FIX: better handling of missing welcome topic in wizard (#6606)

config/locales/server.en.yml

@@ -3999,6 +3999,7 @@ en:
 
       introduction:
         title: "Introduction"
+        disabled: "We couldn’t find any Welcome Topic. Create a topic with `Welcome to Discourse` as title to fix this issue."
 
         fields:
           welcome:

lib/wizard/builder.rb

@@ -46,15 +46,19 @@ class Wizard
       @wizard.append_step('introduction') do |step|
         introduction = IntroductionUpdater.new(@wizard.user)
 
-        step.add_field(id: 'welcome', type: 'textarea', required: true, value: introduction.get_summary)
+        if @wizard.completed_steps?('introduction') && !introduction.get_summary
+          step.disabled = true
+        else
+          step.add_field(id: 'welcome', type: 'textarea', required: true, value: introduction.get_summary)
 
-        step.on_update do |updater|
-          value = updater.fields[:welcome].strip
+          step.on_update do |updater|
+            value = updater.fields[:welcome].strip
 
-          if value.index("\n")
-            updater.errors.add(:welcome, I18n.t("wizard.step.introduction.fields.welcome.one_paragraph"))
-          else
-            introduction.update_summary(value)
+            if value.index("\n")
+              updater.errors.add(:welcome, I18n.t("wizard.step.introduction.fields.welcome.one_paragraph"))
+            else
+              introduction.update_summary(value)
+            end
           end
         end
       end

spec/components/wizard/wizard_builder_spec.rb

@@ -37,4 +37,36 @@ describe Wizard::Builder do
     expect(invites_step.disabled).to be_truthy
   end
 
+  context 'introduction step' do
+    let(:wizard) { Wizard::Builder.new(moderator).build }
+    let(:introduction_step) { wizard.steps.find { |s| s.id == 'introduction' } }
+
+    context 'step has not been completed' do
+      it 'enables the step' do
+        expect(introduction_step.disabled).to be_nil
+      end
+    end
+
+    context 'step has been completed' do
+      before do
+        wizard = Wizard::Builder.new(moderator).build
+        introduction_step = wizard.steps.find { |s| s.id == 'introduction' }
+
+        # manually sets the step as completed
+        logger = StaffActionLogger.new(moderator)
+        logger.log_wizard_step(introduction_step)
+      end
+
+      it 'disables step if no welcome topic' do
+        expect(introduction_step.disabled).to eq(true)
+      end
+
+      it 'enables step if welcome topic is present' do
+        topic = Fabricate(:topic, title: 'Welcome to Discourse')
+        welcome_post = Fabricate(:post, topic: topic, raw: "this will be the welcome topic post\n\ncool!")
+
+        expect(introduction_step.disabled).to be_nil
+      end
+    end
+  end
 end