Load initial database.yml once, and warn if we can't create tasks

For multiple databases we attempt to generate the tasks by reading the
database.yml before the Rails application is booted. This means that we
need to strip out ERB since it could be reading Rails configs.

In some cases like https://github.com/rails/rails/issues/36540 the ERB
is too complex and we can't overwrite with the DummyCompilier we used in
https://github.com/rails/rails/pull/35497. For the complex causes we
simply issue a warning that says we couldn't infer the database tasks
from the database.yml.

While working on this I decided to update the code to only load the
database.yml once initially so that we avoid having to issue the same
warning multiple times. Note that this had no performance impact in my
testing and is merely for not having to save the error off somewhere.
Also this feels cleaner.

Note that this will not break running tasks that exist, it will just
mean that tasks for multi-db like `db:create:other_db` will not be
generated. If the database.yml is actually unreadable it will blow up
during normal rake task calls.

Fixes #36540

activerecord/lib/active_record/railties/databases.rake

@@ -2,6 +2,8 @@
 
 require "active_record"
 
+databases = ActiveRecord::Tasks::DatabaseTasks.setup_initial_database_yaml
+
 db_namespace = namespace :db do
   desc "Set the environment value for the database"
   task "environment:set" => :load_config do
@@ -23,7 +25,7 @@ db_namespace = namespace :db do
       ActiveRecord::Tasks::DatabaseTasks.create_all
     end
 
-    ActiveRecord::Tasks::DatabaseTasks.for_each do |spec_name|
+    ActiveRecord::Tasks::DatabaseTasks.for_each(databases) do |spec_name|
       desc "Create #{spec_name} database for current environment"
       task spec_name => :load_config do
         db_config = ActiveRecord::Base.configurations.configs_for(env_name: Rails.env, spec_name: spec_name)
@@ -42,7 +44,7 @@ db_namespace = namespace :db do
       ActiveRecord::Tasks::DatabaseTasks.drop_all
     end
 
-    ActiveRecord::Tasks::DatabaseTasks.for_each do |spec_name|
+    ActiveRecord::Tasks::DatabaseTasks.for_each(databases) do |spec_name|
       desc "Drop #{spec_name} database for current environment"
       task spec_name => [:load_config, :check_protected_environments] do
         db_config = ActiveRecord::Base.configurations.configs_for(env_name: Rails.env, spec_name: spec_name)
@@ -101,7 +103,7 @@ db_namespace = namespace :db do
   end
 
   namespace :migrate do
-    ActiveRecord::Tasks::DatabaseTasks.for_each do |spec_name|
+    ActiveRecord::Tasks::DatabaseTasks.for_each(databases) do |spec_name|
       desc "Migrate #{spec_name} database for current environment"
       task spec_name => :load_config do
         db_config = ActiveRecord::Base.configurations.configs_for(env_name: Rails.env, spec_name: spec_name)
@@ -142,7 +144,7 @@ db_namespace = namespace :db do
     end
 
     namespace :up do
-      ActiveRecord::Tasks::DatabaseTasks.for_each do |spec_name|
+      ActiveRecord::Tasks::DatabaseTasks.for_each(databases) do |spec_name|
         task spec_name => :load_config do
           raise "VERSION is required" if !ENV["VERSION"] || ENV["VERSION"].empty?
 
@@ -176,7 +178,7 @@ db_namespace = namespace :db do
     end
 
     namespace :down do
-      ActiveRecord::Tasks::DatabaseTasks.for_each do |spec_name|
+      ActiveRecord::Tasks::DatabaseTasks.for_each(databases) do |spec_name|
         task spec_name => :load_config do
           raise "VERSION is required" if !ENV["VERSION"] || ENV["VERSION"].empty?
 
@@ -203,7 +205,7 @@ db_namespace = namespace :db do
     end
 
     namespace :status do
-      ActiveRecord::Tasks::DatabaseTasks.for_each do |spec_name|
+      ActiveRecord::Tasks::DatabaseTasks.for_each(databases) do |spec_name|
         desc "Display status of migrations for #{spec_name} database"
         task spec_name => :load_config do
           db_config = ActiveRecord::Base.configurations.configs_for(env_name: Rails.env, spec_name: spec_name)
@@ -266,7 +268,7 @@ db_namespace = namespace :db do
   end
 
   namespace :abort_if_pending_migrations do
-    ActiveRecord::Tasks::DatabaseTasks.for_each do |spec_name|
+    ActiveRecord::Tasks::DatabaseTasks.for_each(databases) do |spec_name|
       # desc "Raises an error if there are pending migrations for #{spec_name} database"
       task spec_name => :load_config do
         db_config = ActiveRecord::Base.configurations.configs_for(env_name: Rails.env, spec_name: spec_name)

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -141,10 +141,19 @@ module ActiveRecord
         end
       end
 
-      def for_each
+      def setup_initial_database_yaml
         return {} unless defined?(Rails)
 
-        databases = Rails.application.config.load_database_yaml
+        begin
+          Rails.application.config.load_database_yaml
+        rescue
+          $stderr.puts "Rails couldn't infer whether you are using multiple databases from your database.yml and can't generate the tasks for the non-primary databases. If you'd like to use this feature, please simplify your ERB."
+
+          {}
+        end
+      end
+
+      def for_each(databases)
         database_configs = ActiveRecord::DatabaseConfigurations.new(databases).configs_for(env_name: Rails.env)
 
         # if this is a single database application we don't want tasks for each primary database

railties/test/application/rake/dbs_test.rb

@@ -40,6 +40,15 @@ module ApplicationTests
         end
       end
 
+      def db_create_with_warning(expected_database)
+        Dir.chdir(app_path) do
+          output = rails("db:create")
+          assert_match(/Rails couldn't infer whether you are using multiple databases/, output)
+          assert_match(/Created database/, output)
+          assert File.exist?(expected_database)
+        end
+      end
+
       test "db:create and db:drop without database URL" do
         require "#{app_path}/config/environment"
         db_create_and_drop ActiveRecord::Base.configurations[Rails.env]["database"]
@@ -86,6 +95,25 @@ module ApplicationTests
         db_create_and_drop("db/development.sqlite3", environment_loaded: false)
       end
 
+      test "db:create and db:drop show warning but doesn't raise errors when loading YAML with alias ERB" do
+        app_file "config/database.yml", <<-YAML
+          sqlite: &sqlite
+            adapter: sqlite3
+            database: db/development.sqlite3
+
+          development:
+            <<: *<%= ENV["DB"] || "sqlite" %>
+        YAML
+
+        app_file "config/environments/development.rb", <<-RUBY
+          Rails.application.configure do
+            config.database = "db/development.sqlite3"
+          end
+        RUBY
+
+        db_create_with_warning("db/development.sqlite3")
+      end
+
       test "db:create and db:drop don't raise errors when loading YAML containing conditional statements in ERB" do
         app_file "config/database.yml", <<-YAML
           development: