[webkit-dev] Writing a new XML parser with no external libraries

[Jeffrey Pfau]

See responses inline:

On Jun 28, 2011, at 6:26 PM, Adam Barth wrote:

> A question and a comment:
> 
> 1) Will this let us to remove the code for both the libxml2 and the
> QtXml parsers?  I'd certainly much rather have one XML parser than
> three.

This won't replace libxslt or QtXmlPatterns for XSL-T, as they depend on the respective XML libraries. The goal for this XML parser is to be able to replace the core XML parser itself. XSL-T support would have to come later.

> 2) One thing we found very helpful in working on the HTML parser was a
> good test suite.  Presumably there are existing XML parsing test
> suites.  You might consider landing one (or more) of these test suites
> as a first step.
> 
> Adam

I know that W3C provides a test suite, but it's probably not that comprehensive. I can try to find more online; I'm sure that some of the open source projects like libxml2 provide some.

Jeffrey Pfau

> 
> On Tue, Jun 28, 2011 at 6:12 PM, Jeffrey Pfau <jpfau at apple.com> wrote:
>> Currently, WebCore uses libxml2, or, if available, QtXml to parse incoming XML. However, QtXml isn't always available, and using libxml2 exposes its own share of problems. As such, I'm undertaking writing an XML parser that uses no external libraries.
>> 
>> The first step to doing this is to add a new flag that switches off the other two parsers. As the parsers are already independent and can be switched between by checking USE(QXMLSTREAM), I am adding USE(LIBXML2) checks, replacing the #else conditionals, and also a new ENABLE check, tentatively called NEW_XML (although names such as NATIVE_XML or XML_NATIVE, etc, may be more appropriate).
>> 
>> As there will probably be a new slew of files pertaining to XML parsing, I will put these files in WebCore/xml/parser, and move the existing XMLDocumentParser* file into this new directory. As far as I know, the placement of these files in WebCore/dom/ is legacy, and, assuming the build on each platform is changed, it makes sense to move them.
>> 
>> Once all the files are in a logical place, I plan to make a new file for a skeleton of the new XMLDocumentParser, at least to get it to link until a real one is in place, even if the XML parser at that point is just a data sink.
>> 
>> From there, I plan to copy and modify a good chunk of the lower level HTML tokenization and parsing code, and make changes as necessary to make it work on generalized XML, at least until I can generalize the common code in such a way that the HTML and XML tokenizers can be subclasses and use common code. I'd probably do the refactoring at the end.
>> 
>> I'm still exploring the existing parsing code, but I'd probably work my way up from there. I've read a lot of the XML 1.0 spec in preparation, as well, but it doesn't have much on implementation itself. If QtWebKit or parsing people have any comments, concerns, or help, I'd be more than willing to listen--I'm just starting here, and I'm not completely familiar with the codebase.
>> 
>> Although no code is checked in so far, I've started on this list already and have gotten as far as the new flags, a skeleton XMLDocumentParserNew.cpp, and making a tokenizer that compiles and links, but is completely untested.
>> 
>> Jeffrey Pfau
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