Rare NPE in GlyphLayout

[upost]

I have an Android app in the Play Store.
Rarely I see exceptions like this in the Google Play Console:

Exception java.lang.NullPointerException:
  at com.badlogic.gdx.graphics.g2d.GlyphLayout.setText (GlyphLayout.java:176)
  at com.badlogic.gdx.graphics.g2d.GlyphLayout.setText (GlyphLayout.java:95)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.computePrefSize (Label.java:162)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.scaleAndComputePrefSize (Label.java:147)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.getPrefWidth (Label.java:244)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.<init> (Label.java:75)

or:

(RuntimeException root cause)
Caused by java.lang.NullPointerException:
  at com.badlogic.gdx.graphics.g2d.GlyphLayout.calculateWidths (GlyphLayout.java:280)
  at com.badlogic.gdx.graphics.g2d.GlyphLayout.setText (GlyphLayout.java:266)
  at com.badlogic.gdx.graphics.g2d.GlyphLayout.setText (GlyphLayout.java:95)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.computePrefSize (Label.java:162)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.scaleAndComputePrefSize (Label.java:147)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.getPrefWidth (Label.java:244)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.<init> (Label.java:75)

or, maybe related:

Exception java.lang.IndexOutOfBoundsException:
  at com.badlogic.gdx.utils.Array.get (Array.java:155)
  at com.badlogic.gdx.graphics.g2d.GlyphLayout.setText (GlyphLayout.java:215)
  at com.badlogic.gdx.graphics.g2d.GlyphLayout.setText (GlyphLayout.java:101)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.computePrefSize (Label.java:160)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.scaleAndComputePrefSize (Label.java:147)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.getPrefHeight (Label.java:253)
  at com.badlogic.gdx.scenes.scene2d.ui.Label.layout (Label.java:175)
  at com.badlogic.gdx.scenes.scene2d.ui.Widget.validate (Widget.java:88)
  at com.badlogic.gdx.scenes.scene2d.ui.Stack.layout (Stack.java:107)
  at com.badlogic.gdx.scenes.scene2d.ui.WidgetGroup.validate (WidgetGroup.java:104)
  at com.badlogic.gdx.scenes.scene2d.ui.VerticalGroup.layout (VerticalGroup.java:201)
  at com.badlogic.gdx.scenes.scene2d.ui.WidgetGroup.validate (WidgetGroup.java:104)
  at com.badlogic.gdx.scenes.scene2d.ui.ScrollPane.layout (ScrollPane.java:498)

It happens in different labels, but only in labels, and I'm not able to reproduce it.

libgdx version: 1.12.1

Any idea?

[Tom-Ski]

Most likely this is related to threading. Are you doing any multithreading/http requests and mutating state/labels without making sure those mutations are running on main thread?

[upost]

Yes there is stuff happening in the background but I am running UI related code only on the main thread because I know that libgdx is not threadsafe.
At least I hope so. I spent some time to find situations where this could go wrong but found nothing.
I also looked if there's a workaround because this seems to be related to GlyphLayout and nothing else, maybe overriding/implementing an own class which just catches the exception, because it is so rare. It just should not crash the app - if a label is empty once in a lifetime, that'd be acceptable.

[Tom-Ski]

Yeah, it definitely sounds like a multithreading issue. It can be super rare.

I would verify you aren't mutating label states from another thread, you can do this even without getting a crash.
This is a handy util I use when making sure methods are being called correctly.

package com.rockbite.engine.threadutils;

import com.badlogic.gdx.utils.GdxRuntimeException;
import lombok.Getter;
import lombok.Setter;

public class ThreadUtils {
	@Getter
	@Setter
	private static Thread gdxThread;

	public static void gdxThreadSafetyCheck () {
		if (!isOnRightThread()) {
			throw new GdxRuntimeException("Trying to use on non-gdx thread");
		}
	}

	public static void gdxThreadSafetyCheck (Thread accessThread) {
		if (Thread.currentThread() != accessThread) {
			throw new GdxRuntimeException("Trying to use on non-gdx thread");
		}
	}

	public static boolean isOnRightThread () {
		return Thread.currentThread() == gdxThread;
	}
}

You can just ThreadUtils.setGdxThread(Thread.currentThread()), in your create method of application listener. Then use gdxThreadSafetyCheck() anywhere you suspect it may be happening.

You can do this in conditional breakpoints on libgdx methods too with your debugger if you have lots of different access points.

[upost]

Looks interesting, I will certainly look into it, thank you.