[whatwg] StringEncoding open issues

Joshua Bell jsbell at chromium.org
Mon Aug 6 11:29:33 PDT 2012

Regarding the API proposal at: http://wiki.whatwg.org/wiki/StringEncoding

It looks like we've got some developer interest in implementing this, and
need to nail down the open issues. I encourage folks to look over the
"Resolved" issues in the wiki page and make sure the resolutions - gathered
from loose consensus here and offline discussion - are truly resolved or if
anything is not future-proof and should block implementations from
proceeding. Also, look at the "Notes to Implementers" section; this should
be non-controversial but may be non-obvious.

This leaves two open issues: behavior on encoding error, and handling of
Byte Order Marks (BOMs)

== Encoding Errors ==

The proposal builds on Anne's
http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html encoding spec,
which defines when encodings should emit an encoder error. In that spec
(which describes the existing behavior of Web browsers) encoders are used
in a limited fashion, e.g. for encoding form results before submission via
HTTP, and hence the cases are much more restricted than the errors
encountered when browsers are asked to decode content from the wild. As
noted, the encoding process could terminate when an error is emitted.
Alternately (and as is necessary for forms, etc) there is a
use-case-specific escaping mechanism for non-encodable code points.

The proposed TextDecoder object takes a TextDecoderOptions options with a
|fatal| flag that controls the decode behavior in case of error - if
|fatal| is unset (default) a decode error produces a fallback character
(U+FFFD); if |fatal| is set then a DOMException is raised instead.

No such option is currently proposed for the TextEncoder object; the
proposal dictates that a DOMException is thrown if the encoder emits an
error. I believe this is sufficient for V1, but want feedback. For V2 (or
now, if desired), the API could be extended to accept an options object
allowing for some/all of these cases;

* Don't throw, instead emit a standard/encoding-specific replacement
character (e.g. '?')
* Don't throw, instead emit a fixed placeholder character (byte?) sequence
* Don't throw, instead call a user-defined callback and allow it to produce
a replacement "escaped" character sequence, e.g. "&#xXXXX;"

The latter seems the most flexible (superset of the rest) but is probably
overkill for now. Since it can be added in easily later, can we defer until
we have implementer and user feedback?

== Byte Order Marks (BOMs) ==

Once again, the proposal builds on Anne's
http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html encoding spec,
which describes the existing behavior of Web browsers. In the wild,
browsers deal with a variety of mechanisms for indicating the encoding of
documents (server headers, meta tags, XML preludes, etc), many of which are
blatantly incorrect or contradictory. One form is fortunately rarely wrong
- if the document is encoded in UTF-8, UTF-16LE or UTF-16BE and includes
the byte order mark (the encoding-specific serialization of U+FEFF). This
is built into the Encoding spec - given a byte sequence to decode and an
encoding label, the label is ignored if the sequence starts with one of the
three UTF BOMs, and the BOM-indicated encoding is used to decode the rest
of the stream.

The proposed API will have different uses, so it is unclear that this is
necessary or desirable.

At a minimum, it is clear that:

* If one of the UTF encodings is specified AND the BOM matches then the
leading BOM character (U+FEFF) MUST NOT be emitted in the output character
sequence (i.e. it is silently consumed)

Less clear is this behavior in these two cases.

* If one of the UTF encodings is specified AND and a different BOM is
present (e.g. UTF-16LE but a UTF-16BE BOM)
* If one of the non-UTF encodings is specified AND a UTF BOM is present

Options include:
* Nothing special - decoder does what it will with the bytes, possibly
emitting garbage, possibly throwing
* Raise a DOMException
* Switch the decoder from the user-specified encoding to the DOM-specified

The latter seems the most helpful when the proposed API is used as follows:

var s = TextDecoder().decode(bytes); // handles UTF-8 w/o BOM and any UTF
w/ BOM

... but it does seem a little weird when used like this;

var d = TextDecoder('euc-jp');
assert(d.encoding === 'euc-jp');
var s = d.decode(new Uint8Array([0xFE]), {stream: true});
assert(d.encoding === 'euc-jp');
assert(s.length === 0); // can't emit anything until BOM is definitely
s += d.decode(new Uint8Array([0xFF]), {stream: true});
assert(d.encoding === 'utf-16be'); // really?

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