Facilitate use of any regular ERB in database.yml

Commit 37d1429ab1 introduced the DummyERB to avoid loading the environment when
running `rake -T`.

The DummyCompiler simply replaced all output from `<%=` with a fixed string and
removed everything else. This worked okay when it was used for YAML values.
When using `<%=` within a YAML key, it caused an error in the YAML parser,
making it impossible to use ERB as you would expect. For example a
`database.yml` file containing the following should be possible:

  development:
    <% 5.times do |i| %>
    shard_<%= i %>:
      database: db/development_shard_<%= i %>.sqlite3
      adapter: sqlite3
    <% end %>

Instead of using a broken ERB compiler we can temporarily use a
`Rails.application.config` that does not raise an error when configurations are
accessed which have not been set as described in #35468.

This change removes the `DummyCompiler` and uses the standard `ERB::Compiler`.
It introduces the `DummyConfig` which delegates all known configurations to the
real `Rails::Application::Configuration` instance and returns a dummy string for
everything else. This restores the full ERB capabilities without compromising on
speed when generating the rake tasks for multiple databases.

Deprecates `config.active_record.suppress_multiple_database_warning`.

actionmailbox/test/dummy/config/environments/development.rb

@@ -46,10 +46,6 @@ Rails.application.configure do
   # Highlight code that triggered database queries in logs.
   config.active_record.verbose_query_logs = true
 
-  # Show a warning when Rails couldn't parse your database.yml
-  # for multiple databases.
-  config.active_record.suppress_multiple_database_warning = false
-
   # Debug mode disables concatenation and preprocessing of assets.
   # This option may cause significant delays in view rendering with a large
   # number of complex assets.

actiontext/test/dummy/config/environments/development.rb

@@ -46,10 +46,6 @@ Rails.application.configure do
   # Highlight code that triggered database queries in logs.
   config.active_record.verbose_query_logs = true
 
-  # Show a warning when Rails couldn't parse your database.yml
-  # for multiple databases.
-  config.active_record.suppress_multiple_database_warning = false
-
   # Debug mode disables concatenation and preprocessing of assets.
   # This option may cause significant delays in view rendering with a large
   # number of complex assets.

activerecord/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Allow any ERB in the database.yml when creating rake tasks.
+
+    Any ERB can be used in `database.yml` even if it accesses environment
+    configurations.
+
+    Deprecates `config.active_record.suppress_multiple_database_warning`.
+
+    *Eike Send*
+
 *   Add table to error for duplicate column definitions.
 
     If a migration defines duplicate columns for a table, the error message

activerecord/lib/active_record.rb

@@ -336,12 +336,19 @@ module ActiveRecord
   singleton_class.attr_accessor :dump_schemas
   self.dump_schemas = :schema_search_path
 
-  ##
-  # :singleton-method:
-  # Show a warning when Rails couldn't parse your database.yml
-  # for multiple databases.
-  singleton_class.attr_accessor :suppress_multiple_database_warning
-  self.suppress_multiple_database_warning = false
+  def self.suppress_multiple_database_warning
+    ActiveSupport::Deprecation.warn(<<-MSG.squish)
+      config.active_record.suppress_multiple_database_warning is deprecated and will be removed in Rails 7.2.
+      It no longer has any effect and should be removed from the configuration file.
+    MSG
+  end
+
+  def self.suppress_multiple_database_warning=(value)
+    ActiveSupport::Deprecation.warn(<<-MSG.squish)
+      config.active_record.suppress_multiple_database_warning= is deprecated and will be removed in Rails 7.2.
+      It no longer has any effect and should be removed from the configuration file.
+    MSG
+  end
 
   ##
   # :singleton-method:

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -140,15 +140,7 @@ module ActiveRecord
       def setup_initial_database_yaml
         return {} unless defined?(Rails)
 
-        begin
-          Rails.application.config.load_database_yaml
-        rescue
-          unless ActiveRecord.suppress_multiple_database_warning
-            $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
+        Rails.application.config.load_database_yaml
       end
 
       def for_each(databases)

railties/lib/rails/application/configuration.rb

@@ -376,24 +376,34 @@ module Rails
         end
       end
 
-      # Load the database YAML without evaluating ERB. This allows us to
-      # create the rake tasks for multiple databases without filling in the
-      # configuration values or loading the environment. Do not use this
-      # method.
+      # Load the <tt>config/database.yml</tt> to create the Rake tasks for
+      # multiple databases without loading the environment and filling in the
+      # environment specific configuration values.
       #
-      # This uses a DummyERB custom compiler so YAML can ignore the ERB
-      # tags and load the database.yml for the rake tasks.
+      # Do not use this method, use #database_configuration instead.
       def load_database_yaml # :nodoc:
         if path = paths["config/database"].existent.first
-          require "rails/application/dummy_erb_compiler"
+          require "rails/application/dummy_config"
 
-          yaml = DummyERB.new(Pathname.new(path).read).result
+          original_rails_config = Rails.application.config
+          dummy_config = DummyConfig.new(original_rails_config)
+          database_config = {}
 
-          if YAML.respond_to?(:unsafe_load)
-            YAML.unsafe_load(yaml) || {}
-          else
-            YAML.load(yaml) || {}
+          begin
+            Rails.application.config = dummy_config
+
+            yaml = ERB.new(Pathname.new(path).read).result
+
+            if YAML.respond_to?(:unsafe_load)
+              database_config = YAML.unsafe_load(yaml) || {}
+            else
+              database_config = YAML.load(yaml) || {}
+            end
+          ensure
+            Rails.application.config = original_rails_config
           end
+
+          database_config
         else
           {}
         end

railties/lib/rails/application/dummy_config.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class DummyConfig # :nodoc:
+  def initialize(config)
+    @config = config
+  end
+
+  def to_s
+    "DummyConfig"
+  end
+
+  def method_missing(selector, *args, &blk)
+    if @config.respond_to?(selector)
+      @config.send(selector, *args, &blk)
+    else
+      self
+    end
+  end
+end

railties/lib/rails/application/dummy_erb_compiler.rb

@@ -1,18 +0,0 @@
-# frozen_string_literal: true
-
-# These classes are used to strip out the ERB configuration
-# values so we can evaluate the database.yml without evaluating
-# the ERB values.
-class DummyERB < ERB # :nodoc:
-  def make_compiler(trim_mode)
-    DummyCompiler.new trim_mode
-  end
-end
-
-class DummyCompiler < ERB::Compiler # :nodoc:
-  def compile_content(stag, out)
-    if stag == "<%="
-      out.push "_erbout << ''"
-    end
-  end
-end

railties/test/application/configuration_test.rb

@@ -1741,9 +1741,16 @@ module ApplicationTests
       assert_not ActiveRecord.verbose_query_logs
     end
 
-    test "config.active_record.suppress_multiple_database_warning is false by default in development" do
+    test "config.active_record.suppress_multiple_database_warning getter is deprecated" do
       app "development"
-      assert_not ActiveRecord.suppress_multiple_database_warning
+
+      assert_deprecated { ActiveRecord.suppress_multiple_database_warning }
+    end
+
+    test "config.active_record.suppress_multiple_database_warning setter is deprecated" do
+      app "development"
+
+      assert_deprecated { ActiveRecord.suppress_multiple_database_warning = true }
     end
 
     test "config.active_record.use_yaml_unsafe_load is false by default" do

railties/test/application/rake/dbs_test.rb

@@ -40,15 +40,6 @@ 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_config = ActiveRecord::Base.configurations.configs_for(env_name: Rails.env, name: "primary")
@@ -100,6 +91,25 @@ module ApplicationTests
         db_create_and_drop("db/development.sqlite3", environment_loaded: false)
       end
 
+      test "db:create and db:drop don'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_and_drop("db/development.sqlite3", environment_loaded: false)
+      end
+
       test "db:create and db:drop don't raise errors when loading YAML with multiline ERB" do
         app_file "config/database.yml", <<-YAML
           development:
@@ -118,23 +128,21 @@ 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
+      test "db:create and db:drop don't raise errors when loading ERB accessing nested configurations" do
         app_file "config/database.yml", <<-YAML
-          sqlite: &sqlite
-            adapter: sqlite3
-            database: db/development.sqlite3
-
           development:
-            <<: *<%= ENV["DB"] || "sqlite" %>
+            database: db/development.sqlite3
+            adapter: sqlite3
+            other: <%= Rails.application.config.other.value %>
         YAML
 
         app_file "config/environments/development.rb", <<-RUBY
           Rails.application.configure do
-            config.database = "db/development.sqlite3"
+            config.other = OpenStruct.new(value: 123)
           end
         RUBY
 
-        db_create_with_warning("db/development.sqlite3")
+        db_create_and_drop("db/development.sqlite3", environment_loaded: false)
       end
 
       test "db:create and db:drop don't raise errors when loading YAML containing conditional statements in ERB" do

railties/test/application/rake/multi_dbs_test.rb

@@ -958,6 +958,19 @@ module ApplicationTests
         db_create_and_drop_namespace("primary", "db/development.sqlite3")
       end
 
+      test "db:create and db:drop don't raise errors when loading YAML containing ERB in database keys" do
+        app_file "config/database.yml", <<-YAML
+          development:
+            <% 5.times  do |i| %>
+            shard_<%= i %>:
+              database: db/development_shard_<%= i %>.sqlite3
+              adapter: sqlite3
+            <% end  %>
+        YAML
+
+        db_create_and_drop_namespace("shard_3", "db/development_shard_3.sqlite3")
+      end
+
       test "schema generation when dump_schema_after_migration is true schema_dump is false" do
         app_file "config/database.yml", <<~EOS
           development: