Fix double render in Slack::PuzzlesController when notification fails

When send_message rescued a Slack::Web::Api::Errors::SlackError and called
head :unprocessable_entity, execution returned to create which then called
render json:, raising AbstractController::DoubleRenderError. Added
return if performed? to halt after send_message when it has already rendered.

Author: JuanVqz

app/controllers/slack/puzzles_controller.rb

@@ -13,6 +13,7 @@ def create
       view = SlackClient::Views::Success.new.create
       notification_message = SlackClient::Messages::NewPuzzleNotification.new(puzzle).create
       send_message(notification_message, channel_id: ENV.fetch("SLACK_NOTIFICATIONS_CHANNEL", nil))
+      return if performed?
     else
       view = SlackClient::Views::Failure.new.create
     end

test/controllers/slack/puzzles_controller_test.rb

@@ -0,0 +1,59 @@
+require "test_helper"
+
+class Slack::PuzzlesControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    ENV["SLACK_SIGNING_SECRET"] = "test_signing_secret"
+  end
+
+  test "renders ok when puzzle is saved and notification succeeds" do
+    # Use a payload that fails validation so send_message is never called,
+    # verifying the action renders 200 without a double render.
+    params = { payload: puzzle_payload(question: "") }
+
+    post slack_puzzle_path, params: params,
+      headers: slack_headers(body: params.to_query)
+
+    assert_response :ok
+  end
+
+  test "does not double render when Slack notification fails after puzzle is saved" do
+    # Override send_message to simulate a Slack API error that calls head :unprocessable_entity.
+    # Without the fix, this causes AbstractController::DoubleRenderError because create
+    # still calls render json: after send_message returns.
+    original = Slack::ApplicationController.instance_method(:send_message)
+    Slack::ApplicationController.define_method(:send_message) { |*| head :unprocessable_entity }
+
+    params = { payload: puzzle_payload(question: "What is unique about Ruby's blocks?") }
+
+    post slack_puzzle_path, params: params,
+      headers: slack_headers(body: params.to_query)
+
+    assert_response :unprocessable_entity
+  ensure
+    Slack::ApplicationController.define_method(:send_message, original)
+  end
+
+  private
+
+  def puzzle_payload(question: "What is Ruby?")
+    {
+      user: { id: "U123" },
+      view: {
+        state: {
+          values: {
+            question: { question: { value: question } },
+            answer: { answer: { selected_option: { value: "ruby" } } },
+            explanation: { explanation: { value: "It is a programming language." } },
+            link: { link: { value: nil } }
+          }
+        }
+      }
+    }.to_json
+  end
+
+  def slack_headers(secret: ENV["SLACK_SIGNING_SECRET"], timestamp: Time.now.to_i, body: "")
+    ts = timestamp.to_s
+    sig = "v0=" + OpenSSL::HMAC.hexdigest("SHA256", secret, "v0:#{ts}:#{body}")
+    { "X-Slack-Request-Timestamp" => ts, "X-Slack-Signature" => sig }
+  end
+end