[DFDL-WG] Fw: Suggest should be optional feature of DFDL - dfdl:utf16Width='variable' and other corner cases (action 290)

Thu Nov 15 08:32:06 EST 2018

Hi Mike

I've been looking into IBM DFDL's treatment of property dfdl:utf16Width. 
While we claim to support 'variable' and have a few tests that use this, 
there is not the number of tests that I would expect to test the property 
fully. The intent to support 'variable' is clear in the code, though; for 
example, when parsing we check each char for being part of a surrogate 
pair and adjust length accordingly. The code uses java.nio.charset for its 
encoders & decoders, which we wrap in our own class which notes whether 
utf16 is fixed or variable, but this information is not passed to the 
encoder/decoder as there is no way to do so. Hmm. We will add some more 
tests and see if everything is behaving.

Back to your original question, should 'variable' be an optional feature 
of the spec. I have discussed with implementation team members and we 
think that is a sensible thing to do. To handle surrogates does require 
extra code to be written, and for a minimal implementation it should not 
be necessary to do that.

Regards

Steve Hanson
IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh at uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890
Note: I work Tuesday to Friday 

From:   Steve Hanson/UK/IBM
To:     Mike Beckerle <mbeckerle.dfdl at gmail.com>
Cc:     "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>
Date:   04/04/2017 11:56
Subject:        Re: [DFDL-WG] Suggest should be optional feature of DFDL - 
dfdl:utf16Width='variable' and other corner cases

Some light on action 291 - see the last sentence of this extract from the 
original errata document (experience doc 1):

3.9. Section 12.3.5, 7.3.1, 7.3.2.  The spec originally allows lengthKind 
‘pattern’ to be used when the representation of the current element, or of 
a child element, is binary, but imposes restrictions on the encoding that 
can be in force. 

Clarify that the encoding property must be defined for the element (else 
schema definition error), and that a decoding processing error is possible 
if the match of the regex encounters data that does not decode in that 
encoding, dependent on the setting of encodingErrorPolicy. Remove section 
12.3.5.1.

Same clarifications needed for testKind ”pattern” property for asserts and 
discriminators.

For consistency, the restriction that a complex element of specified 
length and lengthUnits ‘characters’ must have children that are all text 
and that have the same encoding as the complex element, is dropped

So that explains how IBM DFDL's error message CTDV1524E came about, it was 
policing a restriction in the original GFD.174 spec, a restriction which 
no longer exists. IBM DFDL has not yet implemented the erratum. It wasn't 
an extra IBM restriction.

Regards

Steve Hanson
IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh at uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890

From:   Steve Hanson/UK/IBM
To:     Mike Beckerle <mbeckerle.dfdl at gmail.com>
Cc:     "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>
Date:   14/09/2016 08:44
Subject:        Re: [DFDL-WG] Suggest should be optional feature of DFDL - 
dfdl:utf16Width='variable' and other corner cases

Actions 290 and 291 raised to investigate further - see minutes.

Regards

Steve Hanson
IBM Integration Bus, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh at uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890

From:   Steve Hanson/UK/IBM
To:     Mike Beckerle <mbeckerle.dfdl at gmail.com>
Cc:     "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>
Date:   13/09/2016 13:14
Subject:        Re: [DFDL-WG] Suggest should be optional feature of DFDL - 
dfdl:utf16Width='variable' and other corner cases

Mike

I am assuming that the processing for utf-16 'fixed' or 'variable' is 
entirely handled by ICU so there should be no coding overhead.

IBM DFDL works ok for dfdl:lengthKind='explicit' for an element of complex 
type with dfdl:lengthUnits='characters' and dfdl:encoding="utf-8". But 
there are conditions the content of the complex type must satisfy 
otherwise an SDE results, such as: 

CTDV1524E : For a complex element, when 'lengthKind' is 'explicit' or 
'prefixed', and 'lengthUnits' is characters, all simple child elements 
must have text representation, 'lengthUnits' set to 'characters' and the 
same encoding. 

So we insist that the properties of the children are consistent with the 
properties of the parent.  If you recall, IBM DFDL does all these kinds of 
validation checks in a pre-processing phase.

That seems a pretty sensible rule but I am not sure if the rule appears in 
the spec as such - I just had a quick look but didn't spot anything.

So I guess I don't see a need for these things to be optional features?

Regards

Steve Hanson
IBM Integration Bus, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh at uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890

From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>
Date:   10/08/2016 18:57
Subject:        [DFDL-WG] Suggest should be optional feature of DFDL - 
dfdl:utf16Width='variable' and other corner cases
Sent by:        "dfdl-wg" <dfdl-wg-bounces at ogf.org>

Given the limited set of required encodings for a conforming DFDL 
processor, I believe dfdl:utf16Width='variable' should be an optional 
feature.

That's just consistency with what is optional already. But it is also 
quite hard to implement. 

There are other situations that are very hard to implement, probably never 
used by real users, yet which are non optional in the spec:

I would suggest that dfdl:lengthKind='explicit' for elements of complex 
type, with dfdl:lengthUnits='characters' and a variable-width encoding 
like utf-8 is very problematic to implement. I am pretty sure IBM DFDL has 
no implementation of this per email threads, and I know I don't want to 
implement this in Daffodil even though we're trying to be very 
comprehensive in the implementation eventually.

I think implementations should be free to just not implement this.  These 
sorts of cases often exist just because we're trying to preserve some 
orthogonality of composition in the language. So it's possible to do quite 
a few things that probably aren't ever needed by anyone, that reflect 
ill-defined data formats, etc.

I'd rather not document a bunch of "non-conformances" for Daffodil or 
other implementations for these sorts of things. I'd like to say we don't 
implement them, but they're optional, and so that's allowed.

Comments?

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | 
www.tresys.com
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