[Python][RDF] Pure Python callables in Define/Filter deadlock with EnableImplicitMT #24140

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Description

A pure-Python callable passed to RDataFrame.Define (or Filter) deadlocks when ROOT.EnableImplicitMT() is on and the event loop has more than one task. The process hangs forever.

The main thread triggers the event loop from Python and keeps the GIL while it waits in TThreadExecutor::ParallelFor. The TBB worker threads block in PyGILState_Ensure when they call the Python callback.

Reproducer

import ROOT

ROOT.EnableImplicitMT(4)

def square(x: float) -> float:
    return x * x

df = ROOT.RDataFrame(10).Define("x", "double(rdfentry_)").Define("y", square, ["x"])
print(df.Sum("y").GetValue())  # never returns

ROOT version

6.42

Installation method

from source

Operating system

linux, fedora

Additional context

No response