Add null check for consumer in MessageListenerProxy

[BewareMyPower]

There is a segmentation fault reported by a user.

So, I recompiled the extension with node-gyp rebuild --debug then attached a debugger and it crashed because of this

(gdb) l 60 void MessageListenerProxy(Napi::Env env, Napi::Function jsCallback, MessageListenerProxyData *data) { 61 Napi::Object msg = Message::NewInstance({}, data->cMessage); 62 Consumer *consumer = data->consumer; 63 delete data; 64 65 jsCallback.Call({msg, consumer->Value()}); 66 } 67 68 void MessageListener(pulsar_consumer_t *cConsumer, pulsar_message_t *cMessage, void *ctx) { 69 ListenerCallback *listenerCallback = (ListenerCallback *)ctx; (gdb) 

On line 65, consumer is actually null

The consumer field is assigned from MessageListenerCallback and the initial value is nullptr. This field could only be modified in Consumer::SetListenerCallback. I didn't look deeper into where it's invoked, but I think it's better to add the null check here for safety.

[gaoran10]

Do we need to print some warning messages when the consumer is a null value?

[BewareMyPower]

No. The initial value is null. And I found the NodeJS wrapper never logs any message. Even if you added a log here, there is still helpless to find in which scenario consumer is null here.

[gaoran10]

I'm not sure if users lose to handle some messages when the consumer is null.

[BewareMyPower]

I think the controversial point is whether should we skip the null consumer.

• Yes: might lose messages (not sure)
• No: fast fail

Adding logs doesn't solve anything. There is no logger in NodeJS client, printing logs to console is meaningless and helpless as I said.

[BewareMyPower]

We need more info to reproduce the segmentation fault. But for now, a safe solution is to avoid accessing a null consumer.

[k2la]

@BewareMyPower
This fix does not address the root cause.
Could you additionally write comments indicating that this modification is workaround?

Also, what versions of client libraries(pulsar-client-node and pulsar-client-cpp) were used when this segmentation fault occurred?

[BewareMyPower]

Yeah. And I'll confirm the version from the user