[squeak-dev] Code simulation error (was Re: I broke the debugger?)

tim Rowledge tim at rowledge.org
Tue Jan 28 02:08:42 UTC 2020

Something pretty weird is happening when the break is hit. I *finally* got a debugger open on a backtrace that includes the problem with Context>at: failing becasue the argument is 0. It's all a bit strange and unless I managed to do something very odd it looks like a fairly serious bug.

I actually caught this because something went wrong in code I added to try to log the dan initial error. Despite that it does appear to be a trace on the #break problem.

debugger>doStep called from #stepOver.
#handleLabelUpdatesIn:whenExecuting: used and does [interruptedProcess completeStep: currentContext] which uses...

 - *we are checking for stackp = 0 which is the very thing that causes problems later with the #pop*

In the #stepToSendOrReturn we use interpretNextInstructionFor: which leads to InterpretV3ClosuresExtension: 7 in: (Object>>break) for: ( aContext sender #on:do:, pc 24 stackp 0 method Object>>break, etc)
 -> doPop
 -> pop  (presumably stackp was 0 here? See above re: #jump)
 -> at: ... but if so why did the error code appear to skip over the first two tests of it?
	<primitive: 210>
	index = 0 ifTrue:[FileStream newFileNamed: 'squeakBreak.log' do:[:f| self errorReportOn: f]].
	index isInteger ifTrue:
		[self errorSubscriptBounds: index].
	index isNumber
		ifTrue: [^self at: index asInteger]"<--- it went here and on the second go around it picked up that index = 0 properly."
		ifFalse: [self errorNonIntegerIndex]

So I *think* that there is an issue in Context>jump where we explicitly check for stackp = 0 but then call code that carefully does a pop via #at:. Something about the primitive: 210 (maybe?) does something weird and the index is both 0 and not 0 - nor even an Integer.

As an interesting bonus, the clause I added to log things when 'index = 0' went very wrong because the 'f' getting passed to the block is apparently the MultiByteFileStream *class* rather than the opened file!

The bit bothering me at the moment is just how this can be a problem that hasn't whacked us before. I haven't been able to cause it with 'normal' code but all I was doing to fall over this was loading & testing the old Plumbing demo code.

Oh, I did just try to see if a newer vm (I am running the 201912311458 ARMv6) would have a fix for the prim 210 but no newer ARM VM will run at all. The latest Mac vm runs but fails with the same huge list of notifiers reporting the error in Context>at: - the fact that there is a *lot* of #at: on the stack is a little more odd.

tim Rowledge; tim at rowledge.org; http://www.rowledge.org/tim
Useful random insult:- If you stand close enough to him, you can hear the ocean

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