NoMethodError: undefined method 'enable_cache' for nil

[pboling]

I have started testing libraries against various released versions of their dependencies, as well as the git HEAD version of the dependencies so I will have advance notice when things break.

This gem works fine with ruby-openid / ruby-openid2 in all released versions I've tested, but on git HEAD I am seeing the following error:

NoMethodError: undefined method 'enable_cache' for nil
    /var/home/pboling/.local/share/mise/installs/ruby/3.4.4/lib/ruby/gems/3.4.0/bundler/gems/rexml-3dc9eca877f8/lib/rexml/xpath_parser.rb:87:in 'REXML::XPathParser#parse'
    /var/home/pboling/.local/share/mise/installs/ruby/3.4.4/lib/ruby/gems/3.4.0/bundler/gems/rexml-3dc9eca877f8/lib/rexml/xpath.rb:66:in 'REXML::XPath.each'
    /var/home/pboling/.local/share/mise/installs/ruby/3.4.4/lib/ruby/gems/3.4.0/bundler/gems/rexml-3dc9eca877f8/lib/rexml/element.rb:1965:in 'REXML::Elements#each'

It is occuring in CI here:
https://github.com/ruby-openid/ruby-openid2/actions/runs/16096185925/job/45419317075#step:6:49

To reproduce, after checking out the repo:

git clone https://github.com/ruby-openid/ruby-openid2.git
cd ruby-openid2
git checkout next
BUNDLE_GEMFILE=gemfiles/dep_heads.gemfile bundle install
TEST=test/test_filters.rb BUNDLE_GEMFILE=gemfiles/dep_heads.gemfile bundle exec rake test

[pboling]

Running against a local checkout of the rexml project:

.......[81, 90] in /var/home/pboling/src/ruby/rexml/lib/rexml/xpath_parser.rb
    81|       if node.is_a?(Array)
    82|         Kernel.warn("REXML::XPath.each, REXML::XPath.first, REXML::XPath.match dropped support for nodeset...", uplevel: 1)
    83|         return [] if node.empty?
    84|         node = node.first
    85|       end
=>  86|       binding.b
    87|       node.document.__send__(:enable_cache) do
    88|         match( path_stack, node )
    89|       end
    90|     end
=>#0    REXML::XPathParser#parse(path="URI", node=<Service> ... </>) at /var/home/pboling/src/ruby/rexml/lib/rexml/xpath_parser.rb:86
  #1    REXML::XPath.each(element=<Service> ... </>, path="URI", namespaces=nil, variables={}, options={}, block=#<Proc:0x00007f9532ca1678 /var/home/pboli...) at /var/home/pboling/src/ruby/rexml/lib/rexml/xpath.rb:66
  # and 22 frames (use `bt' command for all frames)
(rdbg) node
<Service> ... </>
(ruby) node.document
nil
(ruby) node.class
REXML::Element
(ruby) node.parent
nil
(ruby) node.root
<Service> ... </>
(ruby) node.root.parent
nil

Is node.document # => nil expected?

[pboling]

Answer: Yes, it is expected.

From the documentation of the #document method:

    # :call-seq:
    #   document -> document or nil
    #
    # If the element is part of a document, returns that document:
    #
    #   d = REXML::Document.new('<a><b><c/></b></a>')
    #   top_element = d.first
    #   child = top_element.first
    #   top_element.document == d # => true
    #   child.document == d       # => true
    #
    # If the element is not part of a document, returns +nil+:
    #
    #   REXML::Element.new.document # => nil

So the bug is that a nil document is expected but not handled properly.

Should we follow the NullObject pattern here, using a document that is effectively null-ish, but has the shape of a Document?

[pboling]

NullObject pattern would be a much bigger architectural change, so I went with a simple change to just handle a potential nil everywhere.

[pboling]

Superceded by #267