I come across this issue recently when developing a multi-threaded PySide2 (Qt) application. When I'm calling .copy() on a WeakValueDictionary, there is a high chance that my application crashes with the following stack backtrace:

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Traceback (most recent call last):
  File "F:\angr\angr-management\angrmanagement\ui\widgets\qdisasm_graph.py", line 239, in mouseDoubleClickEvent
    block.on_mouse_doubleclicked(event.button(), self._to_graph_pos(event.pos()))
  File "F:\angr\angr-management\angrmanagement\ui\widgets\qblock.py", line 130, in on_mouse_doubleclicked
    obj.on_mouse_doubleclicked(button, pos)
  File "F:\angr\angr-management\angrmanagement\ui\widgets\qinstruction.py", line 128, in on_mouse_doubleclicked
    op.on_mouse_doubleclicked(button, pos)
  File "F:\angr\angr-management\angrmanagement\ui\widgets\qoperand.py", line 162, in on_mouse_doubleclicked
    self.disasm_view.jump_to(self._branch_target, src_ins_addr=self.insn.addr)
  File "F:\angr\angr-management\angrmanagement\ui\views\disassembly_view.py", line 258, in jump_to
    self._jump_to(addr)
  File "F:\angr\angr-management\angrmanagement\ui\views\disassembly_view.py", line 372, in _jump_to
    self._display_function(function)
  File "F:\angr\angr-management\angrmanagement\ui\views\disassembly_view.py", line 343, in _display_function
    vr = self.workspace.instance.project.analyses.VariableRecoveryFast(the_func)
  File "f:\angr\angr\angr\analyses\analysis.py", line 109, in __call__
    oself.__init__(*args, **kwargs)
  File "f:\angr\angr\angr\analyses\variable_recovery\variable_recovery_fast.py", line 618, in __init__
    self._analyze()
  File "f:\angr\angr\angr\analyses\forward_analysis.py", line 557, in _analyze
    self._analysis_core_graph()
  File "f:\angr\angr\angr\analyses\forward_analysis.py", line 580, in _analysis_core_graph
    changed, output_state = self._run_on_node(n, job_state)
  File "f:\angr\angr\angr\analyses\variable_recovery\variable_recovery_fast.py", line 712, in _run_on_node
    input_state = prev_state.merge(input_state, successor=node.addr)
  File "f:\angr\angr\angr\analyses\variable_recovery\variable_recovery_fast.py", line 488, in merge
    merged_register_region = self.register_region.copy().replace(replacements).merge(other.register_region,
  File "f:\angr\angr\angr\keyed_region.py", line 159, in copy
    kr._object_mapping = self._object_mapping.copy()
  File "D:\My Program Files\Python37\lib\weakref.py", line 174, in copy
    for key, wr in self.data.items():
RuntimeError: dictionary changed size during iteration

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I went ahead and read the related methods in Lib\weakref.py, and it seems to me that the WeakValueDictionary.copy() method is missing the protection of an _IterationGuard: It is iterating through self.data.items(), which, might have entries removed because of GC during the iteration.

It seems that this crash can be fixed by wrapping the iteration with `with _IterationGuard(self):`. It worked for me in my tests.

If my above analysis is correct, the following methods all require protection of _IterationGuard (which are currently missing):

- WeakValueDictionary.copy()
- WeakValueDictionary.__deepcopy__()
- WeakKeyDictionary.copy()
- WeakKeyDictionary.__deepcopy__()

Please let me know if this is a legitimate issue, in which case I will be happy to provide a patch. Thanks.