PyOS_setsig currently sets the struct sigaction context.sa_flags = 0 before calling sigaction.

Other virtual machines such as Golang depend on signals using SA_ONSTACK such that signal handlers use a specially allocated stack that runtime sets up for reliability reasons as they use tiny stacks on normal threads.

SA_ONSTACK is a no-op flag in the typical case where no sigaltstack() call has been made to setup an alternate signal handling stack.  (as in 99.99% of all CPython applications)

When a C/C++ extension module is linked with cgo to call into a Golang library, doing this increases reliability.

As much as I try to dissuade anyone from creating and relying on hidden complexity multi-VM-hybrids in a single process like this, some people do, and this one line change helps.

Golang references:
  https://golang.org/pkg/os/signal/#hdr-Go_programs_that_use_cgo_or_SWIG
 and
  https://go-review.googlesource.com/c/go/+/298269/ (which clarifies that SA_RESTART is no longer a requirement. Good. Because Python won't get along well with that one.)

New changeset 02ac6f41e5569ec28d625bb005155903f64cc9ee by Gregory P. Smith in branch 'master':
bpo-43390: Set SA_ONSTACK in PyOS_setsig (GH-24730)
https://github.com/python/cpython/commit/02ac6f41e5569ec28d625bb005155903f64cc9ee

I expect zero fallout from this given the semantics.  SA_ONSTACK really appears to be something that should've been the POSIX default since it was introduced as a feature in ~BSD4.2 in the early 80s.  But it never was.

It'll be good to have in the beta releases to see if anyone pipes up.

We'll be running our interpreter inside Google with this flag enabled.