unix: lift sigaltstack setup to before first install

Author: workingjubilee

library/std/src/sys/pal/unix/stack_overflow.rs

@@ -140,18 +140,19 @@ mod imp {
             unsafe { sigaction(signal, ptr::null_mut(), &mut action) };
             // Configure our signal handler if one is not already set.
             if action.sa_sigaction == SIG_DFL {
+                if !NEED_ALTSTACK.load(Ordering::Relaxed) {
+                    // haven't set up our sigaltstack yet
+                    NEED_ALTSTACK.store(true, Ordering::Relaxed);
+                    let handler = unsafe { make_handler(true) };
+                    MAIN_ALTSTACK.store(handler.data, Ordering::Relaxed);
+                    mem::forget(handler);
+                }
                 action.sa_flags = SA_SIGINFO | SA_ONSTACK;
                 action.sa_sigaction = signal_handler as sighandler_t;
                 // SAFETY: only overriding signals if the default is set
                 unsafe { sigaction(signal, &action, ptr::null_mut()) };
-                NEED_ALTSTACK.store(true, Ordering::Relaxed);
             }
         }
-
-        // SAFETY: mutates our signal stack. shouldn't we install this first?
-        let handler = unsafe { make_handler(true) };
-        MAIN_ALTSTACK.store(handler.data, Ordering::Relaxed);
-        mem::forget(handler);
     }
 
     /// # Safety