Crash in PyROOT in 6.40 (GIL issue?)

[olantwin]

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Description

With 6.40, cppyy releases the GIL for some calls to C++ resulting in a crash when calling back into python to raise in this example a warning.

This results in a segmentation violation. This reproducer can probably be further simplified, anything calling back into python to raise a warning (or for other purposes?) should suffice.

Reproducer

import ROOT
h = ROOT.TH1D("test", "test", 100, 0, 1)  # empty
h.Fit("gaus", "S")  # GILCallO releases GIL, Warning calls back without it → crash

ROOT version

6.40 (from SHiP's stack or LCG dev 4, tested with Python 3.12 and 3.13

Installation method

bits or LCG

Operating system

el9

Additional context

No response

[wdconinc]

I think I encountered this exact same bug and it appears introduced in 6.38.04 as well, via commit 8771c963fec.