Call rb_bug_without_die on CI

when GC.compact's SEGV handler is installed

error.c

@@ -733,7 +733,7 @@ die(void)
 }
 
 void
-rb_bug(const char *fmt, ...)
+rb_bug_without_die(const char *fmt, ...)
 {
     const char *file = NULL;
     int line = 0;
@@ -743,7 +743,15 @@ rb_bug(const char *fmt, ...)
     }
 
     report_bug(file, line, fmt, NULL);
+}
 
+void
+rb_bug(const char *fmt, ...)
+{
+    va_list args;
+    va_start(args, fmt);
+    rb_bug_without_die(fmt, args);
+    va_end(args);
     die();
 }
 

gc.c

@@ -4597,6 +4597,11 @@ static struct sigaction old_sigsegv_handler;
 static void
 read_barrier_signal(int sig, siginfo_t * info, void * data)
 {
+    extern int ruby_on_ci;
+    if (ruby_on_ci) { // read_barrier_handler may crash. Report a backtrace first on CI.
+        extern void rb_bug_without_die(const char *fmt, ...);
+        rb_bug_without_die("died with read_barrier_signal installed");
+    }
     read_barrier_handler((intptr_t)info->si_addr);
 }