Testing validity of Rails feeds with RSpecEdit

Created , updated

It’s actually quite hard to find information about testing feeds because so many pages out there feature the words "feed" nowadays, but some of the first results that I turned up talked about the FeedValidator gem.

The gem actually hits the W3C Feed Validation service, although it uses caching to minimize redundant queries. This approach doesn’t really appeal to me because of the latency of performing a query across the network, as well as the reliance on a third-party service (if the service goes down you can’t run your specs). Also, the need to avoid hammering the service, even with caching, makes running really comprehensive specs prohibitive.

So I started looking for offline validation solutions. My first attempt was using xmllint because it comes installed on both my local (Mac OS X Leopard) machine and my remote (Red Hat) host.

$ curl http://www.atompub.org/2005/07/11/atom.rnc > spec/2005-07-11-atom.rnc
$ curl http://www.atompub.org/2005/08/17/atom.rnc > spec/2005-08-17-atom.rnc

This didn’t work because xmllint obviously doesn’t handle RelaxNG schemas in the "compact" format:

$ xmllint --schema spec/2005-08-17-atom.rnc public/wiki.atom
spec/2005-08-17-atom.rnc:1: parser error : Start tag expected, '<' not found
# -*- rnc -*-
^
Schemas parser error : xmlSchemaParse: could not load 'spec/2005-08-17-atom.rnc'.
WXS schema spec/2005-08-17-atom.rnc failed to compile

So I downloaded Trang to try converting the compact schema into another format:

$ curl -O http://www.thaiopensource.com/download/trang-20030619.zip
$ curl -O http://www.thaiopensource.com/download/jing-20030619.zip
$ unzip trang-20030619.zip 
$ unzip jing-20030619.zip 
$ java -jar trang-20030619/trang.jar spec/2005-08-17-atom.rnc spec/2005-08-17-atom.xsd
$ xmllint --schema spec/2005-08-17-atom.xsd public/wiki.atom 
spec/2005-08-17-atom.xsd:83: element complexType: Schemas parser error : CT local: The content model is not determinist.
spec/2005-08-17-atom.xsd:107: element complexType: Schemas parser error : CT local: The content model is not determinist.
spec/2005-08-17-atom.xsd:258: element complexType: Schemas parser error : CT local: The content model is not determinist.
spec/2005-08-17-atom.xsd:61: element complexType: Schemas parser error : CT 'atomPersonConstruct': The content model is not determinist.
WXS schema spec/2005-08-17-atom.xsd failed to compile

Googling for information on these "non determinist" errors suggested that this might be a bug in the version of libxml I was using, but seeing as it was a bit of a long shot and I don’t really want to go changing that on either of my machines I decided to continue looking for alternative solutions.

Trying another format:

$ java -jar trang-20030619/trang.jar spec/2005-08-17-atom.rnc spec/atom.dtd
/Users/wincent/trabajo/unversioned/wincent.com/src/spec/2005-08-17-atom.rnc:306:13: error: sorry, wildcards are not supported
...

And many more errors omitted from the excerpt… Evidently a DTD cannot express all of the constraints that can be specified in a RelaxNG schema.

So my next attempt was to use Jing, which Trang depends on, to do the validation instead of xmllint:

$ java -jar jing-20030619/bin/jing.jar spec/2005-08-17-atom.rnc public/wiki.atom 
spec/2005-08-17-atom.rnc:1:1: fatal: Content is not allowed in prolog.

Evidently jing doesn’t like the compact syntax either (although it would have if I had read the manual and seen the -c switch!), so we try the XML syntax and voila it works:

$ java -jar jing-20030619/bin/jing.jar spec/2005-08-17-atom.rng public/blog.atom; echo $?
public/blog.atom:7:29: error: bad character content for element
1
$ java -jar jing-20030619/bin/jing.jar spec/2005-08-17-atom.rng public/wiki.atom; echo $?
0

So here we have offline validation with information about problems printed to the standard error and the process exit code indicating failure or success. All we need now is to wrap this up in the appropriate RSpec code.