Hi Jaromir,

thanks for the clarification. If you don't mind I would still wait for a couple of days to see whether Eliot or Marcel or someone else who are longer aboard find anything against your change, but I have been convinced by you. :-) After that, we can merge your open test and eliminate the ifNil checks in the TraceDebugger and also in the penultimate line of Context>>#return:from:.

> In general, some methods are "pure simulation" methods intended to mimic the VM behavior (#step etc) but nothing prevents one from using them for other purposes - like what #runUntilErrorOrReturnFrom: did: to just get rid of some contexts). Is it ok to do that? I tend to think it's not; it's confusing. That's why I made #stepToCalleeOrNil a private method because it's not a "true" simulation method but a sort of hybrid.

Yes, I think I understand your point here, the assumptions and use cases for #stepToCalleeOrNil are too special to expose it to everyone. Clients should mainly use #step, #stepToCallee, or maybe - with care - #runUntilErrorOrReturnFrom: to advance a context I think.

> This is also why I checked Jakob's Git Browser and all tests seem fine.

Wait, Squot is using code simulation?

Best,
Christoph

---
Sent from Squeak Inbox Talk

On 2024-01-02T11:25:57+00:00, mail@jaromir.net wrote:

> Hi Christoph,
>
> correct me if I'm wrong: with the corrected #step semantics (the
> #return:from: fix) one should no longer need to do things like:
>
> self step ifNil: [^ self]
>
> because #step should always return a context, even if attempting to step
> to a nil context:
>
> [] asContext step
>
> In this case it correctly returns the #cannotReturn: context.
>
> I've noticed these nil checks in Trace debugger's #doStepOver or
> #stepToHome.
>
> I hope I haven't overlooked anything :)
>
> Thanks for your thoughts,
> Jaromir
>
>
> On 31-Dec-23 10:42:54 AM, "Jaromir Matas" <mail(a)jaromir.net> wrote:
>
> >Hi Christoph,
> >
> >Yes, that's exactly what I was talking about - the #return:from: fix
> >changes slightly (perhaps it's better to say corrects) the semantics of
> >some stepping methods - #step and #stepToCallee, which allowed
> >illegally stepping into a dead or nil context. They can no longer be
> >used in the manner you showed. Another example was
> >#runUntilErrorOrReturnFrom: - it used #stepToCallee this way so as a
> >workaround I created the #stepToCalleeOrNil method and used in
> >#runUntilErrorOrReturnFrom:
> >
> > [ctxt isDead or: [topContext isNil]] whileFalse: [topContext :=
> >topContext stepToCalleeOrNil].
> >
> >Theoretically there might be some external code (mis)using the
> >incorrect stepping semantics. In Pharo's trunk they were mainly tests
> >and #stepToHome but I haven't checked any external code. But all their
> >tests are green with this change and I guess it's not widespread.
> >
> >This is also why I checked Jakob's Git Browser and all tests seem fine.
> >
> >My opinion is to keep the correct simulation semantics and deal with
> >potential consequences as/if they come. However I don't expect a huge
> >impact as the change only affects border situations.
> >
> >In general, some methods are "pure simulation" methods intended to
> >mimic the VM behavior (#step etc) but nothing prevents one from using
> >them for other purposes - like what #runUntilErrorOrReturnFrom: did: to
> >just get rid of some contexts). Is it ok to do that? I tend to think
> >it's not; it's confusing. That's why I made #stepToCalleeOrNil a
> >private method because it's not a "true" simulation method but a sort
> >of hybrid.
> >
> >What do you think?
> >
> >Thanks for reviewing the fix!
> >Best,
> >Jaromir
> >
> >
> >
> >On 30-Dec-23 11:07:54 PM, christoph.thiede(a)student.hpi.uni-potsdam.de
> >wrote:
> >
> >>Hi Jaromir,
> >>
> >>I found a breaking change in the new behavior of
> >>Context>>#return:from: while using the TraceDebugger:
> >>
> >>In the past we could say:
> >>
> >>c:=[2+3]asContext.
> >>[c]whileNotNil:[c:=cstep].
> >>
> >>With your change, the script runs forever because the last step does
> >>not answer nil as before but activates a new #cannotReturn:.
> >>
> >>This behavior seems not be expected anywhere in the trunk (if my first
> >>search was complete), and you are right that the new behavior aligns
> >>closer to the VM behavior. Still, the old code seemed to explicitly
> >>intend this - see the "newTop ifNotNil:" at the bottom of the method.
> >>
> >>I wonder whether we should keep this. For me it is not a big deal; I
> >>can just change my script like this:
> >>
> >>c:=[2+3]asContext.
> >>[csenderisNiland:[cwillReturn]]whileNotNil:[c:=cstep].
> >>
> >>I just wonder whether this could a breaking or unintended change for
> >>anything else. For [^2] ensure: [] it would not be a big deal, we
> >>could just change the check in question to
> >>(aSenderisDeador:[newTopnotNiland:[newTopisDead]])ifTrue:. I am
> >>tending against restoring the old behavior, but I am unsure. What is
> >>your opinion on this?
> >>
> >>Best,
> >>Christoph
> >>
> >>---
> >>Sent from Squeak Inbox Talk
> >><https://github.com/hpi-swa-lab/squeak-inbox-talk>
> >>
> >>On 2023-12-30T21:13:37+00:00, mail(a)jaromir.net wrote:
> >>
> >> > > nit: You mixed up the order of arguments for #assert:equals:
> >> >
> >> > oops, sorry :) It happens to me all the time; I've never actually
> >> > understood why the strange, almost Yodaesque, order... as if you
> >>asked
> >> > in English:
> >> >
> >> > "Make sure 18 is his age."
> >> >
> >> > Thanks,
> >> > Jaromir
> >> >
> >> >
> >> > On 30-Dec-23 9:13:56 PM,
> >>christoph.thiede(a)student.hpi.uni-potsdam.de
> >> > wrote:
> >> >
> >> > >nit: You mixed up the order of arguments for #assert:equals: (it is
> >> > >assert: expected equals: actual) and could have used it in the
> >>final
> >> > >assert again, but that's clearly no reason to hold back a useful
> >>test.
> >> > >;-) Merged, thanks! :-)
> >> > >
> >> > >Best,
> >> > >Christoph
> >> > >
> >> > >---
> >> > >Sent from Squeak Inbox Talk
> >> > ><https://github.com/hpi-swa-lab/squeak-inbox-talk>
> >> > >
> >> > >On 2023-12-30T17:33:08+00:00, mail(a)jaromir.net wrote:
> >> > >
> >> > > > Hi Christoph,
> >> > > >
> >> > > > Thanks for merging the fixes; I've just sent another test in
> >> > > > KernelTests-jar.448 to complement them.
> >> > > >
> >> > > > Please take a look and if ok I'd appreciate it if you could
> >>merge it
> >> > >as
> >> > > > well.
> >> > > >
> >> > > > Best regards and Happy New Year to you too!
> >> > > > Jaromir
> >> > > >
> >> > > >
> >> > > > On 30-Dec-23 6:15:25 PM,
> >> > >christoph.thiede(a)student.hpi.uni-potsdam.de
> >> > > > wrote:
> >> > > >
> >> > > > >Hi Jaromir, hi all,
> >> > > > >
> >> > > > >finally I have found the time to review these suggestions.
> >> > > > >Kernel-jar.1537, Kernel-jar.1538, and Kernel-jar.1539 look
> >>excellent
> >> > >to
> >> > > > >me as well. Clear, straightforward, useful. :-) I have merged
> >>them
> >> > >into
> >> > > > >the trunk via Kernel-ct.1545.
> >> > > > >
> >> > > > >Regarding DebuggerTests>>test16HandleSimulationError, I have
> >>patched
> >> > >it
> >> > > > >via ToolsTests-ct.125. Nothing to rack your brains over:
> >> > >"thisContext
> >> > > > >pc: nil" just mimicks any kind of unhandled error inside the
> >> > >simulator
> >> > > > >- since we now gently handle this via #cannotReturn:, I just
> >> > >replaced
> >> > > > >it with "thisContext pc: false". :-) Sorry for not clarifying
> >>that
> >> > > > >earlier and letting you speculate.
> >> > > > >
> >> > > > >Thanks for your work, and I already wish you a happy new year!
> >> > > > >
> >> > > > >Best,
> >> > > > >Christoph
> >> > > > >
> >> > > > >---
> >> > > > >Sent from Squeak Inbox Talk
> >> > > > ><https://github.com/hpi-swa-lab/squeak-inbox-talk>
> >> > > > >
> >> > > > >On 2023-11-29T13:31:09+00:00, mail(a)jaromir.net wrote:
> >> > > > >
> >> > > > > > Hi Marcel,
> >> > > > > >
> >> > > > > > > [myself] whether the patch would have been necessary
> >>should the
> >> > > > > > #return:from: had been fixed then
> >> > > > > >
> >> > > > > > Nonsense, I just mixed it up with another issue :)
> >> > > > > >
> >> > > > > >
> >> > > > > > On 29-Nov-23 1:51:21 PM, "Jaromir Matas"
> >><mail(a)jaromir.net>
> >> > >wrote:
> >> > > > > >
> >> > > > > > >Thanks Marcel! This test somehow slipped my attention :)
> >> > > > > > >
> >> > > > > > >The test can no longer work as is. It takes advantage of
> >>the
> >> > > > >erroneous
> >> > > > > > >behavior of #return:from: in the sense that if you simulate
> >> > > > > > >
> >> > > > > > > thisContext pc: nil
> >> > > > > > >
> >> > > > > > >it'll happily return to a dead context (i.e. to thisContext
> >>from
> >> > > > >#pc:
> >> > > > > > >nil context) - which is not what the VM does during
> >>runtime. It
> >> > > > >should
> >> > > > > > >immediately raise an illegal return exception not only
> >>during
> >> > > > >runtime
> >> > > > > > >but also during simulation.
> >> > > > > > >
> >> > > > > > >The test mentions a patch for an infinite debugger chain
> >> > > > > >
> >> >(http://forum.world.st/I-broke-the-debugger-td5110752.html). I
> >> > > > >wonder
> >> > > > > > >whether the problem could have something to do with this
> >> > >simulation
> >> > > > >bug
> >> > > > > > >in return:from:; and a terrible idea occurred to me whether
> >>the
> >> > > > >patch
> >> > > > > > >would have been necessary should the #return:from: had been
> >> > >fixed
> >> > > > >then
> >> > > > > > >;O
> >> > > > > > >
> >> > > > > > >We may potentially come up with more examples like this,
> >>even in
> >> > >the
> >> > > > > > >trunk, where the bug from #return:from: propagated and was
> >>taken
> >> > > > > > >advantage of. I've found and fixed
> >>#runUntilErrorOrReturnFrom:
> >> > >but
> >> > > > >more
> >> > > > > > >can still be surviving undetected...
> >> > > > > > >
> >> > > > > > >I'd place the test into #expectedFailures for now but maybe
> >>it's
> >> > > > >time
> >> > > > > > >to remove it; Christoph should decide :)
> >> > > > > > >
> >> > > > > > >Thanks again,
> >> > > > > > >Jaromir
> >> > > > > > >
> >> > > > > > >
> >> > > > > > >On 29-Nov-23 10:28:38 AM, "Taeumel, Marcel via Squeak-dev"
> >> > > > > > ><squeak-dev(a)lists.squeakfoundation.org> wrote:
> >> > > > > > >
> >> > > > > > >>Hi Jaromir --
> >> > > > > > >>
> >> > > > > > >>Looks good. Still, what about that
> >>#test16HandleSimulationError
> >> > > > >now?
> >> > > > > > >>:-) It is failing with your changes ... how would you
> >>adapt it?
> >> > > > > > >>
> >> > > > > > >>
> >> > > > > > >>
> >> > > > > > >>Best,
> >> > > > > > >>Marcel
> >> > > > > > >>>Am 28.11.2023 01:29:39 schrieb Jaromir Matas
> >> > ><mail(a)jaromir.net>:
> >> > > > > > >>>
> >> > > > > > >>>Hi Eliot, Marcel, all,
> >> > > > > > >>>
> >> > > > > > >>>I've sent a fix Kernel-jar.1539 to the Inbox that solves
> >>the
> >> > > > > > >>>remaining bit of the chain of bugs described in the
> >>previous
> >> > >post.
> >> > > > > > >>>All tests are green now and I think the root cause has
> >>been
> >> > >found
> >> > > > >and
> >> > > > > > >>>fixed.
> >> > > > > > >>>
> >> > > > > > >>>In this last bit I've created a version of stepToCallee
> >>that
> >> > >would
> >> > > > > > >>>identify a potential illegal return to a nil sender and
> >>avoid
> >> > >it.
> >> > > > > > >>>
> >> > > > > > >>>Now this example can be debugged without any problems:
> >> > > > > > >>>
> >> > > > > > >>>[[self halt. ^ 1] on: BlockCannotReturn do: #resume ]
> >>fork
> >> > > > > > >>>
> >> > > > > > >>>If you're happy with the solution in Kernel-jar.1539,
> >> > > > > > >>>Kernel-jar.1538, Kernel-jar.1537 and the test in
> >> > > > >KernelTests-jar.447,
> >> > > > > > >>>could you please double-check and merge, please? (And
> >>remove
> >> > > > > > >>>Kernel-mt.1534 and Tools-jar.1240 from the Inbox)
> >> > > > > > >>>
> >> > > > > > >>>Best,
> >> > > > > > >>>Jaromir
> >> > > > > > >>>
> >> > > > > > >>>
> >> > > > > > >>>
> >> > > > > > >>>On 27-Nov-23 12:09:37 AM, "Jaromir Matas"
> >><mail(a)jaromir.net>
> >> > > > >wrote:
> >> > > > > > >>>
> >> > > > > > >>>>Hi Eliot, Christoph, all
> >> > > > > > >>>>
> >> > > > > > >>>>It looks like there are some more skeletons in the
> >>closet :/
> >> > > > > > >>>>
> >> > > > > > >>>>If you run this example
> >> > > > > > >>>>
> >> > > > > > >>>>[[self halt. ^ 1] on: BlockCannotReturn do: [:ex | ex
> >>resume]
> >> > >]
> >> > > > >fork
> >> > > > > > >>>>
> >> > > > > > >>>>and step over halt and then step over ^1 you get a
> >> > >nonsensical
> >> > > > >error
> >> > > > > > >>>>as a result of decoding nil as an instruction.
> >> > > > > > >>>>
> >> > > > > > >>>>It turns out that the root cause is in the #return:from:
> >> > >method:
> >> > > > >it
> >> > > > > > >>>>only checks whether aSender is dead but ignores the
> >> > >possibility
> >> > > > >that
> >> > > > > > >>>>aSender sender may be nil or dead in which cases the VM
> >>also
> >> > > > > > >>>>responds with sending #cannotReturn, hence I assume the
> >> > >simulator
> >> > > > > > >>>>should do the same. In addition, the VM nills the pc in
> >>such
> >> > > > > > >>>>scenario, so I added the same functionality here too:
> >> > > > > > >>>>
> >> > > > > > >>>>Context >> return: value from: aSender
> >> > > > > > >>>> "For simulation. Roll back self to aSender and return
> >>value
> >> > > > > > >>>>from it. Execute any unwind blocks on the way. ASSUMES
> >> > >aSender is
> >> > > > > > >>>>a sender of self"
> >> > > > > > >>>>
> >> > > > > > >>>> | newTop |
> >> > > > > > >>>> newTop := aSender sender.
> >> > > > > > >>>> (aSender isDead or: [newTop isNil or: [newTop isDead]])
> >> > >ifTrue:
> >> > > > > > >>>> "<--------- this is extended ------"
> >> > > > > > >>>> [^self pc: nil; send: #cannotReturn: to: self with:
> >> > > > > > >>>>{value}]. "<------ pc: nil is added ----"
> >> > > > > > >>>> (self findNextUnwindContextUpTo: newTop) ifNotNil:
> >> > > > > > >>>> "Send #aboutToReturn:through: with nil as the second
> >> > > > > > >>>>argument to avoid this bug:
> >> > > > > > >>>> Cannot #stepOver '^2' in example '[^2] ensure: []'.
> >> > > > > > >>>> See
> >> > > > > >
> >> > > > >
> >> > >
> >> >>>>http://lists.squeakfoundation.org/pipermail/squeak-dev/2022-June/220975.html
> >> > > > > >
> >> > > > >
> >> > >
> >> >>>><http://lists.squeakfoundation.org/pipermail/squeak-dev/2022-June/220975.html>"
> >> > > > > > >>>> [^self send: #aboutToReturn:through: to: self with:
> >>{value.
> >> > > > > > >>>>nil}].
> >> > > > > > >>>> self releaseTo: newTop.
> >> > > > > > >>>> newTop ifNotNil: [newTop push: value].
> >> > > > > > >>>> ^newTop
> >> > > > > > >>>>
> >> > > > > > >>>>In order for this to work #cannotReturn: has to be
> >>modified
> >> > >as in
> >> > > > > > >>>>Kernel-jar.1537:
> >> > > > > > >>>>
> >> > > > > > >>>>Context >> cannotReturn: result
> >> > > > > > >>>>
> >> > > > > > >>>> closureOrNil ifNotNil: [^ self cannotReturn: result to:
> >>self
> >> > > > > > >>>>home sender].
> >> > > > > > >>>> self error: 'Computation has been terminated!'
> >> > > > > > >>>>"<----------- this has to be an Error -----"
> >> > > > > > >>>>
> >> > > > > > >>>>Then it almost works except when you keep stepping over
> >>in
> >> > >the
> >> > > > > > >>>>example above, you get an MNU error on `self previousPc`
> >>in
> >> > > > > > >>>>#cannotReturn:to:` with your solution of the VM crash.
> >>If you
> >> > > > >don't
> >> > > > > > >>>>mind I've amended your solution and added the final
> >>context
> >> > >where
> >> > > > > > >>>>the computation couldn't return along with the pc:
> >> > > > > > >>>>
> >> > > > > > >>>>Context >> cannotReturn: result to: homeContext
> >> > > > > > >>>> "The receiver tried to return result to homeContext
> >>that
> >> > >cannot
> >> > > > > > >>>>be returned from.
> >> > > > > > >>>> Capture the return context/pc in a BlockCannotReturn.
> >>Nil
> >> > >the pc
> >> > > > > > >>>>to prevent repeat
> >> > > > > > >>>> attempts and/or invalid continuation. Answer the result
> >>of
> >> > > > > > >>>>raising the exception."
> >> > > > > > >>>>
> >> > > > > > >>>> | exception previousPc |
> >> > > > > > >>>> exception := BlockCannotReturn new.
> >> > > > > > >>>> previousPc := pc ifNotNil: [self previousPc]. "<-----
> >>here's
> >> > >a
> >> > > > > > >>>>fix ----"
> >> > > > > > >>>> exception
> >> > > > > > >>>> result: result;
> >> > > > > > >>>> deadHome: homeContext;
> >> > > > > > >>>> finalContext: self; "<----- here's the new state, if
> >> > > > > > >>>>that's fine ----"
> >> > > > > > >>>> pc: previousPc.
> >> > > > > > >>>> pc := nil.
> >> > > > > > >>>> ^exception signal
> >> > > > > > >>>>
> >> > > > > > >>>>Unfortunately, this is still not the end of the story:
> >>there
> >> > >are
> >> > > > > > >>>>situations where #runUntilErrorOrReturnFrom: places the
> >>two
> >> > >guard
> >> > > > > > >>>>contexts below the bottom context. And that is a problem
> >> > >because
> >> > > > > > >>>>when the method tries to remove the two guard contexts
> >>before
> >> > > > > > >>>>returning at the end it uses #stepToCalee to do the job
> >>but
> >> > >this
> >> > > > > > >>>>unforotunately was (ab)using the above bug in
> >>#return:from: -
> >> > > > >I'll
> >> > > > > > >>>>explain: #return:from: didn't check whether aSender
> >>sender
> >> > >was
> >> > > > >nil
> >> > > > > > >>>>and as a result it allowed to simulate a return to a
> >>"nil
> >> > > > >context"
> >> > > > > > >>>>which was then (ab)used in the clean-up via #stepToCalee
> >>in
> >> > >the
> >> > > > > > >>>>#runUntilErrorOrReturnFrom:.
> >> > > > > > >>>>
> >> > > > > > >>>>When I fixed the #return:from: bug, the
> >> > > > >#runUntilErrorOrReturnFrom:
> >> > > > > > >>>>cleanup of the guard contexts no longer works in that
> >>very
> >> > > > >special
> >> > > > > > >>>>case where the guard contexts are below the bottom
> >>context.
> >> > > > >There's
> >> > > > > > >>>>one case where this is being used: #terminateAggresively
> >>by
> >> > > > > > >>>>Christoph.
> >> > > > > > >>>>
> >> > > > > > >>>>If I'm right with this analysis, the
> >> > >#runUntilErrorOrReturnFrom:
> >> > > > > > >>>>should get fixed too but I'll be away now for a few days
> >>and
> >> > >I
> >> > > > >won't
> >> > > > > > >>>>be able to respond. If you or Christoph had a chance to
> >>take
> >> > >a
> >> > > > >look
> >> > > > > > >>>>at Kernel-jar.1538 and Kernel-jar.1537 I'd be very
> >>grateful.
> >> > >I
> >> > > > >hope
> >> > > > > > >>>>this super long message at least makes some sense :)
> >> > > > > > >>>>Best,
> >> > > > > > >>>>Jaromir
> >> > > > > > >>>>
> >> > > > > > >>>>[1] Kernel-jar.1538, Kernel-jar.1537
> >> > > > > > >>>>[2] KernelTests-jar.447
> >> > > > > > >>>>
> >> > > > > > >>>>
> >> > > > > > >>>>PS: Christoph,
> >> > > > > > >>>>
> >> > > > > > >>>>With Kernel-jar.1538 + Kernel-jar.1537 your example
> >> > > > > > >>>>
> >> > > > > > >>>>process :=
> >> > > > > > >>>> [(c := thisContext) pc: nil.
> >> > > > > > >>>> 2+3] newProcess.
> >> > > > > > >>>>process runUntil: [:ctx | ctx selector =
> >>#cannotReturn:].
> >> > > > > > >>>>self assert: process suspendedContext sender sender = c.
> >> > > > > > >>>>self assert: process suspendedContext arguments = {c}.
> >> > > > > > >>>>
> >> > > > > > >>>>works fine, I've just corrected your first assert.
> >> > > > > > >>>>
> >> > > > > > >>>>
> >> > > > > > >>>>
> >> > > > > > >>>>
> >> > > > > > >>>>
> >> > > > > > >>>>On 21-Nov-23 6:40:32 PM, "Eliot Miranda"
> >> > > > ><eliot.miranda(a)gmail.com>
> >> > > > > > >>>>wrote:
> >> > > > > > >>>>
> >> > > > > > >>>>>Hi Jaromir,
> >> > > > > > >>>>>
> >> > > > > > >>>>>>On Nov 20, 2023, at 11:51 PM, Jaromir Matas
> >> > > > ><mail(a)jaromir.net>
> >> > > > > > >>>>>>wrote:
> >> > > > > > >>>>>>
> >> > > > > > >>>>>>
> >> > > > > > >>>>>>Hi Eliot,
> >> > > > > > >>>>>>Very elegant! Now I finally got what you meant exactly
> >>:)
> >> > > > >Thanks.
> >> > > > > > >>>>>>
> >> > > > > > >>>>>>Two questions:
> >> > > > > > >>>>>>1. in order for the enclosed test to work I'd need an
> >>Error
> >> > > > > > >>>>>>instead of Processor debugWithTitle:full: call in
> >> > > > >#cannotReturn:.
> >> > > > > > >>>>>>Otherwise I don't know how to catch a plain invocation
> >>of
> >> > >the
> >> > > > > > >>>>>>Debugger:
> >> > > > > > >>>>>>
> >> > > > > > >>>>>>cannotReturn: result
> >> > > > > > >>>>>>
> >> > > > > > >>>>>> closureOrNil ifNotNil: [^ self cannotReturn: result
> >>to:
> >> > >self
> >> > > > > > >>>>>>home sender].
> >> > > > > > >>>>>> self error: 'Computation has been terminated!'
> >> > > > > > >>>>>
> >> > > > > > >>>>>Much nicer.
> >> > > > > > >>>>>
> >> > > > > > >>>>>>2. We are capturing a pc of self which is completely
> >> > >different
> >> > > > > > >>>>>>context from homeContext indeed.
> >> > > > > > >>>>>
> >> > > > > > >>>>>Right. The return is attempted from a specific return
> >> > >bytecode
> >> > > > >in a
> >> > > > > > >>>>>specific block. This is the coordinate of the return
> >>that
> >> > >cannot
> >> > > > >be
> >> > > > > > >>>>>made. This is the relevant point of origin of the
> >>cannot
> >> > >return
> >> > > > > > >>>>>exception.
> >> > > > > > >>>>>
> >> > > > > > >>>>>Why the return fails is another matter:
> >> > > > > > >>>>>- the home context’s sender is a dead context (cannot
> >>be
> >> > > > >resumed)
> >> > > > > > >>>>>- the home context’s sender is nil (home already
> >>returned
> >> > >from)
> >> > > > > > >>>>>- the block activation’s home is nil rather than a
> >>context
> >> > > > >(should
> >> > > > > > >>>>>not happen)
> >> > > > > > >>>>>
> >> > > > > > >>>>>But in all these cases the pc of the home context is
> >> > >immaterial.
> >> > > > > > >>>>>The hike is being returned through/from, rather than
> >>from;
> >> > >the
> >> > > > > > >>>>>home’s pc is not relevant.
> >> > > > > > >>>>>
> >> > > > > > >>>>>>Maybe we could capture self in the exception too to
> >>make it
> >> > > > >more
> >> > > > > > >>>>>>clear/explicit what is going on: what context the
> >>captured
> >> > >pc
> >> > > > >is
> >> > > > > > >>>>>>actually associated with. Just a thought...
> >> > > > > > >>>>>
> >> > > > > > >>>>>Yes, I like that. I also like the idea of somehow
> >>passing
> >> > >the
> >> > > > > > >>>>>block activation’s pc to the debugger so that the
> >>relevant
> >> > > > >return
> >> > > > > > >>>>>expression is highlighted in the debugger.
> >> > > > > > >>>>>
> >> > > > > > >>>>>>
> >> > > > > > >>>>>>Thanks again,
> >> > > > > > >>>>>>Jaromir
> >> > > > > > >>>>>
> >> > > > > > >>>>>You’re welcome. I love working in this part of the
> >>system.
> >> > > > >Thanks
> >> > > > > > >>>>>for dragging me there. I’m in a slump right now and
> >> > >appreciate
> >> > > > >the
> >> > > > > > >>>>>fellowship.
> >> > > > > > >>>>>
> >> > > > > > >>>>>>------ Original Message ------
> >> > > > > > >>>>>>From "Eliot Miranda" <eliot.miranda(a)gmail.com>
> >> > > > > > >>>>>>To "Jaromir Matas" <mail(a)jaromir.net>
> >> > > > > > >>>>>>Cc squeak-dev(a)lists.squeakfoundation.org
> >> > > > > > >>>>>>Date 11/21/2023 2:17:21 AM
> >> > > > > > >>>>>>Subject Re: Re[2]: [squeak-dev] Re: Resuming on
> >> > > > >BlockCannotReturn
> >> > > > > > >>>>>>exception
> >> > > > > > >>>>>>
> >> > > > > > >>>>>>>Hi Jaromir,
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>> see Kernel-eem.1535 for what I was suggesting. This
> >> > >example
> >> > > > > > >>>>>>>now has an exception with the right pc value in it:
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>[[^1] on: BlockCannotReturn do: [:ex| ex pc inspect.
> >>ex
> >> > > > >resume]]
> >> > > > > > >>>>>>>fork
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>The fix is simply
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>Context>>cannotReturn: result to: homeContext
> >> > > > > > >>>>>>> "The receiver tried to return result to homeContext
> >>that
> >> > > > > > >>>>>>>cannot be returned from.
> >> > > > > > >>>>>>> Capture the return pc in a BlockCannotReturn. Nil
> >>the pc
> >> > >to
> >> > > > > > >>>>>>>prevent repeat
> >> > > > > > >>>>>>> attempts and/or invalid continuation. Answer the
> >>result
> >> > >of
> >> > > > > > >>>>>>>raising the exception."
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>> | exception |
> >> > > > > > >>>>>>> exception := BlockCannotReturn new.
> >> > > > > > >>>>>>> exception
> >> > > > > > >>>>>>> result: result;
> >> > > > > > >>>>>>> deadHome: homeContext;
> >> > > > > > >>>>>>> pc: self previousPc.
> >> > > > > > >>>>>>> pc := nil.
> >> > > > > > >>>>>>> ^exception signal
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>The VM crash is now avoided. The debugger displays
> >>the
> >> > >method,
> >> > > > > > >>>>>>>but does not highlight the offending pc, which is no
> >>big
> >> > >deal.
> >> > > > >A
> >> > > > > > >>>>>>>suitable defaultHandler for B lockCannotReturn may be
> >>able
> >> > >to
> >> > > > >get
> >> > > > > > >>>>>>>the debugger to highlight correctly on opening. Try
> >>the
> >> > > > > > >>>>>>>following examples:
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>[[^1] on: BlockCannotReturn do: #resume] fork.
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>[[^1] on: BlockCannotReturn do: [:ex| ex pc inspect.
> >>ex
> >> > > > >resume]]
> >> > > > > > >>>>>>>fork
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>[[^1] value] fork.
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>They al; seem to behave perfectly acceptably to me.
> >>Does
> >> > >this
> >> > > > > > >>>>>>>fix work for you?
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>On Fri, Nov 17, 2023 at 3:14 PM Jaromir Matas
> >> > > > ><mail(a)jaromir.net>
> >> > > > > > >>>>>>>wrote:
> >> > > > > > >>>>>>>>Hi Eliot,
> >> > > > > > >>>>>>>>
> >> > > > > > >>>>>>>>How about to nil the pc just before making the
> >>return:
> >> > > > > > >>>>>>>>```
> >> > > > > > >>>>>>>>Context >> #cannotReturn: result
> >> > > > > > >>>>>>>>
> >> > > > > > >>>>>>>> self push: self pc. "backup the pc for the sake of
> >> > > > > > >>>>>>>>debugging"
> >> > > > > > >>>>>>>> closureOrNil ifNotNil: [^self cannotReturn: result
> >>to:
> >> > >self
> >> > > > > > >>>>>>>>home sender; pc: nil].
> >> > > > > > >>>>>>>> Processor debugWithTitle: 'Computation has been
> >> > >terminated!'
> >> > > > > > >>>>>>>>translated full: false
> >> > > > > > >>>>>>>>```
> >> > > > > > >>>>>>>>The nilled pc should not even potentially interfere
> >>with
> >> > >the
> >> > > > > > >>>>>>>>#isDead now.
> >> > > > > > >>>>>>>>
> >> > > > > > >>>>>>>>I hope this is at least a step in the right
> >>direction :)
> >> > > > > > >>>>>>>>
> >> > > > > > >>>>>>>>However, there's still a problem when debugging the
> >> > > > >resumption
> >> > > > > > >>>>>>>>of #cannotReturn because the encoders expect a
> >>reasonable
> >> > > > >index.
> >> > > > > > >>>>>>>>I haven't figured out yet where to place a nil check
> >>-
> >> > >#step,
> >> > > > > > >>>>>>>>#stepToSendOrReturn... ?
> >> > > > > > >>>>>>>>
> >> > > > > > >>>>>>>>Thanks again,
> >> > > > > > >>>>>>>>Jaromir
> >> > > > > > >>>>>>>>
> >> > > > > > >>>>>>>>
> >> > > > > > >>>>>>>>------ Original Message ------
> >> > > > > > >>>>>>>>From "Eliot Miranda" <eliot.miranda(a)gmail.com>
> >> > > > > > >>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net>
> >> > > > > > >>>>>>>>Date 11/17/2023 8:36:50 PM
> >> > > > > > >>>>>>>>Subject Re: [squeak-dev] Re: Resuming on
> >> > >BlockCannotReturn
> >> > > > > > >>>>>>>>exception
> >> > > > > > >>>>>>>>
> >> > > > > > >>>>>>>>>Hi Jaromir,
> >> > > > > > >>>>>>>>>
> >> > > > > > >>>>>>>>>>On Nov 17, 2023, at 7:05 AM, Jaromir Matas
> >> > > > ><mail(a)jaromir.net>
> >> > > > > > >>>>>>>>>>wrote:
> >> > > > > > >>>>>>>>>>
> >> > > > > > >>>>>>>>>>
> >> > > > > > >>>>>>>>>>Eliot, hi again,
> >> > > > > > >>>>>>>>>>
> >> > > > > > >>>>>>>>>>Please disregard my previous comment about nilling
> >>the
> >> > > > > > >>>>>>>>>>contexts that have returned. We are indeed talking
> >> > >about
> >> > > > >the
> >> > > > > > >>>>>>>>>>context directly under the #cannotReturn context
> >>which
> >> > >is
> >> > > > > > >>>>>>>>>>totally different from the home context in another
> >> > >thread
> >> > > > > > >>>>>>>>>>that's gone.
> >> > > > > > >>>>>>>>>>
> >> > > > > > >>>>>>>>>>I may still be confused but would nilling the pc
> >>of the
> >> > > > > > >>>>>>>>>>context directly under the cannotReturn context
> >>help?
> >> > > > >Here's
> >> > > > > > >>>>>>>>>>what I mean:
> >> > > > > > >>>>>>>>>>```
> >> > > > > > >>>>>>>>>>Context >> #cannotReturn: result
> >> > > > > > >>>>>>>>>>
> >> > > > > > >>>>>>>>>> closureOrNil ifNotNil: [^self pc: nil;
> >>cannotReturn:
> >> > > > > > >>>>>>>>>>result to: self home sender].
> >> > > > > > >>>>>>>>>> Processor debugWithTitle: 'Computation has been
> >> > > > > > >>>>>>>>>>terminated!' translated full: false.
> >> > > > > > >>>>>>>>>>```
> >> > > > > > >>>>>>>>>>Instead of crashing the VM invokes the debugger
> >>with
> >> > >the
> >> > > > > > >>>>>>>>>>'Computation has been terminated!' message.
> >> > > > > > >>>>>>>>>>
> >> > > > > > >>>>>>>>>>Does this make sense?
> >> > > > > > >>>>>>>>>
> >> > > > > > >>>>>>>>>Nearly. But it loses the information on what the pc
> >> > >actually
> >> > > > > > >>>>>>>>>is, and that’s potentially vital information. So
> >>IMO the
> >> > >ox
> >> > > > > > >>>>>>>>>should only be nilled between the BlockCannotReturn
> >> > > > >exception
> >> > > > > > >>>>>>>>>being created and raised.
> >> > > > > > >>>>>>>>>
> >> > > > > > >>>>>>>>>[But if you try this don’t be surprised if it
> >>causes a
> >> > >few
> >> > > > > > >>>>>>>>>temporary problems. It looks to me that without a
> >>little
> >> > > > > > >>>>>>>>>refactoring this could easily cause an infinite
> >> > >recursion
> >> > > > > > >>>>>>>>>around the sending of isDead. I’m sure you’ll be
> >>able to
> >> > >fix
> >> > > > > > >>>>>>>>>the code to work correctly]
> >> > > > > > >>>>>>>>>
> >> > > > > > >>>>>>>>>>Thanks,
> >> > > > > > >>>>>>>>>>Jaromir
> >> > > > > > >>>>>>>>>>
> >> > > > > > >>>>>>>>>>
> >> > > > > > >>>>>>>>>>------ Original Message ------
> >> > > > > > >>>>>>>>>>From "Jaromir Matas" <mail(a)jaromir.net>
> >> > > > > > >>>>>>>>>>To "Eliot Miranda" <eliot.miranda(a)gmail.com>;
> >>"The
> >> > > > > > >>>>>>>>>>general-purpose Squeak developers list"
> >> > > > > > >>>>>>>>>><squeak-dev(a)lists.squeakfoundation.org>
> >> > > > > > >>>>>>>>>>Date 11/17/2023 10:15:17 AM
> >> > > > > > >>>>>>>>>>Subject [squeak-dev] Re: Resuming on
> >>BlockCannotReturn
> >> > > > > > >>>>>>>>>>exception
> >> > > > > > >>>>>>>>>>
> >> > > > > > >>>>>>>>>>>Hi Eliot,
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>------ Original Message ------
> >> > > > > > >>>>>>>>>>>From "Eliot Miranda" <eliot.miranda(a)gmail.com>
> >> > > > > > >>>>>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net>
> >> > > > > > >>>>>>>>>>>Cc "The general-purpose Squeak developers list"
> >> > > > > > >>>>>>>>>>><squeak-dev(a)lists.squeakfoundation.org>
> >> > > > > > >>>>>>>>>>>Date 11/16/2023 11:52:45 PM
> >> > > > > > >>>>>>>>>>>Subject Re: Re[2]: [squeak-dev] Re: Resuming on
> >> > > > > > >>>>>>>>>>>BlockCannotReturn exception
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>Hi Jaromir,
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>On Thu, Nov 16, 2023 at 2:22 PM Jaromir Matas
> >> > > > > > >>>>>>>>>>>><mail(a)jaromir.net> wrote:
> >> > > > > > >>>>>>>>>>>>>Hi Nicolas, Eliot,
> >> > > > > > >>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>here's what I understand is happening (see the
> >> > >enclosed
> >> > > > > > >>>>>>>>>>>>>screenshot):
> >> > > > > > >>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>1) we fork a new process to evaluate [^1]
> >> > > > > > >>>>>>>>>>>>>2) the new process evaluates [^1] which means
> >> > > > >instruction
> >> > > > > > >>>>>>>>>>>>>18 is being evaluated, hence pc points to
> >> > >instruction 19
> >> > > > > > >>>>>>>>>>>>>now
> >> > > > > > >>>>>>>>>>>>>3) however, the home context where ^1 should
> >>return
> >> > >to
> >> > > > >is
> >> > > > > > >>>>>>>>>>>>>gone by this time (the process that executed
> >>the
> >> > >fork
> >> > > > >has
> >> > > > > > >>>>>>>>>>>>>already returned - notice the two up arrows in
> >>the
> >> > > > >debugger
> >> > > > > > >>>>>>>>>>>>>screenshot)
> >> > > > > > >>>>>>>>>>>>>4) the VM can't finish the instruction and
> >>returns
> >> > > > >control
> >> > > > > > >>>>>>>>>>>>>to the image via placing the #cannotReturn:
> >>context
> >> > >on
> >> > > > >top
> >> > > > > > >>>>>>>>>>>>>of the [^1] context
> >> > > > > > >>>>>>>>>>>>>5) #cannotReturn: evaluation results in
> >>signalling
> >> > >the
> >> > > > >BCR
> >> > > > > > >>>>>>>>>>>>>exception which is then handled by the #resume
> >> > >handler
> >> > > > > > >>>>>>>>>>>>> (in our debugged case the [:ex | self halt. ex
> >> > >resume]
> >> > > > > > >>>>>>>>>>>>>handler)
> >> > > > > > >>>>>>>>>>>>>6) ex resume is evaluated, however, this means
> >> > > > >requesting
> >> > > > > > >>>>>>>>>>>>>the VM to evaluate instruction 19 of the [^1]
> >> > >context -
> >> > > > > > >>>>>>>>>>>>>which is past the last instruction of the
> >>context
> >> > >and
> >> > > > >the
> >> > > > > > >>>>>>>>>>>>>crash ensues
> >> > > > > > >>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>I wonder whether such situations could/should
> >>be
> >> > > > >prevented
> >> > > > > > >>>>>>>>>>>>>inside the VM or whether such an expectation is
> >> > >wrong
> >> > > > >for
> >> > > > > > >>>>>>>>>>>>>some reason.
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>As Nicolas says, IMO this is best done at the
> >>image
> >> > > > >level.
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>It could be prevented in the VM, but at great
> >>cost,
> >> > >and
> >> > > > >only
> >> > > > > > >>>>>>>>>>>>partially. The performance issue is that the
> >>last
> >> > > > >bytecode
> >> > > > > > >>>>>>>>>>>>in a method is not marked in any way, and that
> >>to
> >> > > > >determine
> >> > > > > > >>>>>>>>>>>>the last bytecode the bytecodes must be
> >>symbolically
> >> > > > > > >>>>>>>>>>>>evaluated from the start of the method. See
> >> > >implementors
> >> > > > >of
> >> > > > > > >>>>>>>>>>>>endPC at the image level (which defer to the
> >>method
> >> > > > >trailer)
> >> > > > > > >>>>>>>>>>>>and implementors of endPCOf: in the VMMaker
> >>code.
> >> > >Doing
> >> > > > >this
> >> > > > > > >>>>>>>>>>>>every time execution commences is prohibitively
> >> > > > >expensive.
> >> > > > > > >>>>>>>>>>>>The "only partially" issue is that following the
> >> > >return
> >> > > > > > >>>>>>>>>>>>instruction may be other valid bytecodes, but
> >>these
> >> > >are
> >> > > > >not
> >> > > > > > >>>>>>>>>>>>a continuation.
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>Consider the following code in some block:
> >> > > > > > >>>>>>>>>>>> [self expression ifTrue:
> >> > > > > > >>>>>>>>>>>> [^1].
> >> > > > > > >>>>>>>>>>>> ^2
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>The bytecodes for this are
> >> > > > > > >>>>>>>>>>>> pushReceiver
> >> > > > > > >>>>>>>>>>>> send #expression
> >> > > > > > >>>>>>>>>>>> jumpFalse L1
> >> > > > > > >>>>>>>>>>>> push 1
> >> > > > > > >>>>>>>>>>>> methodReturnTop
> >> > > > > > >>>>>>>>>>>>L1
> >> > > > > > >>>>>>>>>>>> push 2
> >> > > > > > >>>>>>>>>>>> methodReturnTop
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>Clearly if expression is true these should be
> >>*no*
> >> > > > > > >>>>>>>>>>>>continuation in which ^2 is executed.
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>Well, in that case there's a bug because the
> >> > >computation
> >> > > > >in
> >> > > > > > >>>>>>>>>>>the following example shouldn't continue past the
> >>[^1]
> >> > > > >block
> >> > > > > > >>>>>>>>>>>but it silently does:
> >> > > > > > >>>>>>>>>>>`[[true ifTrue: [^ 1]] on: BlockCannotReturn do:
> >> > >#resume ]
> >> > > > > > >>>>>>>>>>>fork`
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>The bytecodes are
> >> > > > > > >>>>>>>>>>> push true
> >> > > > > > >>>>>>>>>>> jumpFalse L1
> >> > > > > > >>>>>>>>>>> push 1
> >> > > > > > >>>>>>>>>>> returnTop
> >> > > > > > >>>>>>>>>>>L1
> >> > > > > > >>>>>>>>>>> push nil
> >> > > > > > >>>>>>>>>>> blockReturn
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>So even if the VM did try and detect whether the
> >> > >return
> >> > > > >was
> >> > > > > > >>>>>>>>>>>>at the last block method, it would only work for
> >> > >special
> >> > > > > > >>>>>>>>>>>>cases.
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>It seems to me the issue is simply that the
> >>context
> >> > >that
> >> > > > > > >>>>>>>>>>>>cannot be returned from should be marked as dead
> >>(see
> >> > > > > > >>>>>>>>>>>>Context>>isDead) by setting its pc to nil at
> >>some
> >> > >point,
> >> > > > > > >>>>>>>>>>>>presumably after copying the actual return pc
> >>into
> >> > >the
> >> > > > > > >>>>>>>>>>>>BlockCannotReturn exception, to avoid ever
> >>trying to
> >> > > > >resume
> >> > > > > > >>>>>>>>>>>>the context.
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>Does this mean, in other words, that every
> >>context
> >> > >that
> >> > > > > > >>>>>>>>>>>returns should nil its pc to avoid being
> >>"wrongly"
> >> > > > > > >>>>>>>>>>>reused/executed in the future, which concerns
> >> > >primarily
> >> > > > >those
> >> > > > > > >>>>>>>>>>>being referenced somewhere hence potentially
> >> > >executable in
> >> > > > > > >>>>>>>>>>>the future, is that right?
> >> > > > > > >>>>>>>>>>>Hypothetical question: would nilling the pc
> >>during
> >> > >returns
> >> > > > > > >>>>>>>>>>>"fix" the example?
> >> > > > > > >>>>>>>>>>>Thanks a lot for helping me understand this.
> >> > > > > > >>>>>>>>>>>Best,
> >> > > > > > >>>>>>>>>>>Jaromir
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>Thanks,
> >> > > > > > >>>>>>>>>>>>>Jaromir
> >> > > > > > >>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>><bdxuqalu.png>
> >> > > > > > >>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>------ Original Message ------
> >> > > > > > >>>>>>>>>>>>>From "Eliot Miranda"
> >><eliot.miranda(a)gmail.com>
> >> > > > > > >>>>>>>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net>; "The
> >> > > > >general-purpose
> >> > > > > > >>>>>>>>>>>>>Squeak developers list"
> >> > > > > > >>>>>>>>>>>>><squeak-dev(a)lists.squeakfoundation.org>
> >> > > > > > >>>>>>>>>>>>>Date 11/16/2023 6:48:43 PM
> >> > > > > > >>>>>>>>>>>>>Subject Re: [squeak-dev] Re: Resuming on
> >> > > > >BlockCannotReturn
> >> > > > > > >>>>>>>>>>>>>exception
> >> > > > > > >>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>Hi Jaromir,
> >> > > > > > >>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>On Nov 16, 2023, at 3:23 AM, Jaromir Matas
> >> > > > > > >>>>>>>>>>>>>>><mail(a)jaromir.net> wrote:
> >> > > > > > >>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>Hi Nicloas,
> >> > > > > > >>>>>>>>>>>>>>>No no, I don't have any practical scenario in
> >> > >mind,
> >> > > > >I'm
> >> > > > > > >>>>>>>>>>>>>>>just trying to understand why the VM is
> >> > >implemented
> >> > > > >like
> >> > > > > > >>>>>>>>>>>>>>>this, whether there were a reason to leave
> >>this
> >> > > > > > >>>>>>>>>>>>>>>possibility of a crash, e.g. it would slow
> >>down
> >> > >the VM
> >> > > > >to
> >> > > > > > >>>>>>>>>>>>>>>try to prevent such a dumb situation (who
> >>would
> >> > >resume
> >> > > > > > >>>>>>>>>>>>>>>from BCR in his right mind? :) ) - or perhaps
> >>I
> >> > >have
> >> > > > > > >>>>>>>>>>>>>>>overlooked some good reason to even keep this
> >> > >behavior
> >> > > > >in
> >> > > > > > >>>>>>>>>>>>>>>the VM. That's all.
> >> > > > > > >>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>Let’s first understand what’s really
> >>happening.
> >> > > > >Presumably
> >> > > > > > >>>>>>>>>>>>>>at tone point a context is resumed those pc is
> >> > >already
> >> > > > >at
> >> > > > > > >>>>>>>>>>>>>>the block return bytecode (effectively,
> >>because it
> >> > > > >crashes
> >> > > > > > >>>>>>>>>>>>>>in JITted code, but I bet the stack vm will
> >>crash
> >> > >also,
> >> > > > > > >>>>>>>>>>>>>>but not as cleanly - it will try and execute
> >>the
> >> > >bytes
> >> > > > >in
> >> > > > > > >>>>>>>>>>>>>>the encoded method trailer). So which method
> >> > >actually
> >> > > > > > >>>>>>>>>>>>>>sends resume, and to what, and what state is
> >> > >resume’s
> >> > > > > > >>>>>>>>>>>>>>receiver when resume is sent?
> >> > > > > > >>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>Thanks for your reply.
> >> > > > > > >>>>>>>>>>>>>>>Regards,
> >> > > > > > >>>>>>>>>>>>>>>Jaromir
> >> > > > > > >>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>------ Original Message ------
> >> > > > > > >>>>>>>>>>>>>>>From "Nicolas Cellier"
> >> > > > > > >>>>>>>>>>>>>>><nicolas.cellier.aka.nice(a)gmail.com>
> >> > > > > > >>>>>>>>>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net>; "The
> >> > > > > > >>>>>>>>>>>>>>>general-purpose Squeak developers list"
> >> > > > > > >>>>>>>>>>>>>>><squeak-dev(a)lists.squeakfoundation.org>
> >> > > > > > >>>>>>>>>>>>>>>Date 11/16/2023 7:20:20 AM
> >> > > > > > >>>>>>>>>>>>>>>Subject Re: [squeak-dev] Resuming on
> >> > >BlockCannotReturn
> >> > > > > > >>>>>>>>>>>>>>>exception
> >> > > > > > >>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>Hi Jaromir,
> >> > > > > > >>>>>>>>>>>>>>>>Is there a scenario where it would make
> >>sense to
> >> > > > >resume
> >> > > > > > >>>>>>>>>>>>>>>>a BlockCannotReturn?
> >> > > > > > >>>>>>>>>>>>>>>>If not, I would suggest to protect at image
> >>side
> >> > >and
> >> > > > > > >>>>>>>>>>>>>>>>override #resume.
> >> > > > > > >>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>Le mer. 15 nov. 2023, 23:42, Jaromir Matas
> >> > > > > > >>>>>>>>>>>>>>>><mail(a)jaromir.net> a écrit :
> >> > > > > > >>>>>>>>>>>>>>>>>Hi Eliot, Christoph, All,
> >> > > > > > >>>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>>It's known the following example crashes
> >>the VM.
> >> > >Is
> >> > > > > > >>>>>>>>>>>>>>>>>this an intended behavior or a "tolerated
> >>bug"?
> >> > > > > > >>>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>>`[[^ 1] on: BlockCannotReturn do: #resume]
> >>fork`
> >> > > > > > >>>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>>I understand why it crashes: the non-local
> >> > >return
> >> > > > >has
> >> > > > > > >>>>>>>>>>>>>>>>>nowhere to return to and so resuming the
> >> > >computation
> >> > > > > > >>>>>>>>>>>>>>>>>leads to a crash. But why not raise another
> >>BCR
> >> > > > > > >>>>>>>>>>>>>>>>>exception to prevent the crash? Potential
> >> > >infinite
> >> > > > > > >>>>>>>>>>>>>>>>>loop? Perhaps I'm just missing the purpose
> >>of
> >> > >this
> >> > > > > > >>>>>>>>>>>>>>>>>behavior...
> >> > > > > > >>>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>>Thanks for an explanation.
> >> > > > > > >>>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>>Best,
> >> > > > > > >>>>>>>>>>>>>>>>>Jaromir
> >> > > > > > >>>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>>--
> >> > > > > > >>>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>>Jaromir Matas
> >> > > > > > >>>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>
> >> > > > > > >>>>>>>>>>>>--
> >> > > > > > >>>>>>>>>>>>_,,,^..^,,,_
> >> > > > > > >>>>>>>>>>>>best, Eliot
> >> > > > > > >>>>>>>>>><Context-cannotReturn.st>
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>
> >> > > > > > >>>>>>>--
> >> > > > > > >>>>>>>_,,,^..^,,,_
> >> > > > > > >>>>>>>best, Eliot
> >> > > > > > >>>>>><ProcessTest-testResumeAfterBCR.st>