Rescue Database errors during active_record.define_attribute_methods

This initializer should be best effort. If for some reason the
database in unhealthy, it is better to move forward with the
boot process than to crash.

activerecord/lib/active_record/railtie.rb

@@ -146,7 +146,7 @@ To keep using the current cache store, you can turn off cache versioning entirel
               current_version = begin
                 ActiveRecord::Migrator.current_version
               rescue ActiveRecordError => error
-                warn "Failed to load the schema cache because of #{error.class}: #{error.message}"
+                warn "Failed to validate the schema cache because of #{error.class}: #{error.message}"
                 nil
               end
               next if current_version.nil?
@@ -167,18 +167,31 @@ To keep using the current cache store, you can turn off cache versioning entirel
       config.after_initialize do
         ActiveSupport.on_load(:active_record) do
           if app.config.eager_load
+            begin
               descendants.each do |model|
                 # SchemaMigration and InternalMetadata both override `table_exists?`
                 # to bypass the schema cache, so skip them to avoid the extra queries.
                 next if model._internal?
 
-              # If there's no connection yet, or the schema cache doesn't have the columns
-              # hash for the model cached, `define_attribute_methods` would trigger a query.
-              next unless model.connected? && model.connection.schema_cache.columns_hash?(model.table_name)
+                # If the schema cache was loaded from a dump, we can use it without connecting
+                schema_cache = model.connection_pool.schema_cache
 
+                # If there's no connection yet, we avoid connecting.
+                schema_cache ||= model.connected? && model.connection.schema_cache
+
+                # If the schema cache doesn't have the columns
+                # hash for the model cached, `define_attribute_methods` would trigger a query.
+                if schema_cache && schema_cache.columns_hash?(model.table_name)
                   model.define_attribute_methods
                 end
               end
+            rescue ActiveRecordError => error
+              # Regardless of wether there was already a connection or not, we rescue any database
+              # error because it is critical that the application can boot even if the database
+              # is unhealthy.
+              warn "Failed to define attribute methods because of #{error.class}: #{error.message}"
+            end
+          end
         end
       end
     end

railties/test/application/initializers/frameworks_test.rb

@@ -1,10 +1,12 @@
 # frozen_string_literal: true
 
 require "isolation/abstract_unit"
+require "env_helpers"
 
 module ApplicationTests
   class FrameworksTest < ActiveSupport::TestCase
     include ActiveSupport::Testing::Isolation
+    include EnvHelpers
 
     def setup
       build_app
@@ -214,10 +216,24 @@ module ApplicationTests
       assert !defined?(ActiveRecord::Base) || ActiveRecord.autoload?(:Base)
     end
 
+    test "can boot with an unhealthy database" do
+      rails %w(generate model post title:string)
+
+      switch_env("DATABASE_URL", "mysql2://127.0.0.1:1") do
+        require "#{app_path}/config/environment"
+      end
+    end
+
     test "use schema cache dump" do
       rails %w(generate model post title:string)
       rails %w(db:migrate db:schema:cache:dump)
+
+      add_to_config <<-RUBY
+        config.eager_load = true
+      RUBY
+
       require "#{app_path}/config/environment"
+
       assert ActiveRecord::Base.connection.schema_cache.data_sources("posts")
     ensure
       ActiveRecord::Base.connection.drop_table("posts", if_exists: true) # force drop posts table for test.
@@ -226,20 +242,65 @@ module ApplicationTests
     test "expire schema cache dump" do
       rails %w(generate model post title:string)
       rails %w(db:migrate db:schema:cache:dump db:rollback)
+
+      add_to_config <<-RUBY
+        config.eager_load = true
+      RUBY
+
       require "#{app_path}/config/environment"
       assert_not ActiveRecord::Base.connection.schema_cache.data_sources("posts")
     end
 
-    test "does not expire schema cache dump if check_schema_cache_dump_version is false" do
+    test "expire schema cache dump if the version can't be checked because the database is unhealthy" do
+      rails %w(generate model post title:string)
+      rails %w(db:migrate db:schema:cache:dump)
+
       add_to_config <<-RUBY
-        config.active_record.check_schema_cache_dump_version = false
+        config.eager_load = true
       RUBY
+
+      switch_env("DATABASE_URL", "mysql2://127.0.0.1:1") do
+        require "#{app_path}/config/environment"
+
+        assert_nil ActiveRecord::Base.connection_pool.schema_cache
+        assert_raises ActiveRecord::ConnectionNotEstablished do
+          ActiveRecord::Base.connection.execute("SELECT 1")
+        end
+      end
+    end
+
+    test "does not expire schema cache dump if check_schema_cache_dump_version is false" do
       rails %w(generate model post title:string)
       rails %w(db:migrate db:schema:cache:dump db:rollback)
+
+      add_to_config <<-RUBY
+        config.eager_load = true
+        config.active_record.check_schema_cache_dump_version = false
+      RUBY
+
       require "#{app_path}/config/environment"
       assert ActiveRecord::Base.connection_pool.schema_cache.data_sources("posts")
     end
 
+    test "does not expire schema cache dump if check_schema_cache_dump_version is false and the database unhealthy" do
+      rails %w(generate model post title:string)
+      rails %w(db:migrate db:schema:cache:dump db:rollback)
+
+      add_to_config <<-RUBY
+        config.eager_load = true
+        config.active_record.check_schema_cache_dump_version = false
+      RUBY
+
+      switch_env("DATABASE_URL", "mysql2://127.0.0.1:1") do
+        require "#{app_path}/config/environment"
+
+        assert ActiveRecord::Base.connection_pool.schema_cache.data_sources("posts")
+        assert_raises ActiveRecord::ConnectionNotEstablished do
+          ActiveRecord::Base.connection.execute("SELECT 1")
+        end
+      end
+    end
+
     test "active record establish_connection uses Rails.env if DATABASE_URL is not set" do
       require "#{app_path}/config/environment"
       orig_database_url = ENV.delete("DATABASE_URL")