[DFDL-WG] Proposed Errata Language: Issue 156 - ICU fallback mappings - character encoding/decoding errors

[Mike Beckerle]

Below is proposed errata language for Issue 156 (ICU fallback mappings, and
character encoding/decoding errors)

* Errata - Character encoding/decoding errors*
 * *
A format property is added to section 11 (properties common to both content
and framing) at the bottom of the table.

The property is encodingErrorPolicy it is an Enum. Valid values are '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 with one replacement
character for any incorrect fragment of an encoding.

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.
* *

The following specific situations involving UTF-16, UTF-16LE, and UTF-16BE
do not cause an encoding error:

   - *utf-16 and unpaired surrogate code-point*
   - *utf-16 and out-of-order surrogate code-point pair*
   - *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*

In all these cases the code-point becomes part of the DFDL Information Item
for the string.

*For Unparsing/Encoding Errors*

The following are kinds of errors when unparsing 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.)

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.)
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