add unconditional warning before handling StackOverflow

Since this is not modeled by the exception logic, and it can interrupt
arbitrary program state or corrupt locks (leading to hangs and other
issues), as well as just frequently segfaulting afterwards, give a
printed message as soon as we notice things are going badly before
attempting to recover.

src/signal-handling.c

@@ -309,6 +309,11 @@ static void jl_check_profile_autostop(void)
     }
 }
 
+static void stack_overflow_warning(void)
+{
+    jl_safe_printf("Warning: detected a stack overflow; program state may be corrupted, so further execution might be unreliable.\n");
+}
+
 #if defined(_WIN32)
 #include "signals-win.c"
 #else

src/signals-mach.c

@@ -353,7 +353,7 @@ kern_return_t catch_mach_exception_raise(
     // XXX: jl_throw_in_thread or segv_handler will eventually check this, but
     //      we would like to avoid some of this work if we could detect this earlier
     // if (jl_has_safe_restore(ptls2)) {
-    //     jl_throw_in_thread(ptls2, thread, jl_stackovf_exception);
+    //     jl_throw_in_thread(ptls2, thread, NULL);
     //     return KERN_SUCCESS;
     // }
     if (jl_atomic_load_acquire(&ptls2->gc_state) == JL_GC_STATE_WAITING)
@@ -385,6 +385,7 @@ kern_return_t catch_mach_exception_raise(
         return KERN_FAILURE;
     jl_value_t *excpt;
     if (is_addr_on_stack(jl_atomic_load_relaxed(&ptls2->current_task), (void*)fault_addr)) {
+        stack_overflow_warning();
         excpt = jl_stackovf_exception;
     }
     else if (is_write_fault(exc_state)) // false for alignment errors

src/signals-unix.c

@@ -404,6 +404,7 @@ JL_NO_ASAN static void segv_handler(int sig, siginfo_t *info, void *context)
     if (ct->eh == NULL)
         sigdie_handler(sig, info, context);
     if ((sig != SIGBUS || info->si_code == BUS_ADRERR) && is_addr_on_stack(ct, info->si_addr)) { // stack overflow and not a BUS_ADRALN (alignment error)
+        stack_overflow_warning();
         jl_throw_in_ctx(ct, jl_stackovf_exception, sig, context);
     }
     else if (jl_is_on_sigstack(ct->ptls, info->si_addr, context)) {

src/signals-win.c

@@ -244,6 +244,7 @@ LONG WINAPI jl_exception_handler(struct _EXCEPTION_POINTERS *ExceptionInfo)
         case EXCEPTION_STACK_OVERFLOW:
             if (ct->eh != NULL) {
                 ptls->needs_resetstkoflw = 1;
+                stack_overflow_warning();
                 jl_throw_in_ctx(ct, jl_stackovf_exception, ExceptionInfo->ContextRecord);
                 return EXCEPTION_CONTINUE_EXECUTION;
             }