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>