Merging to release-4-lts: [TT-7127] Fix goplugin invalid memory addre…

…ss or nil pointer dereference (#5589) (#5594)

[TT-7127] Fix goplugin invalid memory address or nil pointer dereference
(#5589)

https://tyktech.atlassian.net/browse/TT-7127

Closes #4198 #4197

Signed-off-by: Chenyang Yan <memory.yancy@gmail.com>
Co-authored-by: Chenyang Yan <memory.yancy@gmail.com>

[TT-7127]:
https://tyktech.atlassian.net/browse/TT-7127?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

---------

Co-authored-by: Tit Petric <tit.petric@monotek.net>
Co-authored-by: Tit Petric <tit@tyk.io>

gateway/mw_go_plugin.go

@@ -156,13 +156,16 @@ func (m *GoPluginMiddleware) goPluginFromRequest(r *http.Request) (*GoPluginMidd
 	return perPathPerMethodGoPlugin.(*GoPluginMiddleware), found
 }
 func (m *GoPluginMiddleware) ProcessRequest(w http.ResponseWriter, r *http.Request, conf interface{}) (err error, respCode int) {
-	// if a Go plugin is found for this path, override the base handler and logger:
 	logger := m.logger
 	handler := m.handler
+	successHandler := m.successHandler
+
 	if !m.APILevel {
+		// if a Go plugin is found for this path, override the base handler and logger
 		if pluginMw, found := m.goPluginFromRequest(r); found {
 			logger = pluginMw.logger
 			handler = pluginMw.handler
+			successHandler = &SuccessHandler{BaseMiddleware: m.BaseMiddleware}
 		} else {
 			return nil, http.StatusOK // next middleware
 		}
@@ -226,7 +229,7 @@ func (m *GoPluginMiddleware) ProcessRequest(w http.ResponseWriter, r *http.Reque
 			m.logger.WithError(err).Error("Failed to process request with Go-plugin middleware func")
 		default:
 			// record 2XX to analytics
-			m.successHandler.RecordHit(r, Latency{Total: int64(ms)}, rw.statusCodeSent, rw.getHttpResponse(r))
+			successHandler.RecordHit(r, Latency{Total: int64(ms)}, rw.statusCodeSent, rw.getHttpResponse(r))
 
 			// no need to continue passing this request down to reverse proxy
 			respCode = mwStatusRespond