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Re: [scxml] semantics of nested history in parallel state

From: Jacob Beard <jbeard4@cs.mcgill.ca>
Date: Tue, 1 Mar 2011 19:16:23 -0500
Message-ID: <AANLkTi=g6LkD7a9ikTPrhGfi-toHcu=HBJsby7a6vvvH@mail.gmail.com>
To: Jim Barnett <Jim.Barnett@alcatel-lucent.com>
Cc: www-voice <www-voice@w3.org>
Hi Jim,

Unfortunately, it seems I was mistaken, the change does not fix the
test case I presented.

In fact, I realized that, given the current step algorithm, the test
case I presented fails during initialization, which does not have
anything to do with history.

Please consider the following new test case:

<scxml xmlns="http://www.w3.org/2005/07/scxml" profile="ecmascript"
version="1.0" name="scxmlRoot">
	<initial>
		 <transition target="A2 B2"/>
	</initial>

	<parallel id="P">

		 <state id="A" initial="A1">
			  <state id="A1"/>

			  <state id="A2"/>
		 </state>

		 <state id="B" initial="B1">
			  <state id="B1"/>

			  <state id="B2"/>
		 </state>

		 <transition event="e1" target="C"/>
	</parallel>

	<state id="C">
		 <transition event="e2" target="H"/>
	</state>
</scxml>


(gist: https://gist.github.com/850186 )

Here's a trace of the calls to addStatesToEnter, along with their parameters:

addStatesToEnter: s : A2, root : scxmlRoot
addStatesToEnter: s : B, root : P
addStatesToEnter: s : B1, root : B
addStatesToEnter: s : B2, root : scxmlRoot

I think the problem here is similar to what I noticed involving
history and nested parallel states:

On the initial default transition, the step algorithm calls
addStatesToEnter with A2 bound to parameter s, and H bound to
parameter root. A2 is a basic state, so it gets added to
statesToEnter. Then, for each ancestor anc of A2, anc is added to
statesToEnter and if anc is a parallel state, any child of anc that
does not have a descendant on statesToEnter is added to statesToEnter.
 In this case, P is an ancestor of A2, and its child compound state B
does not yet have a descendant on statesToEnter (B2 has not yet been
added to statesToEnter, as we have not yet returned from this
recursive call). So, addStatesToEnter is called with B bound to s, and
P bound to root. B is a compound state, so B's initial state, B1, will
be added to statesToEnter.  Later on, we will return from the
recursive call, and addStatesToEnter will be called for B2. B2 will be
added to statesToEnter, and so both B1 and B2 will have been added to
statesToEnter, which will later on cause the statechart to enter the
illegal basic configuration [A2, B1, B2].



I think the problem also persists in cases involving history inside of
parallel. Please consider the following case:


<scxml xmlns="http://www.w3.org/2005/07/scxml" profile="ecmascript"
version="1.0" name="scxmlRoot">
	<initial>
		 <transition target="P"/>
	</initial>

	<parallel id="P">
		 <history id="H" type="deep"/>

		 <state id="A" initial="A1">
			<state id="A1">
				<transition target="A2" event="e1" />
			</state>
	

			<state id="A2"/>
		 </state>

		 <state id="B" initial="B1">
			<state id="B1">
				<transition target="B2" event="e1" />
			</state>

			<state id="B2"/>
		 </state>

		 <transition event="e2" target="C"/>
	</parallel>

	<state id="C">
		 <transition event="e3" target="H"/>
	</state>
</scxml>

(gist: https://gist.github.com/850188 )


Given that e1 has been called, bringing the state machine into
configuration [A2,B2], and e2 has been called, bringing the state
machine into configuration [C] and capturing [A2,B2] as history of H,
here's the trace of calls to addStatesToEnter on event e3:

addStatesToEnter: s : H, root : scxmlRoot
addStatesToEnter: s : A2, root : scxmlRoot
addStatesToEnter: s : B, root : P
addStatesToEnter: s : B1, root : B
addStatesToEnter: s : B2, root : scxmlRoot

Because the LCA for the transition from C to H is scxmlRoot, this
means that the call to getProperAncestors(A2,scxmlRoot) will still
include P in the returned ancestor set, and the trace will be the same
as the one I outlined in the previous test case.


I'd appreciate any guidance you can offer into this. Thanks,

Jake

On Tue, Mar 1, 2011 at 5:24 PM, Jacob Beard <jbeard4@cs.mcgill.ca> wrote:
> Hi Jim,
>
> Thanks for the quick reply. The modified algorithm works as expected.
>
> Best,
>
> Jake
>
> On Tue, Mar 1, 2011 at 5:08 PM, Jim Barnett
> <Jim.Barnett@alcatel-lucent.com> wrote:
>> Jacob,
>>  We have discovered a bug in the algorithm that is probably involved in
>> this case.  The history state should not be passed as 'root' on the
>> recursive call to addStatesToEnter.  The new version of the procedure
>> keeps the same value of 'root' on the recursive call:
>>
>> procedure addStatesToEnter(s,root,statesToEnter,statesForDefaultEntry):
>>    if isHistoryState(s):
>>        if historyValue[s.id]:
>>            for s0 in historyValue[s.id]:
>>
>> addStatesToEnter(s0,root,statesToEnter,statesForDefaultEntry)
>>
>> This revision will be in the next published version of the spec.  Try it
>> out and see if it helps.  If not, please report back to us, and we will
>> investigate further.
>>
>> - Jim
>>
>> -----Original Message-----
>> From: www-voice-request@w3.org [mailto:www-voice-request@w3.org] On
>> Behalf Of Jacob Beard
>> Sent: Sunday, February 27, 2011 8:11 PM
>> To: www-voice
>> Subject: [scxml] semantics of nested history in parallel state
>>
>> Hi,
>>
>> I'm currently working on an implementation of the SCXML step
>> algorithm, and I've run into a situation which is somewhat unclear.
>> Please consider the following scenario:
>>
>> <scxml>
>>        <initial>
>>                <transition target="A2 B2"/>
>>        </initial>
>>
>>        <parallel id="P">
>>                <history id="H" type="deep"/>
>>
>>                <state id="A" initial="A1">
>>                        <state id="A1"/>
>>
>>                        <state id="A2"/>
>>                </state>
>>
>>                <state id="B" initial="B1">
>>                        <state id="B1"/>
>>
>>                        <state id="B2"/>
>>                </state>
>>
>>                <transition event="e1" target="C"/>
>>        </parallel>
>>
>>        <state id="C">
>>                <transition event="e2" target="H"/>
>>        </state>
>> </scxml>
>>
>> The state machine starts in basic configuration [A2,B2].
>>
>> Given event e1, the state machine transitions to C, so the basic
>> configuration is [C].
>>
>> Given event e2, the state machine transitions to H. It has already
>> captured the previous configuration [A2,B2]. The step algorithm then
>> calls addStatesToEnter with A2 bound to parameter s, and H bound to
>> parameter root. A2 is a basic state, so it gets added to current
>> statesToEnter. Then, for each ancestor anc of A2, add anc to
>> statesToEnter and if anc is a parallel state, any child of anc that
>> does not have a descendant on statesToEnter is added to statesToEnter.
>> In this case, P is an ancestor of A2, and its child compound state B
>> does not yet have a descendant on statesToEnter (B2 has not yet been
>> added to statesToEnter, as we have not yet returned from this
>> recursive call). So, addStatesToEnter is called with B bound to s, and
>> P bound to root. B is a compound state, so B's initial state, B1, will
>> be added to statesToEnter.
>>
>> Later on, we will return from the recursive call, and addStatesToEnter
>> will be called for B2. B2 will be added to statesToEnter, and so both
>> B1 and B2 will have been added to statesToEnter, which will later on
>> cause the statechart to enter an illegal configuration.
>>
>> It seems I have probably misunderstood the specification, but it's not
>> clear how. I would appreciate any advice as to what I might be
>> missing.
>>
>> Thanks,
>>
>> Jake
>>
>>
>>
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>>
>
Received on Wednesday, 2 March 2011 00:17:11 GMT

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