Debugging a React Native crash in release - react-native

Question

I'm using react-native-tcp and am experiencing a crash on Android that I'm unable to debug. Unfortunately, this only happens in release.
The stack trace I get is as follows:
E/AndroidRuntime: FATAL EXCEPTION: mqt_native_modules
Process: com.xx.xx, PID: 22986
com.facebook.react.common.JavascriptException: unable to find socket, stack:
d#664:708
_onError#664:4582
<unknown>#664:3790
value#77:1364
value#53:2778
<unknown>#53:1013
<unknown>#53:106
value#53:985
at com.facebook.react.modules.core.ExceptionsManagerModule.showOrThrowError(ExceptionsManagerModule.java:99)
at com.facebook.react.modules.core.ExceptionsManagerModule.reportFatalException(ExceptionsManagerModule.java:83)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.facebook.react.bridge.BaseJavaModule$JavaMethod.invoke(BaseJavaModule.java:345)
at com.facebook.react.cxxbridge.JavaModuleWrapper.invoke(JavaModuleWrapper.java:136)
at com.facebook.react.bridge.queue.NativeRunnable.run(Native Method)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at com.facebook.react.bridge.queue.MessageQueueThreadHandler.dispatchMessage(MessageQueueThreadHandler.java:31)
at android.os.Looper.loop(Looper.java:145)
at com.facebook.react.bridge.queue.MessageQueueThreadImpl$3.run(MessageQueueThreadImpl.java:196)
at java.lang.Thread.run(Thread.java:818)
Now, the Javascript stack trace is pretty cryptic (presumably due to minification and bundling), but it helpfully includes _onError, which in react-native-tcp is implemented as follows:
TcpSocket.prototype._onError = function(error: string): void {
this._debug('received', 'error');
this.emit('error', normalizeError(error));
this.destroy();
};
The normalizeError call above maps to the d function in the stacktrace. I know this because I examined the bundled file generated during a build.
I understand why my code flow leads to an unable to find socket event, but what I don't understand is how this event eventually leads to an application crash. If it helps, this event is generated in the native (Android) component of react-native-tcp and propagates into the JS side.
I expected such an event to be reported to the listeners (if any) rather than crash the application.
Is it possible that events reported via React Native lead to a crash in some conditions - e.g. if there are no listeners, or if the event is an Error object, or something else?
How would I go about debugging this, given that it happens only in release?

Answer 1

I was unable to find an efficient debugging technique, but was able to debug the issue I had with good ol' prints.
The problem was that NodeJS (and apparently also React Native) have a behavior which I found peculiar: when an error event is emitted and there is no error listener, an unhandled exception is thrown. This is documented in the NodeJS documentation:
When an error occurs within an EventEmitter instance, the typical action is
for an 'error' event to be emitted. These are treated as special cases
within Node.js.
If an EventEmitter does not have at least one listener registered for the
'error' event, and an 'error' event is emitted, the error is thrown, a stack
trace is printed, and the Node.js process exits.
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js
My problem is that I was removing the error listener in my code, because of a react-native-tcp issue that causes the error callback to be called twice when connect fails because no one is listening on the port.