[DFDL-WG] Fw: Action 249: 16.6 Forward Progress Requirement

[Steve Hanson]

Updated below to reflect comments from WG call: 

16.6 Forward Progress Requirement 
To prevent an infinite loop, the parsing of an array that is potentially 
unbounded must terminate when the following are true: 
* The occurrence is a point of uncertainty;
* The position in the data does not move during the parsing of the 
occurrence (including any associated Separator, PrefixSeparator or 
PostfixSeparator region);
* The occurrence is known-to-exist with empty representation or nil 
representation. 

An array is potentially unbounded if any of the following are true: 
* dfdl:occursCountKind is 'stopValue' 
* dfdl:occursCountKind is 'parsed'
* dfdl:occursCountKind is 'implicit' and XSDL maxOccurs is unbounded 

When dfdl:occursCountKind 'stopValue' this results in a processing error 
because the stop value will never be encountered. 

Further, to prevent unnecessary consumption of resources for large bounded 
values of XSDL maxOccurs, the parsing of an array must terminate when the 
following are true: 
* dfdl:occursCountKind is 'implicit'; 
* The occurrence is a point of uncertainty;
* The position in the data does not move during the parsing of the 
occurrence (including any associated Separator, PrefixSeparator or 
PostfixSeparator region);
* The occurrence is known-to-exist with empty representation. 

Regards
 
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
----- Forwarded by Steve Hanson/UK/IBM on 17/02/2014 15:54 -----

From:   Steve Hanson/UK/IBM
To:     Tim Kimber/UK/IBM at IBMGB, 
Cc:     dfdl-wg at ogf.org, dfdl-wg-bounces at ogf.org
Date:   03/02/2014 14:54
Subject:        Re: [DFDL-WG] Fw: Spec question: 16.6 Forward Progress 
Requirement


Tim

'Optional occurrence' is already defined in the Glossary with a different 
meaning. 

I've spent some time thinking through this issue again, and I'd prefer not 
to say that this is a processing error. Although net effect is the same we 
have to be very careful that the words here are consistent with section 9. 
Note that we don't have to worry about known-not-to-exist (missing/absent) 
as this terminates the array anyway by definition. The circumstance we are 
really discussing here is known-to-exist. 

16.6 Forward Progress Requirement 
The parsing of an array that is potentially unbounded terminates when the 
position in the data does 
not move during the parsing of an occurrence that is a point of 
uncertainty (including any associated Separator, PrefixSeparator or 
PostfixSeparator region), even if the occurrence is known-to-exist. This 
is to prevent an infinite loop. An array is potentially unbounded if any 
of the following are true: 

* dfdl:occursCountKind is 'stopValue' 
* dfdl:occursCountKind is 'parsed'
* dfdl:occursCountKind is 'implicit' and XSDL maxOccurs is unbounded 

When dfdl:occursCountKind 'stopValue' this results in a processing error 
because the stop value will never be encountered. 

Note: When dfdl:occursCountKind is 'implicit' and XSDL maxOccurs is not 
unbounded, and the position in the data does not move during the parsing 
of an occurrence that is a point of uncertainty (including any associated 
Separator, PrefixSeparator or PostfixSeparator region), and the occurrence 
is known-to-exist, and the occurrence does not give rise to a value in the 
infoset, implementations may terminate the parsing of the array. This is 
to prevent the unnecessary consumption of resources for large values of 
XSDL maxOccurs.

Note that the last (bounded) paragraph above goes back on what we said 
about nil representation. For consistency with section 9, nil should not 
terminate the array. This might result in excessive resource consumption, 
but that is necessary to create the infoset implied by the schema.

Regards

Steve Hanson
Architect, IBM Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848



From:   Tim Kimber/UK/IBM at IBMGB
To:     dfdl-wg at ogf.org, 
Date:   29/01/2014 15:12
Subject:        [DFDL-WG] Fw: Spec question: 16.6 Forward Progress 
Requirement
Sent by:        dfdl-wg-bounces at ogf.org



Proposed revised wording: 

16.6 Forward Progress Requirement 
When parsing an optional occurrence of an array element, if the element is 
found to have a zero-length representation and its associated separator 
region is also zero-length then it is a processing error. 

An optional occurrence is defined as follows: 
- any occurrence of an element with occursCountKind='implicit' and index > 
XSDL minOccurs 
- any occurrence of an element with occursCountKind='parsed' or 
'stopValue' 

If occursCountKind is 'stopValue' then the element will not be a point of 
uncertainty and the processing error will invalidate the entire array. For 
occursCountKind='implicit' or 'parsed' the processing error will terminate 
the array and will thus avoid either redundant processing or an infinite 
loop ( when XSDL maxOccurs='unbounded'). 

Feel free to propose better terminology. I'm not sure whether 'optional 
occurrence' has been used elsewhere in the specification with a different 
meaning. 
The final paragraph is conveying important information but can be reworded 
if necessary. 

regards,

Tim Kimber, DFDL Team,
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 37246742

----- Forwarded by Tim Kimber/UK/IBM on 29/01/2014 14:42 ----- 

From:        Tim Kimber/UK/IBM 
To:        dfdl-wg at ogf.org, 
Date:        28/01/2014 14:24 
Subject:        Spec question: 16.6 Forward Progress Requirement 


16.6 Forward Progress Requirement 
It is a processing error when an array is potentially unbounded and the 
position in the data does 
not move during the parsing of an occurrence of the element including any 
associated separator. 
This is to prevent an infinite loop. An array is potentially unbounded if 
any of the following are 
true: 
* dfdl:occursCountKind is 'parsed' or 'stopValue' 
* dfdl:occursCountKind is 'implicit' and XSDL maxOccurs is 'unbounded' 

Q1: Is this paragraph intended to 
a) safely terminate the parsing of the array, allowing the parser to 
proceed with whatever follows in the DFDL schema or 
b) cause the parser to discard the entire array along with everything that 
was parsed within the nearest point of uncertainty? 
If b) then I think a Schema Definition Error would be more appropriate ( 
because backing out the entire array implies that none of the occurrences 
were well-formed and some later branch might be the correct option ). 
Although... 

Q2: If the array has occursCountKind='stopValue' then none of the 
occurrences is an actual point of uncertainty because only stopValue can 
terminate the array. If paragraph 16.1 is invoked then a processing error 
will be thrown. But the occurrence is not a point of uncertainty so the 
entire array will be treated as badly-formed. In other words, behaviour 
will be as described in option b) above. I think that is correct in this 
case because the array's stopValue was never found. 

Q3: Should this infinite-loop detection be performed only after at least 
minOccurs occurrences of the array have been processed successfully? Until 
then, I think an empty occurrence should cause the default value to be put 
into the info set. 

regards,

Tim Kimber, DFDL Team,
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 37246742

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 
741598. 
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 
741598. 
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
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Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 
741598. 
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 
741598. 
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
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