Class | Chef::Mixin::WhyRun::ResourceRequirements
In: | lib/chef/mixin/why_run.rb
Parent: | Object

## ResourceRequirements

ResourceRequirements provides a framework for making assertions about the host system‘s state. It also provides a mechanism for making assumptions about what the system‘s state might have been when running in why run mode.

For example, consider a recipe that consists of a package resource and a service resource. If the service‘s init script is installed by the package, and Chef is running in why run mode, then the service resource would fail when attempting to run `/etc/init.d/software-name status`. In order to provide a more useful approximation of what would happen in a real chef run, we want to instead assume that the service was created but isn‘t running. The logic would look like this:

        # Hypothetical service provider demonstrating why run assumption logic.
        # This isn't the actual API, it just shows the logic.
        class HypotheticalServiceProvider < Chef::Provider

          def load_current_resource
            # Make sure we have the init script available:
            if ::File.exist?("/etc/init.d/some-service"
              # If the init script exists, proceed as normal:
              status_cmd = shell_out("/etc/init.d/some-service status")
              if status_cmd.success?
                @current_resource.status(:running)
              else
                @current_resource.status(:stopped)
              end
            else
              if whyrun_mode?
                # If the init script is not available, and we're in why run mode,
                # assume that some previous action would've created it:
                log("warning: init script '/etc/init.d/some-service' is not available")
                log("warning: assuming that the init script would have been created, assuming the state of 'some-service' is 'stopped'")
                @current_resource.status(:stopped)
              else
                raise "expected init script /etc/init.d/some-service doesn't exist"
              end
            end
          end

        end

In short, the code above does the following:

- runs a test to determine if a requirement is met: `::File.exist?("/etc/init.d/some-service"`
- raises an error if the requirement is not met, and we‘re not in why run mode.
- if we are in why run mode, print a message explaining the situation, and run some code that makes an assumption about what the state of the system would be. In this case, we also skip the normal `load_current_resource` logic
- when the requirement is met, we run the normal `load_current_resource` logic

ResourceRequirements encapsulates the above logic in a more declarative API.

### Examples

Assertions and assumptions should be created through the WhyRun#assert method, which gets mixed in to providers. See that method‘s documentation for examples.

### Methods

action_blocked? assert events new run

### Classes and Modules

Class Chef::Mixin::WhyRun::ResourceRequirements::Assertion

### Public Class methods

new(resource, run_context)

### Public Instance methods

action_blocked?(action)

Check to see if a given action is blocked by a failed assertion

Takes the action name to be verified.

assert(*actions) {|assertion| ...}

Define a new Assertion.

Takes a list of action names for which the assertion should be made.

#### Examples:

A File provider that requires the parent directory to exist:

      assert(:create, :create_if_missing) do |a|
        parent_dir = File.basename(@new_resource.path)
        a.assertion { ::File.directory?(parent_dir) }
        a.failure_message(Exceptions::ParentDirectoryDoesNotExist,
                          "Can't create file #{@new_resource.path}: parent directory #{parent_dir} doesn't exist")
        a.why_run("assuming parent directory #{parent_dir} would have been previously created"
      end

A service provider that requires the init script to exist:

      assert(:start, :restart) do |a|
        a.assertion { ::File.exist?(@new_resource.init_script) }
        a.failure_message(Exceptions::MissingInitScript,
                          "Can't check status of #{@new_resource}: init script #{@new_resource.init_script} is missing")
        a.why_run("Assuming init script would have been created and service is stopped") do
          @current_resource.status(:stopped)
        end
      end

A File provider that will error out if you don‘t have permissions do delete the file, *even in why run mode*:

      assert(:delete) do |a|
        a.assertion { ::File.writable?(@new_resource.path) }
        a.failure_message(Exceptions::InsufficientPrivileges,
                          "You don't have sufficient privileges to delete #{@new_resource.path}")
      end

A Template provider that will prevent action execution but continue the run in whyrun mode if the template source is not available.

      assert(:create, :create_if_missing) do |a|
        a.assertion { File::exist?(@new_resource.source) }
        a.failure_message Chef::Exceptions::TemplateError, "Template #{@new_resource.source} could not be found exist."
        a.whyrun "Template source #{@new_resource.source} does not exist. Assuming it would have been created."
        a.block_action!
      end

      assert(:delete) do |a|
        a.assertion { ::File.writable?(@new_resource.path) }
        a.failure_message(Exceptions::InsufficientPrivileges,
                          "You don't have sufficient privileges to delete #{@new_resource.path}")
      end

events()
run(action)

Run the assertion and assumption logic.

[Validate]