bpo-43390: Set SA_ONSTACK in PyOS_setsig (pythonGH-24730)

This is friendlier to other in-process code that an extension module or
embedding use could pull in such as CGo where tiny stacks are the norm
and sigaltstack() has been used to provide for signal handlers.

Without this, signals received by a process using tiny stacks may lead
to stack overflow crashes.

Misc/NEWS.d/next/Core and Builtins/2021-03-03-17-58-49.bpo-43390.epPpwV.rst

@@ -0,0 +1,6 @@
+CPython now sets the ``SA_ONSTACK`` flag in ``PyOS_setsig`` for the VM's
+default signal handlers.  This is friendlier to other in-process code that
+an extension module or embedding use could pull in (such as Golang's cgo)
+where tiny thread stacks are the norm and ``sigaltstack()`` has been used to
+provide for signal handlers.  This is a no-op change for the vast majority
+of processes that don't use sigaltstack.

Python/pylifecycle.c

@@ -2884,7 +2884,10 @@ PyOS_setsig(int sig, PyOS_sighandler_t handler)
     struct sigaction context, ocontext;
     context.sa_handler = handler;
     sigemptyset(&context.sa_mask);
-    context.sa_flags = 0;
+    /* Using SA_ONSTACK is friendlier to other C/C++/Golang-VM code that
+     * extension module or embedding code may use where tiny thread stacks
+     * are used.  https://bugs.python.org/issue43390 */
+    context.sa_flags = SA_ONSTACK;
     if (sigaction(sig, &context, &ocontext) == -1)
         return SIG_ERR;
     return ocontext.sa_handler;