[Vm-dev] Weird rehashing problem

Max Leske maxleske at gmail.com
Fri Oct 24 14:29:43 UTC 2014


I have a rather strange problem with rehasing that occurred simultaneously multiple times on the same day but not since. Luckily I’ve saved an image with the debugger in it. The mean thing is: that doesn’t help me at all because from what I can tell either the Debugger is broken or there’s a VM problem.

The image is a Pharo 1.3 image. The problem occurred on two different VMs independently, an NBCog VM (Croquet Closure Cog VM [CoInterpreter VMMaker-oscog-EstebanLorenzano.166] 21.0) and a Squeak VM (Squeak3.10.2 of '5 June 2008' [latest update: #7179]).
The operation during which this happened was the loading of an ImageSegment (in both instances).

Here’s what happened. I got a SubscriptOutOfBounds error during rehashing of a Dictionary. When I look at the debugger it’s clear why the exception got signaled but not how that situation could ever happen.

These are the top three contexts on the stack:


This is the #noCheckAdd: method”

noCheckAdd: anObject
	"Must be defined separately for Dictionary because (self findElementOrNil:) expects a key, not an association.  9/7/96 tk"

	array at: (self findElementOrNil: anObject key) put: anObject.
	tally := tally + 1

and here’s #at:put: where the exception is signaled:

at: index put: value 
	<primitive: 61>
	index isInteger ifTrue:
		[self class isVariable
			ifTrue: [(index >= 1 and: [index <= self size])
					ifTrue: [self errorImproperStore]
					ifFalse: [self errorSubscriptBounds: index]] <——— signaled
			ifFalse: [self errorNotIndexable]].
	index isNumber
		ifTrue: [^self at: index asInteger put: value]
		ifFalse: [self errorNonIntegerIndex]

Here’s the weird thing: according to the debugger ‘array' in #noCheckAdd: IS NOT THE SAME INSTANCE as ‘self’ in #at:put:!!!!!! ????????? When I look at the pointers to both arrays, the correct one is pointed to by the dictionary while the other is only being referenced by the exception context:

Bad array: 
1: SubscriptOutOfBounds: 88
2: Array(Object)>>errorSubscriptBounds:
3: Array(Object>>at:put:
4: an Inspector

Good array:
1: aCBConfiguration(size 107)

I really have no clue how this could have happened. I’d appreciate any thoughts you might have…


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