[android] RuntimeException when app gets paused and returns to foreground

[haveneersrobin]

Description

We see the following errors in our Sentry logging:

java.lang.NullPointerException: Attempt to invoke virtual method 'void java.net.DatagramSocket.send(java.net.DatagramPacket)' on a null object reference
    at com.tradle.react.c.a(UdpSenderTask.java:3)
    at com.tradle.react.c.doInBackground(UdpSenderTask.java:1)
    at android.os.AsyncTask$2.call(AsyncTask.java:333)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:245)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1162)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:636)
    at java.lang.Thread.run(Thread.java:764)
java.lang.RuntimeException: An error occurred while executing doInBackground()
    at android.os.AsyncTask$3.done(AsyncTask.java:353)
    at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:383)
    at java.util.concurrent.FutureTask.setException(FutureTask.java:252)
    at java.util.concurrent.FutureTask.run(FutureTask.java:271)
    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:245)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1162)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:636)
    at java.lang.Thread.run(Thread.java:764)

This error has occured for multiple users with different devices (all Android). The error always occurs after the activity was paused (e.g. app put to background) and the user returns to the application.

I can not share the exact code but I believe the error occurs if the user suspends the app before a response was received for a UDP broadcast message and then returns to the app.

Current behavior

An error is thrown

Expected behavior

No error is thrown, null reference is handled gracefully (?)

Relevant information

OS	Android 8, 9, 10 and 11 (Nokia 5.4, Nokia 8, Samsung Galaxy A51, Samsung Galaxy S20)
react-native	0.64 and newer
react-native-udp	4.1.3 (only this version I believe)

[gibo77]

How long was the App waiting for a response before the user exited the App?

[haveneersrobin]

Not long, the Sentry logs for the starting of the UDP broadcast and the

{
   screen: MainActivity, 
   state: paused
}

event, have the exact same timestamp... Which makes me wonder: is it possible the app is crashing?

[eliaslecomte]

I also had a look at this and created this pr: #197.

Given that it's a nullpointer exception it looks like either:

• A rogue AsyncTask, which is deprecated
• The close function has been called but there are still ongoing send packets

I've added both a safeguard and replaced the AsyncTask with a better alternative.

[Rapsssito]

Fixed by #197.