Hi @mwilkins,
thank you for the simple reproducer.
This is now ROOT-9948.
As a temporary workaround, note that if you construct RDataFrame as
ROOT.RDataFrame('DecayTree', 'root://some/path/ForSharing.root')
instead of
f = ROOT.TFile.Open('root://some/path/ForSharing.root')
t = f.Get('DecayTree')
df1 = ROOT.RDataFrame(t)
things work correctly even with multi-threading enabled, as the problem is in the logic we have in place to deduce a TTree name in a TFile from a TTree object.
Cheers,
Enrico