check workflow safety before executing

closes FOO-1314
flag=none

TEST PLAN:
  1) try to publish a content tag in
     with an assignment in a state like "duplicating"
  2) you should get a 4xx and no sentry error

Change-Id: I8654402e4b4a7da0fdce6a2e471669449821b0aa
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/254829
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Michael Ziwisky <mziwisky@instructure.com>
QA-Review: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>

app/controllers/context_module_items_api_controller.rb

@@ -491,7 +491,11 @@ class ContextModuleItemsApiController < ApplicationController
 
       if params[:module_item].has_key?(:published)
         if value_to_boolean(params[:module_item][:published])
-          @tag.publish
+          if module_item_publishable?(@tag)
+            @tag.publish
+          else
+            return render json: {message: "item can't be published"}, status: :unprocessable_entity
+          end
         else
           if module_item_unpublishable?(@tag)
             @tag.unpublish

app/helpers/context_modules_helper.rb

@@ -94,7 +94,8 @@ module ContextModulesHelper
   end
 
   def module_item_publishable?(item)
-    true
+    return true if item.nil? || !item.content || !item.content.respond_to?(:can_publish?)
+    item.content.can_publish?
   end
 
   def prerequisite_list(prerequisites)

app/models/assignment.rb

@@ -3133,6 +3133,11 @@ class Assignment < ActiveRecord::Base
     Assignments::NeedsGradingCountQuery.new(self).manual_count
   end
 
+  def can_publish?
+    return true if new_record?
+    ['unpublished', 'published'].include?(workflow_state)
+  end
+
   def can_unpublish?
     return true if new_record?
     return @can_unpublish unless @can_unpublish.nil?

spec/helpers/context_modules_helper_spec.rb

@@ -56,6 +56,39 @@ describe ContextModulesHelper do
     end
   end
 
+  describe "module_item_publishable?" do
+    it "should return true for an itemless item like a subheader" do
+      item = t_module.add_item(type: 'context_module_sub_header')
+      expect(module_item_publishable?(item)).to be_truthy
+    end
+
+    it "returns true for an item that doesn't respond to can_publish?" do
+      tag = t_module.content_tags.build
+      tag.tag_type = 'context_module'
+      tag.content = Thumbnail.new
+      expect(module_item_publishable?(tag)).to be_truthy
+    end
+
+    it "returns the content's can_publish?" do
+      assignment = t_course.assignments.create(workflow_state: 'unpublished')
+      student_in_course(:course => t_course)
+      item = t_module.add_item(type: 'assignment', id: assignment.id)
+      expect(assignment.can_publish?).to be_truthy
+      expect(module_item_publishable?(item)).to be_truthy
+      assignment.publish!
+      item.content.reload
+      item.reload
+      expect(assignment.can_publish?).to be_truthy
+      expect(module_item_publishable?(item)).to be_truthy
+      assignment.workflow_state = 'duplicating'
+      assignment.save!
+      item.content.reload
+      item.reload
+      expect(assignment.can_publish?).to be_falsey
+      expect(module_item_publishable?(item)).to be_falsey
+    end
+  end
+
   describe "module_item_translated_content_type" do
     it 'returns "" for nil item' do
       expect(module_item_translated_content_type(nil)).to eq ''