Description

As originally report at cms-sw/cmssw#47287 (comment),
there is a race condition in the setting of TClass::fStreamerImpl for emulated classes. For those class
the TClass property are not final (they can be updated from 'new' information coming from the interpreter
and the current implementation of the check if this update is needed has a race condition that leads to
the use of the wrong StreamerImpl in some (rare) cases as seen in the original report.

Reproducer

root-project/roottest#1262

ROOT version

All version prior to v6.36/00, v6.34.06, v6.32.12