[DFDL-WG] Issue 156 - ICU fallback mappings - character encoding/decoding errors (version 2 - modified per call 2011-12-06)

[Mike Beckerle]

*Issue 156 - ICU fallback mappings - character encoding/decoding errors*

(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 errors. It specifies what to do when
there is no mapping to Unicode (which is, frankly, a very unlikely
situation today) meaning a character is legally decoded, but then has no
mapping.

*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 fails to work:


*During unparsing, characters that are unrepresentable in the target
encoding will be replaced by the replacement character for that encoding.*

Sounds symmetric and expedient, but the problem is that some character
encodings have no reserved replacement character, and we expect that DFDL
users will need a variety of different choices for how to deal with
characters that cannot be encoded.

*Suggested Resolution: Summary
*

   - DFDL property dfdl:inputEncodingErrorPolicy with values 'skip',
   'error', 'replace'
   - DFDL property dfdl:outputEncodingErrorPolicy with values 'skip',
   'error', 'replace'
   - DFDL annotation element. Example: <dfdl:encodingReplacementCharacter
   encoding="ASCII" character="%#x7c;"/>

*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: 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.

Note for errata: The language in section 4.1.2 Item 5 about decoding data
into infoset Unicode has to change of course as well.

*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:outputEncodingErrorPolicy.

If the policy is 'error' then a processing error occurs (both case 1 and
case 2)

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, and by the dfdl:encodingReplacementCharacter annotation
element.

If provided, the dfdl:encodingReplacementCharacter annotation can appear
anywhere that DFDL annotations are allowed.

It has two attributes, which are 'encoding', and 'character'. The
'encoding' attribute specifies the encoding for which a replacement
character is being specified. This takes the same values as the
dfdl:encoding format property.
The 'character' attribute specifies a DFDL literal string specifying
exactly one character. This character is used as the replacement character
for the specified encoding whenever that encoding is in use, and the
dfdl:encodingReplacementCharacter annotation is in scope according to the
usual scoping rules.

There are these cases to consider when policy is 'replace'

   1. there is no standard replacement character defined as part of the
   encoding specification, and there is no dfdl:encodingReplacementCharacter
   annotation element.
   2. there is a standard replacement character defined as part of the
   encoding, and there is no dfdl:encodingReplacementCharacter annotation
   element.
   3. there is a dfdl:encodingReplacementCharacter annotation element

In case 1, since no replacement is possible, a processing error occurs.
In case 2, the standard replacement character is used to replace the
unmapped or error data.
In case 3, the character specified by the dfdl:encodingReplacementCharacter
annotation is used to replace the unmapped or error data. Note
specifically, if the character set has a standard replacement character,
the dfdl:encodingReplacementCharacter annotation can be used to override
use of the standard replacement character.

In these cases 2, and 3, it is still possible to be unable to output the
replacement character if there is not enough room for its encoding. This
situation is always a processing error.

Note for errata: The language in section 4.1.2 about encoding data from
infoset Unicode has to change as well.
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