faulthandler.register(SIGUSR2, ..., chain=True) writes the stack dump and
then re-raises the signal to its previous handler. SIGUSR2's default
disposition is "terminate", so the diagnostic hook added for #70344 kills the
very process an operator is trying to inspect (rc = -12), which is what the
#110437 reporter hit while introspecting a long-lived gateway. Nothing else
in the gateway installs a SIGUSR2 handler, so there is nothing to chain to.
Test: a child interpreter runs the real _start_install_faulthandler, receives
SIGUSR2, and must still be alive with a dump in gateway_faulthandler.log.
Red on origin/main (rc=-12), green with chain=False. Docs: the stack-dump
signal is now documented next to the event-loop watchdog.
teknium1
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