whitekeyboard opened a new issue #1100:
URL: https://github.com/apache/dubbo-go/issues/1100
注册中心为nacos，provider频繁上线时，consumer会出现以下异常：
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x9edf17]
goroutine 278 [running]:
github.com/apache/dubbo-go/protocol/protocolwrapper.buildInvokerChain(0x0,
0x0, 0x103ff2f, 0x10, 0xc00020d000, 0xc000010201)
src/github.com/apache/dubbo-go/protocol/protocolwrapper/protocol_filter_wrapper.go:71
+0x37
github.com/apache/dubbo-go/protocol/protocolwrapper.(*ProtocolFilterWrapper).Refer(0xc000436ed0,
0xc00016cd80, 0x1036eac, 0x6)
src/github.com/apache/dubbo-go/protocol/protocolwrapper/protocol_filter_wrapper.go:62
+0x74
github.com/apache/dubbo-go/registry/directory.(*RegistryDirectory).cacheInvoker(0xc00016ca80,
0xc00016ccc0, 0x1c, 0xc000509b00)
src/github.com/apache/dubbo-go/registry/directory/directory.go:299
+0x562
github.com/apache/dubbo-go/registry/directory.(*RegistryDirectory).cacheInvokerByEvent(0xc00016ca80,
0xc0005099e0, 0x1b, 0xc000509a80, 0x1, 0x1)
src/github.com/apache/dubbo-go/registry/directory/directory.go:185
+0x11d
github.com/apache/dubbo-go/registry/directory.(*RegistryDirectory).refreshInvokers(0xc00016ca80,
0xc000308020, 0x1, 0x1)
src/github.com/apache/dubbo-go/registry/directory/directory.go:133
+0x11a
created by
github.com/apache/dubbo-go/registry/directory.(*RegistryDirectory).Notify
src/github.com/apache/dubbo-go/registry/directory/directory.go:107
+0x5d
```
初步定位了一下：
```
// Notify monitor changes from registry,and update the cacheServices
func (dir *RegistryDirectory) Notify(event *registry.ServiceEvent) {
if event == nil {
return
}
go dir.refreshInvokers(event)
}
```
provider上下线时，此处是go协程处理，同一个interface的下线处理和上线处理可能并行处理invoker, 导致panic。
出现概率：偶现
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