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 '�' (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. 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.)