W3C home > Mailing lists > Public > public-qt-comments@w3.org > October 2002

Re: escape-uri() issues

From: Mike Brown <mike@skew.org>
Date: Mon, 21 Oct 2002 14:50:17 -0400 (EDT)
Message-Id: <200210211850.g9LIo8vt006139@chilled.skew.org>
To: "Kay, Michael" <Michael.Kay@softwareag.com>
CC: Mike Brown <mike@skew.org>, public-qt-comments@w3.org, Ashok Malhotra <ashokma@microsoft.com>

Kay, Michael wrote:
> I'm not sure what your message refers to when it says that UTF-8 is defined
> only for "new" schemes.

RFC 2718, section 2.2.5.

I should have also mentioned RFC 2277.

> I agree that like everything to do with URIs, the
> lack of clarity in the specs is quite extraordinary, and I also accept there
> are legacy situations where other encodings are used. But it seems to me
> that UTF-8 is clearly the intended direction, and I think this is what we
> should support in the W3C recommendations. 

Yes, UTF-8 is clearly the intended direction, which is why it should be the
default and required to be supported, even if other encodings are allowed.

> There's no harm in EXSLT
> supporting other encodings if you think it useful.

But I suppose there is harm in xf:escape-uri() supporting other encodings,
since this is explicitly forbidden in new W3C specs by
http://www.w3.org/TR/charmod/#sec-UniqueEncoding (though this is still a WD,
apparently). If you want to fall back on that, then fine.

To restate my case, the real problem is mainly in the HTML/HTTP space, where
ASCII, iso-8859-1, or nonspecific encodings are assumed. This situation has
*not* been corrected by any specs, except one edge case in HTML 4.01, and
therefore it is inaccurate to say that these are "legacy" situations.

Right now, it is *not* forbidden (nor mandated, for that matter) for every
modern HTML user agent to do what they do when presented with an HTML form
that uses x-www-urlencoded for data submission: use the encoding of the
document containing the form as the basis for the URI-style escaping of the
form data.

Consequently, it is not forbidden for server-side APIs such as Java's
javax.servlet.ServletRequest.getParameter() or
javax.servlet.http.HttpUtils.parsePostData() to fail to specify how non-ASCII
escaped octets are to be decoded, or for implementations of these APIs to make
assumptions like iso-8859-1 or platform default encoding, which they often do.

Until this situation is addressed to meet the goal of standardizing on UTF-8,
we are going to continue to see URIs and URI references that use encodings
other than UTF-8 for their non-ASCII characters. It seems to be the last
hurdle, but it's a big one. Microsoft will not change IE to do the right thing
if their customers have built mission-critical apps around platform-default
encoding -- this is the reason they gave me for why they don't send the
';charset=foo' parameter in MIME-encoded form data submissions!

Nevertheless, I think there is a precedent for the W3C to retroactively
address this issue (e.g., in HTTP/1.1, the IETF strongly encouraged 1.0
implementations to support the Host header), but of course this is not the
forum for that.

> > 3. IRIs are not supported. In an IRI, the ASCII characters 
> > that would be escaped in a URI are still escaped, but 
> > non-ASCII characters are not escaped (forgive me if that's an 
> > overstatement). In the EXSLT proposal, I suggest using an 
> > empty string for an encoding name (in an optional final 
> > argument) to indicate that non-ASCII characters are not to be escaped.
> I'm not sure what you're referring to as your definition of "IRI".


See also

These were mentioned on the EXSLT list by Craig Stewart.

> [...]
> This seems to suggest that an IRI reference doesn't require any characters
> to be escaped. But I suppose that's misleading: you still need to escape the
> characters that have a special meaning (such as "#"), if you are using them
> without that special meaning.

I'm not sure about this. In URIs, "#" is a special case because it is only
part of the URI reference syntax. In a plain URI, or in the part of a URI
reference before the 'fragment', it is always escaped because it is
disallowed, being neither 'reserved' nor 'unreserved'.

But in IRIs and IRI references, "#" is an 'idelims' which *is* allowed. "Note
that the space character and various delimiters are allowed in IRIs and IRI
references", it even says. This brings to light a possible flaw in the IRI
draft. As far as I can tell, there's no way to know, based on the grammar,
where the ifragment begins, if an IRI reference contains more than one "#". 
I'll ask Martin if there's something I'm missing.

> So this seems to suggest that there should be
> a third option, effectively escape-reserved="only", which only escapes the
> reserved characters and nothing else? On the other hand, escaping other
> characters seems to do no harm. I'm having trouble seeing a real use case
> for additional options in a function that's already complex enough as it is.

Perhaps xf:escape-uri() should call its second argument
$escape-reserved-and-mark, or split them up into $escape-reserved and
$escape-mark. Even better, though no simpler, define classes of characters
'reserved' 'mark' 'non-ascii' 'unreserved' and whatever is needed for IRIs,
and make the argument be a string of tokens that name those classes.

  xf:escape-uri('/~mike/résumé','reserved mark non-ascii')
  = '%2F%7Emike%2Fr%C3%A9sum%C3%A9'
  ...suitable for embedding in the query part of an http URL

  = '%2F~mike%2Frésumé'
  ...suitable for use in the query part of an http IRI reference (I think)

The use case where escaping the 'mark' characters is harmful is probably in
the realm of equivalence ("http://foo/~mike/" != "http://foo/%7Emike/").
Whether this is ever important in the real world, I don't know.

My position has always been that it's a bad idea to encourage people to
(continue to) think that they ought to be able to slap together a string and
URI-ify the whole thing at once through some magic function that's supposed to
know what components are being used for what purpose. If we eliminate that use
case as bad practice, then it seems that escaping functions can be given more
precision and a lot of this guesswork goes away.

> We decided at the last meeting to add a function string-to-codepoints()
> which gives you a sequence of integers representing the Unicode values of
> the characters in the string. This rather takes the pressure of escape-uri()
> since it will become possible for users to write implementations of encoding
> variants that we've chosen not to support.

That's a good idea. An equivalent function in EXSLT for the XSLT 1.0
implementations may require some additional thought.

> > 4. I prefer the function name "uri-escape" over "escape-uri", 
> > as the former implies a type of action (URI-style escaping of 
> > something), and the latter implies an action on a subject 
> > (some kind of escaping of a URI), which would be more accurate.
> Our general style of function naming, insofar as we have one, is verb-noun.
> Of course there's a certain imprecision here, in that we aren't applying
> escaping to the URI, but to a string that is destined to form part of a URI,
> but I've always argued that we can't pack the entire specification of the
> function into its name.

Fair enough, but I'd argue that consistency is less important than accuracy.
If the opportunity exists to fix it now so we won't be stuck with having to
maintain this convention in the future for backward compatibility or
convention's sake, it seems prudent to do so. Witness EXSLT's
str:encode-uri(), which is an unfortunate name we're now stuck with. The
intent with the proposal is to fix what's wrong with this function, but the
changes we propose are not quite drastic enough to warrant a separate function
with a new name, especially if we want to keep in sync with / base it on XPath
2.0's xf:escape-uri() function, which is what you said we should do. :)

   - Mike
  mike j. brown                   |  xml/xslt: http://skew.org/xml/
  denver/boulder, colorado, usa   |  resume: http://skew.org/~mike/resume/
Received on Tuesday, 22 October 2002 04:20:49 UTC

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