scons: RuntimeError: dictionary changed size during iteration

Scons 4.0.1 on Gentoo Linux. I didn’t get and answer on Discord, so posting here

  File "/usr/lib/python3.8/site-packages/SCons/Environment.py", line 1407, in Clone
    clone._dict = semi_deepcopy_dict(self._dict, ['BUILDERS'])
  File "/usr/lib/python3.8/site-packages/SCons/Util.py", line 539, in semi_deepcopy_dict
    for key, val in x.items():
RuntimeError: dictionary changed size during iteration:

My company uses Scons for a huge project, so it’s sometimes hard to say what’s going on or how Scons was invoked. But quite likely the code above was executed in a thread and I don’t know if it’s eligible.

I believe the easiest fix is to get a list of keys and not items iterator.

something like this probably

def semi_deepcopy_dict(x, exclude = [] ):
    copy = {}
    for key in list(x.keys()):
        # The regular Python copy.deepcopy() also deepcopies the key,
        # as follows:
        #
        #    copy[semi_deepcopy(key)] = semi_deepcopy(val)
        #
        # Doesn't seem like we need to, but we'll comment it just in case.
        if key not in exclude:
            copy[key] = semi_deepcopy(x[key])
    return copy