Revised to include better characterization of ICU behavior based on some experiments I ran. Basically, ICU tolerates isolated surrogates in 16-bit strings, but the UTF-8 conversion requires surrogates to be properly paired.

This is "standard" behavior, i.e., when encoding into UTF-8, any non-iso10646 character code or surrogate pairing is an error. Similarly when decoding.

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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.)

(Modified per workgropu discussion on 2011-12-13 - did experiments to answer the TBDs in here.)

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'
  • Clarify that DFDL Infoset allows any 16-bit codepoint, not just those allowed by ISO 10646
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 '�' (U+FFFD) is substituted for the offending errors, one replacement character for each invalid encoding error. This can be one per byte if there is a series of all-illegal bytes, or it can be fewer replacement characters if a multi-byte sequence encoding a character has an error in the later bytes. For example, in UTF-8, if a 4-byte character has an error in the last byte of the 4, then a single replacement character is created. Conversely, if the first byte of a 4-byte character encoding has been corrupted, then one might get as many as 4 replacement characters. 

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.  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. For example, for UTF-8 encoding, the standard substitution character is represented by 3 bytes. If there is no room for 3 bytes, then it is a processing error.

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.)