bpo-43390: Set SA_ONSTACK in PyOS_setsig

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;