucket().Z,)
+                    firstbucket = node.firstbucket()
+                    print('firstbucket: %s  (embedded: %s)' % (firstbucket, firstbucket.embedded_in_tree))
+                    if not firstbucket.embedded_in_tree:
+                        zstate += (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)
@@ -434,6 +447,10 @@ def Restructure(ztree, newStructure):
     assert tnew.Z is ztree
     assert len(kv) == 0     # all keys must have been popped
 
+    print()
+    print('new struct:')
+    print(StructureOf(ztree))
+    print()
     _bcheck(ztree)  # verify ztree after our tweaks
     tstruct = StructureOf(ztree)
     if tstruct != newStructure: