lua cluster_specifier: fix crash in getCluster() (envoyproxy#37073)

Envoy would crash when the Lua garbage collector ran if `getCluster()`
had been called.

This was added in envoyproxy#36998

Signed-off-by: Greg Greenway <ggreenway@apple.com>

source/extensions/router/cluster_specifiers/lua/lua_cluster_specifier.h

@@ -66,6 +66,8 @@ class ClusterWrapper : public Filters::Common::Lua::BaseLuaObject<ClusterWrapper
     };
   }
 
+  void onMarkDead() override { cluster_.reset(); }
+
 private:
   DECLARE_LUA_FUNCTION(ClusterWrapper, luaNumConnections);
   DECLARE_LUA_FUNCTION(ClusterWrapper, luaNumRequests);
@@ -74,7 +76,7 @@ class ClusterWrapper : public Filters::Common::Lua::BaseLuaObject<ClusterWrapper
   Upstream::ClusterInfoConstSharedPtr cluster_;
 };
 
-using ClusterRef = Filters::Common::Lua::LuaRef<ClusterWrapper>;
+using ClusterRef = Filters::Common::Lua::LuaDeathRef<ClusterWrapper>;
 
 class RouteHandleWrapper : public Filters::Common::Lua::BaseLuaObject<RouteHandleWrapper> {
 public:
@@ -91,7 +93,10 @@ class RouteHandleWrapper : public Filters::Common::Lua::BaseLuaObject<RouteHandl
   // All embedded references should be reset when the object is marked dead. This is to ensure that
   // we won't do the resetting in the destructor, which may be called after the referenced
   // coroutine's lua_State is closed. And if that happens, the resetting will cause a crash.
-  void onMarkDead() override { headers_wrapper_.reset(); }
+  void onMarkDead() override {
+    headers_wrapper_.reset();
+    clusters_.clear();
+  }
 
 private:
   /**

test/extensions/router/cluster_specifiers/lua/lua_cluster_specifier_test.cc

@@ -265,6 +265,9 @@ TEST_F(LuaClusterSpecifierPluginTest, ClusterMethods) {
   Http::TestRequestHeaderMapImpl headers{{":path", "/"}};
   auto route = plugin_->route(mock_route, headers);
   EXPECT_EQ("pass", route->routeEntry()->clusterName());
+
+  // Force the runtime to gc and destroy all the userdata.
+  config_->perLuaCodeSetup()->runtimeGC();
 }
 
 } // namespace Lua