[DFDL-WG] Issue 156 - ICU fallback mappings - character encoding/decoding errors (version 2 - modified per call 2011-12-06)
Mike Beckerle
mbeckerle.dfdl at gmail.com
Wed Dec 14 15:52:52 EST 2011
Tim, do you think you were thinking of this encoding
(CESU-8)<http://en.wikipedia.org/wiki/CESU-8>(pronounced "sez you") of
surrogate pairs as 2 3-byte UTF-8 sequences?
I believe there is also this hack by which code point 0 is encoded as two
bytes instead of just a 0. Not sure why this was needed, but it was a Java
object-serialization convention.
I was expecting that the ICU UTF-8 parser would deal with these, but it
traps them as errors. Using the callback hook one could change it to handle
them, or an encoding description that is more flexible could be created.
On parsing, being able to accept everything possible seems good.
The big concern is what to generate on unparse. E.g., for a floating
surrogate, generate CESU-8 3-byte sequence? or error out/substitute? For a
surrogate pair, generate two 3-byte CESU sequences for 6 byte total, or the
UTF-8 standard 4-byte encoding?
Or, perhaps we're just trying to squeeze too much into one encoding, and we
actually need a strict and a tolerant variant of UTF-8? Like maybe people
should say CESU-8 if that's what they mean?
...mikeb
On Wed, Dec 14, 2011 at 5:29 AM, Tim Kimber <KIMBERT at uk.ibm.com> wrote:
> This is a little picky, but as the whole point is to tighten up the
> spec....
>
> UTF-8 characters should only ever be 1,2, or 3 bytes in length.
>
> In some applications a single Unicode character that is outside of the BMP
> ( so needs to be a surrogate pair in UTF-16 ) can end up as a pair of
> 2-byte UTF-8 characters. So the end result is 4 bytes of UTF-8 for a single
> Unicode character. But that's frowned upon by the Unicode consortium. The
> application should convert the single Unicode character to a single 3-byte
> UTF-8 character.
>
> regards,
>
> Tim Kimber, Common Transformation Team,
> Hursley, UK
> Internet: kimbert at uk.ibm.com
> Tel. 01962-816742
> Internal tel. 246742
>
>
>
>
> From: Steve Hanson/UK/IBM at IBMGB
> To: Mike Beckerle <mbeckerle.dfdl at gmail.com>
> Cc: dfdl-wg at ogf.org, Andreas Martens1/UK/IBM at IBMGB
> Date: 14/12/2011 07:45
> Subject: Re: [DFDL-WG] Issue 156 - ICU fallback mappings -
> character encoding/decoding errors (version 2 - modified per call
> 2011-12-06)
> Sent by: dfdl-wg-bounces at ogf.org
> ------------------------------
>
>
>
> Mike, I think this proposal looks good and provides an adequate solution
> for DFDL 1.0. Let's discuss further on today's WG call.
>
> Regards
>
> Steve Hanson
> Architect, Data Format Description Language (DFDL)
> Co-Chair, *OGF DFDL Working Group* <http://www.ogf.org/dfdl/>
> IBM SWG, Hursley, UK*
> **smh at uk.ibm.com* <smh at uk.ibm.com>
> tel:+44-1962-815848
>
>
>
> From: Mike Beckerle <mbeckerle.dfdl at gmail.com>
> To: Steve Hanson/UK/IBM at IBMGB
> Cc: Andreas Martens1/UK/IBM at IBMGB, dfdl-wg at ogf.org
> Date: 07/12/2011 15:02
> Subject: Re: [DFDL-WG] Issue 156 - ICU fallback mappings -
> character encoding/decoding errors (version 2 - modified per call
> 2011-12-06)
> ------------------------------
>
>
>
> Alright, I was able to convince myself that a substitution character is
> available, and associated with the IANA character set ID aliases. Even
> us-ascii has one (\x1A) E.g., *
> http://demo.icu-project.org/icu-bin/convexp?conv=US-ASCII&s=ALL*<http://demo.icu-project.org/icu-bin/convexp?conv=US-ASCII&s=ALL>
>
> So our original language that said to just use "the replacement character
> for the encoding" was actually correct!
>
> Revised proposal below. Basically, it's just error, skip or replace flag
> for encoding error policy. We still have to figure out the TBDs in there
> with respect to how many substitution/replacements will occur, and what to
> do about some of these Unicode-encoding related issues.
>
> ...mikeb
>
> ---------------------------------------------------------------------------
> *
> Issue 156 - ICU fallback mappings - character encoding/decoding errors*
>
> (modified per email thread on standardized ICU substitution/replacement
> characters)
> (Modified per workgroup discussion on 2011-12-06 - removed rationale and
> discussion, simplified to just the minimum. Note couple of important TBDs
> in here. Topics we forgot to discuss.)*
>
> Summary*
>
> DFDL currently does not have adequate capability to handle encoding and
> decoding errors. Language in the spec is incorrect/infeasible to implement.
> ICU provides mechanisms giving degree of control over this issue, the
> question is whether and how to embrace those mechanisms, or provide some
> other alternative solution.*
>
> Discussion*
>
> This language in section 4.1.2 about character set decoding/encoding just
> doesn't work:
>
> This first part is unacceptable because it fails to specify what happens
> when the decoding fails because of data errors. *
>
> During parsing, characters whose value is unknown or unrepresentable in
> ISO 10646 are replaced by the Unicode Replacement Character U+FFFD. *
>
> This second part also is inadequate:*
>
> During unparsing, characters that are unrepresentable in the target
> encoding will be replaced by the replacement character for that encoding.*
>
> This needs a citation for where these replacement characters are
> specified. It also needs to specify what happens in certain error
> situations.
> *
> Suggested Resolution: Summary*
>
> - DFDL property dfdl:encodingErrorPolicy with values 'skip', 'error',
> 'replace'
>
> *For Parsing/Decoding Errors*
>
> There are two errors that can occur when decoding characters into
> Unicode/ISO 10646.
> 1. the data is broken - invalid byte sequences that don't match
> the definition of the encoding are encountered.
> 2. not enough bytes are found to make up the entire encoding of a
> character. That is, a fragment of a valid encoding is found.
>
> The behavior in these cases is controlled by dfdl:inputEncodingErrorPolicy.
>
> If 'replace', then the Unicode *replacement character*<http://en.wikipedia.org/wiki/Replacement_character>'�' (U+FFFD) is substituted for the offending bytes, one replacement
> character for each invalid byte, one replacement character for any fragment
> of an encoding.*
>
> (TBD: Should this say 'byte' or 'unit' ?? I.e., in UTF-16BE, will ICU
> error callback occur once for a broken codepoint, or once per byte?)
>
> (TBD: Assumptions to validate: I am assuming here that if there are 6
> invalid bytes, none of which can validly be unit 1 of the encoding of any
> character, that ICU will call the error hook either (a) 6 times, or (b)
> once but notifying about all 6 bad units - but providing a way for the
> hook-writer to say they want to substitute 6 characters for the 6 units.
>
> I am also assuming in the end-of-data fragment case that the ICU hook gets
> called once for the fragment, not once per byte of the fragment.)
>
> (TBD: We did not discuss on the call on Dec 6th, the issue of errors in
> unicode encodings. While there are no encodings where a properly encoded
> character is unmapped to unicode, the unicode UTF encodings themselves can
> contains things that are errors. Here's a short list of some things that
> can happen:*
>
> - *utf-16 and unpaired surrogate code-point*
> - *utf-16 and out-of-order surrogate code-point pair*
> - *utf-8 parsing and 3-byte encoding of a surrogate code-point is found
> *
> - *utf-8 unparsing and code-point of an isolated surrogate is to be
> encoded.*
> - *utf-8 decoding, and if you assemble the bits the usual way, you get
> a code point out of range (higher than 0x10FFFF)*
> - *utf-8 encoding, and code-point to encode is higher than 0x10FFFF. *
> - *utf-16 encoding utf16Width='fixed' and a surrogate code point is
> encountered*
> - *utf-16 byte-order-marks found not at the beginning of the data*
>
> *We have an option here to be 'tolerant' of unicode-encoding foibles. We
> can preserve isolated surrogates in a natural way if we wish. I believe
> many Unicode and UTF implementations tolerate these situations. For example
> the standard Java utf-8 decoder/encoder InputStreamReader and
> OutputStreamWriter, is tolerant of incorrectly paired and isolated
> surrogate code points in the Java string data. *
>
> *I do not know what ICU does in these cases, i.e., if it provides us
> enough flexibility to do whatever we want, or if it doesn't even detect
> some of these things as errors.)*
>
>
> If 'skip' then the invalid byte sequences are dropped/ignored. No
> corresponding characters are created in the DFDL infoset.
>
> If 'error' then a processing error occurs.
>
> It is suggested that if a DFDL user wants to preserve information
> containing data where the encodings have these kinds of errors, that they
> model such data as xs:hexBinary, or as a xs:string, but using an encoding
> such as iso-8859-1 which preserves all bytes.*
>
> Suggested Resolution - Unparsing/Encoding Errors*
>
> The following are kinds of errors when encoding characters:
> 1. no mapping provided by the encoding specification.
> 2. not enough room to output the entire encoding of the character
> (e.g., need 2 bytes for a DBCS, but only 1 byte remains in the available
> length.
> The behavior in these cases is controlled by dfdl:encodingErrorPolicy.
>
> If the policy is 'error' then a processing error occurs.
>
> If the policy is 'skip' then the character is skipped. No character is
> encoded to be output for case 1, and no partial character is attempted in
> case 2.
>
> If the policy is 'replace' then the behavior is determined by the encoding
> specification.
>
> Each encoding has a replacement/substitution character specified by the
> ICU. These can be found conveniently in the *ICU Converter Explorer.*<http://demo.icu-project.org/icu-bin/convexp> This character is substituted for the unmapped character or the character
> that has too large an encoding (errors 1, and 2 above).
>
> It is a processing error if it is not possible to output the replacement
> character because there is not enough room for its representation.
>
> It is a processing error if a character encoding does not provide a
> substitution/replacement character definition and one is needed because of
> dfdl:encodingErrorPolicy='replace'. (This would be rare, but could occur if
> a DFDL implementation allows many encodings beyond the minimum set.)
>
>
>
>
>
> ------------------------------
> *
> *
>
> *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
> *
>
>
>
>
>
>
>
--
Mike Beckerle | OGF DFDL WG Co-Chair
Tel: 781-330-0412
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