Don't call JS_GC() after js.exec() until new background threads exit

Hopefully this will fix the occasional crash in the testsuites
(and presumably, occasional crash in other things that use background
threads with js.exec(), if there are any).

src/sbbs3/exec.cpp

@@ -531,6 +531,7 @@ js_OperationCallback(JSContext *cx)
 	return ret;
 }
 
+extern JSClass js_global_class; // Hurr derr
 int sbbs_t::js_execfile(const char *cmd, const char* startup_dir, JSObject* scope, JSContext* js_cx, JSObject* js_glob)
 {
 	char*                     p;
@@ -554,7 +555,10 @@ int sbbs_t::js_execfile(const char *cmd, const char* startup_dir, JSObject* scop
 	struct js_runq_entry *    rq_head;
 	struct js_runq_entry *    rq_tail;
 	struct js_listener_entry *listeners;
+	global_private_t *gptp;
 
+	if ((gptp = (global_private_t*)js_GetClassPrivate(js_cx, js_glob, &js_global_class)) == NULL)
+		return -1;
 
 	if (js_cx == NULL)
 		js_cx = this->js_cx;
@@ -694,6 +698,8 @@ int sbbs_t::js_execfile(const char *cmd, const char* startup_dir, JSObject* scop
 	js_callback.rq_tail = NULL;
 	listeners = js_callback.listeners;
 	js_callback.listeners = NULL;
+
+	unsigned orig_bgcount = gptp->bg_count;
 	if (!JS_ExecuteScript(js_cx, js_scope, js_script, &rval))
 		result = -1;
 	js_handle_events(js_cx, &js_callback, &terminated);
@@ -742,6 +748,10 @@ int sbbs_t::js_execfile(const char *cmd, const char* startup_dir, JSObject* scop
 		JS_RemoveValueRoot(js_cx, &old_js_exec_path);
 		JS_RemoveValueRoot(js_cx, &old_js_scope);
 	}
+	while (gptp->bg_count > orig_bgcount) {
+		if (sem_wait(&gptp->bg_sem) == 0)
+			gptp->bg_count--;
+	}
 
 	JS_GC(js_cx);