only_if / not_if command guard with lazy behavior

[ericnorris]

Describe the Enhancement:

When using only_if / not_if with a String, i.e. passing in a command instead of Ruby code, it does not appear easily possible to lazily evaluate node attributes.

For example, the following code will evaluate the value of node['some-attribute'] at compile-time:

file "foo" do
  content "bar"
  only_if "some-command #{node['some-attribute']}"
end

It'd be nice to have something like the following, but I realize that implementing this literally may be confusing since only_if already lazily evaluates Ruby code (see also #10243).

file "foo" do
  content "bar"
  only_if lazy { "some-command #{node['some-attribute']}" }
end

Describe the Need:

I don't know exactly how common this need is, but it seems like there's an opportunity for syntactic sugar here that would make writing these kinds of things more pleasant.

Current Alternative

It is possible for a user to define a custom resource, or do something like only_if { shell_out("some-command #{node['some-attribute']}").status.success? }.

Can We Help You Implement This?:

Sure, I think the relevant code is here:

chef/lib/chef/resource/conditional.rb

Lines 56 to 77 in 4ce99c4

	def configure
	case @command
	when String, Array
	@guard_interpreter = Chef::GuardInterpreter.for_resource(@parent_resource, @command, @command_opts)
	@block = nil
	when nil
	# We should have a block if we get here
	# Check to see if the user set the guard_interpreter on the parent resource. Note that
	# this error will not be raised when using the default_guard_interpreter
	if @parent_resource.guard_interpreter != @parent_resource.default_guard_interpreter
	msg = "#{@parent_resource.name} was given a guard_interpreter of #{@parent_resource.guard_interpreter}, "
	msg << "but not given a command as a string. guard_interpreter does not support blocks (because they just contain ruby)."
	raise ArgumentError, msg
	end
	@guard_interpreter = nil
	@command, @command_opts = nil, nil
	else
	# command was passed, but it wasn't a String
	raise ArgumentError, "Invalid only_if/not_if command, expected a string: #{command.inspect} (#{command.class})"
	end
	end

The tough part would be deciding what the syntax should be, because lazy might not be a good fit since it could be confusing. I imagine one way might be to have the user pass an Array, and have the guard_interpreter first de-lazy any array elements that are lazy, for example:

file "foo" do
  content "bar"
  only_if ['some-command', lazy { node['some-attribute'] }]
end