DEV: Check Zeitwerk eager loading in GitHub CI (#20699)

In production, `eager_load=true`. This sometimes leads to boot errors which are not present in dev/test environments. Running `zeitwerk:check` in CI will help us to pick up on any errors early.

This commit also introduces a `DISCOURSE_ZEITWERK_EAGER_LOAD` environment variable to make it easier to toggle the behaviour when developing locally.

.github/workflows/tests.yml

@@ -159,6 +159,22 @@ jobs:
           path: tmp/turbo_rspec_runtime.log
           key: rspec-runtime-backend-core
 
+      - name: Run Zeitwerk check
+        if: matrix.build_type == 'backend'
+        env:
+          LOAD_PLUGINS: ${{ (matrix.target == 'plugins') && '1' || '0' }}
+        run: |
+          if ! bin/rails zeitwerk:check --trace; then
+            echo
+            echo "---------------------------------------------"
+            echo
+            echo "::error::'bin/rails zeitwerk:check' failed - the app will fail to boot with 'eager_load=true' (e.g. in production)."
+            echo "To reproduce locally, run 'bin/rails zeitwerk:check'."
+            echo "Alternatively, you can run your local server/tests with the 'DISCOURSE_ZEITWERK_EAGER_LOAD=1' environment variable."
+            echo
+            exit 1
+          fi
+
       - name: Core RSpec
         if: matrix.build_type == 'backend' && matrix.target == 'core'
         run: bin/turbo_rspec --verbose

config/environments/development.rb

@@ -9,7 +9,7 @@
   config.cache_classes = false
   config.file_watcher = ActiveSupport::EventedFileUpdateChecker
 
-  config.eager_load = false
+  config.eager_load = ENV["DISCOURSE_ZEITWERK_EAGER_LOAD"] == "1"
 
   # Use the schema_cache.yml file generated during db:migrate (via db:schema:cache:dump)
   config.active_record.use_schema_cache_dump = true

config/environments/test.rb

@@ -44,7 +44,7 @@
   config.assets.compile = true
   config.assets.digest = false
 
-  config.eager_load = false
+  config.eager_load = ENV["DISCOURSE_ZEITWERK_EAGER_LOAD"] == "1"
 
   if ENV["RAILS_ENABLE_TEST_LOG"]
     config.logger = Logger.new(STDOUT)