feature flag state-change callbacks

to enable for your feature, in your feature definition, use
  after_state_change_proc: ->(context, old_state, new_state) { ... }

test plan:
 - specs pass
 - a future commit will make use of this feature
   and can be tested directly

fixes CNVS-10357

Change-Id: I5510545802fd181707bc217e179ba7102eec9d37
Reviewed-on: https://gerrit.instructure.com/28570
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Bracken Mosbacker <bracken@instructure.com>

app/controllers/feature_flags_controller.rb

@@ -244,6 +244,9 @@ class FeatureFlagsController < ApplicationController
       end
 
       if new_flag.save
+        if prior_state != new_flag.state && feature_def.after_state_change_proc.is_a?(Proc)
+          feature_def.after_state_change_proc.call(@context, prior_state, new_flag.state)
+        end
         render json: feature_flag_json(new_flag, @context, @current_user, session)
       else
         render json: new_flag.errors.to_json, status: :bad_request

lib/feature.rb

@@ -17,7 +17,7 @@
 #
 
 class Feature
-  ATTRS = [:feature, :display_name, :description, :applies_to, :state, :root_opt_in, :enable_at, :beta, :development, :release_notes_url, :custom_transition_proc]
+  ATTRS = [:feature, :display_name, :description, :applies_to, :state, :root_opt_in, :enable_at, :beta, :development, :release_notes_url, :custom_transition_proc, :after_state_change_proc]
   attr_reader *ATTRS
 
   def initialize(opts = {})
@@ -63,6 +63,7 @@ class Feature
     beta: false,          # 'beta' tag shown in UI
     development: false,   # 'development' tag shown in UI
     release_notes_url: 'http://example.com/',
+
     # optional: you can supply a Proc to attach warning messages to and/or forbid certain transitions
     # see lib/feature/draft_state.rb for example usage
     custom_transition_proc: ->(user, context, from_state, transitions) do
@@ -70,7 +71,11 @@ class Feature
         transitions['on']['warning'] = I18n.t('features.automatic_essay_grading.enable_warning',
           'Enabling this feature after some students have submitted essays may yield inconsistent grades.')
       end
-    end
+    end,
+
+    # optional hook to be called before after a feature flag change
+    # queue a delayed_job to perform any nontrivial processing
+    after_state_change_proc:  ->(context, old_state, new_state) { ... }
   }
 =end
 

spec/apis/v1/feature_flags_api_spec.rb

@@ -471,5 +471,36 @@ describe "Feature Flags API", type: :request do
       end
     end
   end
-end
 
+  describe "after_state_change_proc" do
+    let(:t_state_changes) { [] }
+
+    before do
+      Feature.stubs(:definitions).returns({
+          'custom_feature' => Feature.new(feature: 'custom_feature', applies_to: 'Course', state: 'allowed',
+                after_state_change_proc: ->(context, from_state, to_state) do
+                  t_state_changes << [context.id, from_state, to_state]
+                end
+          )
+      })
+    end
+
+    it "should fire when creating a feature flag to enable an allowed feature" do
+      expect {
+        api_call_as_user(t_root_admin, :put, "/api/v1/courses/#{t_course.id}/features/flags/custom_feature?state=on",
+           { controller: 'feature_flags', action: 'update', format: 'json', course_id: t_course.to_param, feature: 'custom_feature', state: 'on' })
+      }.to change(t_state_changes, :size).by(1)
+      t_state_changes.last.should eql [t_course.id, 'allowed', 'on']
+    end
+
+    it "should fire when changing a feature flag's state" do
+      t_course.disable_feature! 'custom_feature'
+      expect {
+        api_call_as_user(t_root_admin, :put, "/api/v1/courses/#{t_course.id}/features/flags/custom_feature?state=on",
+           { controller: 'feature_flags', action: 'update', format: 'json', course_id: t_course.to_param, feature: 'custom_feature', state: 'on' })
+      }.to change(t_state_changes, :size).by(1)
+      t_state_changes.last.should eql [t_course.id, 'off', 'on']
+    end
+  end
+
+end