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
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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 :)
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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
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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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