Hi Jaromir,

On 2024-02-25T13:53:42+00:00, mail@jaromir.net wrote:

> Hi Christoph,
>
> thanks again for these examples. I really look forward to trying to
> crack them :)
>
> In the meantime, I wonder if you could help me cleanup my recent Inbox
> changesets:
>
> If you're happy with Kernel-jar.1555, Kernel-jar.1554 and
> Kernel-jar.1553 they can be merged. These are related to the problems
> you observed in the Simulation Studio. All of them are IMO simple bugs
> that need fixing.

I still wonder - yet I am no way convinced - whether we should fix this issue on a general level rather than avoiding temporarily incomplete context stacks. If it does not block you anywhere else, could we maybe continue that discussion in https://github.com/squeak-smalltalk/squeak-object-memory/issues/112 first? :-)

>
> This one: Kernel-jar.1550 is a bugfix too - if you're ok with it, it's
> ready for merging.

Thanks, merged!

>
> Kernel-jar.1552, Kernel-jar.1545, and Tools-jar.1240 can be moved to
> Treated.

Done.

>
> As for Kernel-jar.1551, I have a better version I'm looking forward to
> showing you :)

Yay!

>
> Sorry for throwing all this at you; many thanks for helping me to clean
> this up.
>
> Best regards,
> Jaromir

Best,
Christoph

>
>
> On 24-Feb-24 10:38:37 PM, christoph.thiede(a)student.hpi.uni-potsdam.de
> wrote:
>
> >Hi Jaromir, all,
> >
> >just found another oddity with debugging non-local returns:
> >
> >Debug it in a workspace, then select the second method from the stack
> >in the debugger (CompiledMethod>>valueWithReceiver:arguments:) and
> >press Through:
> >
> > [sender:=thisContextswapSender:nil.
> > ^1]value.
> >
> >Expected: A BlockCannotReturn error
> >Actual: The method has returned 1!
> >
> >It seems that the VM "checks" the validity of the entire stack up to
> >the sender-to-return-to while the simulator essentially just uses "self
> >home sender". I wonder what's the best way to fix this. Insert
> >something like this at the beginning of Context>>#return:from:?
> >
> > newSender:=selffindContextSuchThat:[:ea|ea==aSender].
> >
> >Or would this break sideways returns for any scenario I currently don't
> >see? I have to confess I do not know whether and when we support them
> >at all ...
> >
> >Best,
> >Christoph
> >
> >---
> >Sent from Squeak Inbox Talk
> ><https://github.com/hpi-swa-lab/squeak-inbox-talk>
> >
> >On 2024-02-01T17:27:21+00:00, mail(a)jaromir.net wrote:
> >
> > > Hi Christoph,
> > >
> > > I still owe you an explanation of the mechanics of the bug (detailed
> > > description for future reference - especially for me):
> > >
> > >
> > > On 13-Jan-24 9:52:19 PM,
> >christoph.thiede(a)student.hpi.uni-potsdam.de
> > > wrote:
> > >
> > > >Hi Jaromir,
> > > >
> > > >do you have any idea why the former behavior would also stop when
> >the
> > > >context activates a new method?
> > > The key to understanding the issue with #runUntilErrorOrReturnFrom:
> >in
> > > this particular example is that before returning, when stepping until
> > > the guard contexts inserted by #runUntilErrorOrReturnFrom: are gone,
> >the
> > > stepping finalizes the execution of #resumeEvaluating:through: and it
> > > finally terminates all contexts including the guard context inserted
> >by
> > > #runUntilErrorOrReturnFrom: which will satisfiy the condition `ctxt
> > > isDead` at the end of #runUntilErrorOrReturnFrom: BUT
> > > #resumeEvaluating:through: still has to execute `aBlock value` which
> > > will become the intermediate point where contexts switch (stack top
> > > context changes), hence #stepToCalleeOrNil returns and the above
> > > mentioned condition is checked - resulting in the observed premature
> > > return from #runUntilErrorOrReturnFrom:.
> > > If we replace #stepToCalleeOrNil with #stepToSenderOrNil the stepping
> >in
> > > #runUntilErrorOrReturnFrom: will only stop when the stack goes down
> > > which is exactly was was intended. (i.e. the bug manifests in
> >#stepOver
> > > but it's a general deficiency in #runUntilErrorOrReturnFrom:)
> > >
> > > >
> > > >
> > > >Otherwise, I agree that seeing that #resume:through: context in the
> > > >debugger is probably not required in this situation.
> > > >
> > > >Best,
> > > >Christoph
> > > >
> > > >PS: Here's another bug if you haven't it on your radar already: In
> >the
> > > >same expression ([^2] ensure: []), step through, through, over so
> >you
> > > >end in FullBlockClosure(BlockClosure)>>ensure:. Step over, over,
> >over,
> > > >over to move beyond aBlock value. At the last step, you will get
> > > >another BCR in a second debugger. :-)
> > >
> > > Yeah, a nice one. I've already wondered why... I'll investigate.
> >Thanks
> > > for the push :)
> > >
> > > >
> > > >
> > > >---
> > > >Sent from Squeak Inbox Talk
> > > ><https://github.com/hpi-swa-lab/squeak-inbox-talk>
> > > >
> > > >On 2024-01-09T19:46:15+00:00, mail(a)jaromir.net wrote:
> > > >
> > > > > Hi Christoph, all
> > > > >
> > > > > I've just sent a minor fix to the Inbox I missed previously -
> > > > > Kernel-jar.1550.
> > > > >
> > > > > if you debug and do step through to ^2 and then step over in
> > > > > [^2] ensure: []
> > > > > the debugger incorrectly stops at #resume:through:.
> > > > >
> > > > > This is an old issue predating any of my changes. I'm sending the
> >fix
> > > >as
> > > > > part of this thread because it's closely related (but
> >independent).
> > > > >
> > > > > I think it can be safely merged.
> > > > >
> > > > > Thanks,
> > > > > 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>

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