Monkey patching the Template resource to do validation

default.rb

#
# Cookbook Name:: template_checker
# Recipe:: default
#
# Copyright 2011, Opscode, Inc
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#


template "/tmp/validation_test.txt" do
  source "temp.erb"
  verify_template {|template_path|
    # The block is passed the temporary path of the rendered
    # template
    Chef::Log.info "Testing #{template_path}" 
    # Do some test here, return true if it passes, false if it fails
    true
  }
end


#An example of how you might run a shell command as your test

template "/tmp/validation_test.txt" do
  source "temp.erb"
  verify_template {|template_path|
    Chef::Log.info "Testing #{template_path}" 
    # Run a command and test its return value
    `some_command #{template_path}`
    $? == 0
  }
end

template_checker.rb

# This is a library file.  Place in libraries/

class Chef::Resource::Template < Chef::Resource::File
  def verify_template(&block)
    if block_given? and block
      @block=block
    else
      @block
    end
  end
end



class Chef::Provider::Template < Chef::Provider::File

  def action_create
    render_with_context(template_location) do |rendered_template|
      rendered(rendered_template)
      if ::File.exist?(@new_resource.path) && content_matches?
        Chef::Log.debug("#{@new_resource} content has not changed.")
        set_all_access_controls(@new_resource.path)
      else
        check_template(rendered_template) if @new_resource.verify_template
        backup
        set_all_access_controls(rendered_template.path)
        FileUtils.mv(rendered_template.path, @new_resource.path)
        Chef::Log.info("#{@new_resource} updated content")
        @new_resource.updated_by_last_action(true)
      end
    end
  end
  
  def check_template(rendered_template)
    unless @new_resource.verify_template.call(rendered_template.path)
      raise Chef::Exceptions::ValidationFailed, "Could not validate rendered template #{rendered_template.path}"
    end
  end
  
end

Users of this should note that this monkey patch is not compatible with why-run mode and was written against an older version of Chef.