I don't think it's a good idea to change any code to work around a 
temporary VM bug.
I have seen this error in other cases as well (sockets) and I decided to 
revert the VM to 3397.
I haven't reported the bug yet, because I don't have a reproducible case. 
The only symptom I saw is that the instance variables get mixed with 
temporaries (including method arguments).

Levente

On Tue, 20 Oct 2015, commits at source.squeak.org wrote:

> Marcel Taeumel uploaded a new version of Graphics to project Squeak 4.6:
> http://source.squeak.org/squeak46/Graphics-mt.316.mcz
>
> ==================== Summary ====================
>
> Name: Graphics-mt.316
> Author: mt
> Time: 20 October 2015, 11:40:19.755 am
> UUID: 41cf59e1-6149-7e4b-9d40-485a1f4798ef
> Ancestors: Graphics-mt.313
>
> Fixes loading of PNG files for recent (> #3397) v3 CogVMs.
>
> There seems to be a bug in the VM that, at some point, manifests itself by replacing z, which is the current ZLib stream, with self, which is the PNGReadWriter. Any indirection with a BlockClosure will do. I chose to create an interval and then calling #do: on that interval. Another way would be to wrap the body/argument of #to:do: into another block like this:
>
> 0 to: height-1 do: [:y | [ ... ] value ].
>
> This fix might affect performance a tiny little bit and might be removed as soon as the VM bug gets fixed.
>
> Note that there might be other places in the code affected by this bug.
>
> =============== Diff against Graphics-mt.313 ===============
>
> Item was changed:
>  ----- Method: PNGReadWriter>>processNonInterlaced (in category 'chunks') -----
>  processNonInterlaced
>  	| z filter temp copyMethod debug |
>  	debug := self debugging.
>  	copyMethod := #(copyPixelsGray: nil copyPixelsRGB: copyPixelsIndexed:
>  		  copyPixelsGrayAlpha: nil copyPixelsRGBA:) at: colorType+1.
>  	debug ifTrue: [ Transcript cr; nextPutAll: 'NI chunk size='; print: idatChunkStream position ].
>  	z := ZLibReadStream
>  		on: idatChunkStream originalContents
>  		from: 1
>  		to: idatChunkStream position.
>  	prevScanline := ByteArray new: bytesPerScanline.
>  	thisScanline := ByteArray new: bytesPerScanline.
> + 	(0 to: height-1) do: [ :y |
> - 	0 to: height-1 do: [ :y |
>  		filter := z next.
>  		debug ifTrue:[filtersSeen add: filter].
>  		thisScanline := z next: bytesPerScanline into: thisScanline startingAt: 1.
>  		(debug and: [ thisScanline size < bytesPerScanline ]) ifTrue: [ Transcript nextPutAll: ('wanted {1} but only got {2}' format: { bytesPerScanline. thisScanline size }); cr ].
>  		filter = 0 ifFalse:[self filterScanline: filter count: bytesPerScanline].
>  		self perform: copyMethod with: y.
>  		temp := prevScanline.
>  		prevScanline := thisScanline.
>  		thisScanline := temp.
>  		].
>  	z atEnd ifFalse:[self error:'Unexpected data'].
>  	debug ifTrue: [Transcript  nextPutAll: ' compressed size='; print: z position  ].
>  !
>
>
>