print exceptions like MRI

src/main.cc

@@ -6,11 +6,40 @@ using namespace std;
 
 RUBY_GLOBAL_SETUP
 
+static const char * ruby_string(VALUE obj)
+{
+    VALUE s = rb_funcall(obj, rb_intern("to_s"), 0);
+    return StringValueCStr(s);
+}
+
+static void print_exception(VALUE e)
+{
+    VALUE c = rb_funcall(e, rb_intern("class"), 0);
+    VALUE m = rb_funcall(e, rb_intern("message"), 0);
+    VALUE bt = rb_funcall(e, rb_intern("backtrace"), 0);
+    if ((TYPE(bt) == T_ARRAY) && (RARRAY_LEN(bt) > 0))
+    {
+        fprintf(stderr,
+                "%s: %s (%s)\n",
+                ruby_string(rb_ary_entry(bt, 0)),
+                ruby_string(m),
+                ruby_string(c));
+        for (int i = 1; i < RARRAY_LEN(bt); i++)
+        {
+            fprintf(stderr,
+                    "        from %s\n",
+                    ruby_string(rb_ary_entry(bt, i)));
+        }
+    }
+    else
+    {
+        fprintf(stderr, "%s (%s)\n", ruby_string(m), ruby_string(c));
+    }
+}
+
 static void handle_bootstrap_error()
 {
-    VALUE e = rb_errinfo();
+    print_exception(rb_errinfo());
-    VALUE s = rb_funcall(e, rb_intern("message"), 0);
-    fprintf(stderr, "%s\n", StringValueCStr(s));
     rb_set_errinfo(Qnil);
 }