Load YAML for rake tasks without parsing ERB

This change adds a new method that loads the YAML for the database
config without parsing the ERB. This may seem odd but bear with me:

When we added the ability to have rake tasks for multiple databases we
started looping through the configurations to collect the namespaces so
we could do `rake db:create:my_second_db`. See rails#32274.

This caused a problem where if you had `Rails.config.max_threads` set in
your database.yml it will blow up because the environment that defines
`max_threads` isn't loaded during `rake -T`. See rails#35468.

We tried to fix this by adding the ability to just load the YAML and
ignore ERB all together but that caused a bug in GitHub's YAML loading
where if you used multi-line ERB the YAML was invalid. That led us to
reverting some changes in rails#33748.

After trying to resolve this a bunch of ways `@tenderlove` came up with
replacing the ERB values so that we don't need to load the environment
but we also can load the YAML.

This change adds a DummyCompiler for ERB that will replace all the
values so we can load the database yaml and create the rake tasks.
Nothing else uses this method so it's "safe".

DO NOT use this method in your application.

Fixes rails#35468

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -142,7 +142,7 @@ def create_all
       end
 
       def for_each
-        databases = Rails.application.config.database_configuration
+        databases = Rails.application.config.load_database_yaml
         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/lib/rails/application/configuration.rb

@@ -184,6 +184,26 @@ def paths
         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.
+      #
+      # This uses a DummyERB custom compiler so YAML can ignore the ERB
+      # tags and load the database.yml for the rake tasks.
+      def load_database_yaml # :nodoc:
+        if path = paths["config/database"].existent.first
+          require "rails/application/dummy_erb_compiler"
+
+          yaml = Pathname.new(path)
+          erb = DummyERB.new(yaml.read)
+
+          YAML.load(erb.result)
+        else
+          {}
+        end
+      end
+
       # Loads and returns the entire raw configuration of database from
       # values stored in <tt>config/database.yml</tt>.
       def database_configuration

railties/lib/rails/application/dummy_erb_compiler.rb

@@ -0,0 +1,16 @@
+# 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)
+    out.push "_erbout << 'dummy_compiler'"
+  end
+end

railties/test/application/rake/dbs_test.rb

@@ -54,26 +54,17 @@ def db_create_and_drop(expected_database, environment_loaded: true)
       test "db:create and db:drop respect environment setting" do
         app_file "config/database.yml", <<-YAML
           development:
-            database: db/development.sqlite3
+            database: <%= Rails.application.config.database %>
             adapter: sqlite3
         YAML
 
         app_file "config/environments/development.rb", <<-RUBY
           Rails.application.configure do
-            config.read_encrypted_secrets = true
+            config.database = "db/development.sqlite3"
           end
         RUBY
 
-        app_file "lib/tasks/check_env.rake", <<-RUBY
-          Rake::Task["db:create"].enhance do
-            File.write("tmp/config_value", Rails.application.config.read_encrypted_secrets)
-          end
-        RUBY
-
-        db_create_and_drop("db/development.sqlite3", environment_loaded: false) do
-          assert File.exist?("tmp/config_value")
-          assert_equal "true", File.read("tmp/config_value")
-        end
+        db_create_and_drop("db/development.sqlite3", environment_loaded: false)
       end
 
       def with_database_existing