`I18nBackend#available_locales` does not rescue `ActiveRecord::ConnectionNotEstablished`

[takahashim]

Summary

• Decidim::TermCustomizer::I18nBackend::Implementation#available_locales only rescues ActiveRecord::StatementInvalid.
• But ActiveRecord::ConnectionNotEstablished is not rescued when the application cannot connect to the database.
• This causes problems in environments without a database, for example when running assets:precompile while building a Docker image.

Current implementation

decidim-module-term_customizer/lib/decidim/term_customizer/i18n_backend.rb

Lines 15 to 20 in ae8e832

	# Get available locales from the translations hash
	def available_locales
	Translation.available_locales
	rescue ::ActiveRecord::StatementInvalid
	[]
	end

This backend is added to I18n::Backend::Chain in the engine initializer.

Therefore, a database query can happen whenever I18n.available_locales is called.

Why the current rescue is not enough

In the ActiveRecord exception hierarchy, ConnectionNotEstablished is not a subclass of StatementInvalid.

ActiveRecord::ActiveRecordError
└─ AdapterError
   ├─ StatementInvalid           # rescued
   │  ├─ NoDatabaseError         # rescued
   │  └─ QueryAborted
   │     └─ ConnectionFailed     # rescued
   └─ ConnectionNotEstablished   # not rescued
      └─ DatabaseConnectionError # not rescued

Cases already handled

• The database table does not exist because migrations have not been run -> StatementInvalid
• The database itself does not exist -> NoDatabaseError

Case not handled

• A connection to the database cannot be established -> ConnectionNotEstablished

The last case can happen while building a container, or in a CI environment without database credentials.

Steps to reproduce

1. Create a Decidim application with decidim-term_customizer.
2. Make the database unreachable, for example by stopping the database server or setting DATABASE_URL to a non-existing host.
3. Run:

bin/rails runner 'I18n.available_locales'

The same issue also occurs with:

bin/rails assets:precompile

ActiveRecord::ConnectionNotEstablished is propagated from available_locales, and the process fails.

Expected behavior

The term customizer backend returns no locales, and the process continues.