Concurrency issues

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I have come across two concurrency issues:

E/AndroidRuntime( 2602): FATAL EXCEPTION: GLThread
E/AndroidRuntime( 2602): java.lang.IndexOutOfBoundsException: Invalid index 1, 
size is 0
E/AndroidRuntime( 2602):    at 
java.util.ArrayList.throwIndexOutOfBoundsException(ArrayList.java:257)
E/AndroidRuntime( 2602):    at java.util.ArrayList.get(ArrayList.java:311)
E/AndroidRuntime( 2602):    at 
org.anddev.andengine.entity.Entity.onManagedDraw(Entity.java:898)
E/AndroidRuntime( 2602):    at 
org.anddev.andengine.entity.scene.Scene.onManagedDraw(Scene.java:230)
E/AndroidRuntime( 2602):    at 
org.anddev.andengine.entity.Entity.onDraw(Entity.java:782)
E/AndroidRuntime( 2602):    at 
org.anddev.andengine.engine.Engine.onDrawScene(Engine.java:507)
E/AndroidRuntime( 2602):    at 
org.anddev.andengine.engine.Engine.onDrawFrame(Engine.java:499)
E/AndroidRuntime( 2602):    at 
org.anddev.andengine.opengl.view.RenderSurfaceView$Renderer.onDrawFrame(RenderSu
rfaceView.java:148)
E/AndroidRuntime( 2602):    at 
org.anddev.andengine.opengl.view.GLSurfaceView$GLThread.guardedRun(GLSurfaceView
.java:617)
E/AndroidRuntime( 2602):    at 
org.anddev.andengine.opengl.view.GLSurfaceView$GLThread.run(GLSurfaceView.java:5
49)

E/AndroidRuntime(23153): java.lang.IndexOutOfBoundsException: Invalid index 9, 
size is 1
E/AndroidRuntime(23153):    at 
java.util.ArrayList.throwIndexOutOfBoundsException(ArrayList.java:257)
E/AndroidRuntime(23153):    at java.util.ArrayList.remove(ArrayList.java:406)
E/AndroidRuntime(23153):    at 
org.anddev.andengine.opengl.texture.TextureManager.updateTextures(TextureManager
.java:134)
E/AndroidRuntime(23153):    at 
org.anddev.andengine.engine.Engine.onDrawFrame(Engine.java:493)
E/AndroidRuntime(23153):    at 
org.anddev.andengine.opengl.view.RenderSurfaceView$Renderer.onDrawFrame(RenderSu
rfaceView.java:148)
E/AndroidRuntime(23153):    at 
org.anddev.andengine.opengl.view.GLSurfaceView$GLThread.guardedRun(GLSurfaceView
.java:617)
E/AndroidRuntime(23153):    at 
org.anddev.andengine.opengl.view.GLSurfaceView$GLThread.run(GLSurfaceView.java:5
49)

This happens because detachChildren() and TextureManager.unloadTexture()
are not synchronized with the entity or the engine. So, when these are
called, the GL thread and TextureManager.updateTextures() or onManagedDraw()
can be in the process of looping through the elements... - boom!

When looking through the code, it is evident that there are many similar
concurrency issues in AndEngine that need fixing.

Original issue reported on code.google.com by adam.waldenberg@gmail.com on 17 Jul 2011 at 5:14

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@Adam, how many concurrency issues did you notice in the code? Starting a game 
and deciding which game engine to use - concurrency issues are a bit worrisome.

Original comment by mar...@gmail.com on 5 Sep 2011 at 12:57

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@markww, There are quite a few. Some of them have been fixed in this commit:

http://code.google.com/p/andengine/source/detail?r=8866cde61cf59ce189fa129453d25
ebe0fa438f5

But there are more concurrency issues in there... Some related to the Engine 
class, modifiers and entities in the engine. We have fixed most of these issues 
by extending the Engine class and adding custom locks, but there is more in 
there... 

Some of these are very hard to trigger...
We still have a few random crashes related to this that we have not looked into 
yet.
AndEngine is still a good choice though :).

Original comment by adam.waldenberg@gmail.com on 5 Sep 2011 at 9:20

[GoogleCodeExporter]

Hi Adam,

Thanks for the update, do you know if the other concurrency issues are being 
worked on?

Related - will builds of the project as jars (in the download section) no 
longer be supported? I thought I downloaded precompiled jars from there in 
April, don't see anything in the downloads sections anymore. Would be nice if 
official jar snapshots were made whenever bug fixes were submitted.

Thanks

Original comment by mar...@gmail.com on 5 Sep 2011 at 2:05

[GoogleCodeExporter]

Errors like this are also caused by the way Entity iterates through its 
children.

In particular, it caches the number of children before starting. This means 
that if a child removes itself during its onUpdate function, the cached value 
will now be too high and the iteration will go out of bounds.

In my experience, it is quite common to want to remove a child when a modifier 
finishes. However, the listener's onModifierFinished function is called during 
onUpdate, so doing so causes an exception. The same applies to update handlers.

I can't think of an efficient solution for this, but an inefficient solution is 
to recheck the array size after updating each child. (Actually, I also added 
synchronized blocks in the methods that change the array and try/catch blocks 
in a couple other places, so you might have to do all of those to get the 
benefit.)

Original comment by player3...@gmail.com on 8 Sep 2011 at 7:51

[GoogleCodeExporter]

@markww, No idea :). And as far as I know there are no precompiled snapshots.

@player3.14, We added a lock in a custom Engine class to fix all these issues 
and simply override onDrawFrame in Engine. Then it's just a matter of 
synchronizing against the lock in the right methods in our extended 
Scene/Entity classes.

It has worked so far... But I have a feeling we will run into something sooner 
or later that will force us to modify the AndEngine code ;).

Original comment by adam.waldenberg@gmail.com on 11 Sep 2011 at 10:23