Add ability to change the names of the default handlers

When I wrote the `connected_to` and `connects_to` API's I wrote them
with the idea in mind that it didn't really matter what the
handlers/roles were called as long as those connecting to the roles knew
which one wrote and which one read.

With the introduction of the middleware Rails begins to assume it's
`writing` and `reading` and there's no room for other roles. At GitHub
we've been using this method for a long time so we have a ton of legacy
code that uses different handler names `default` and `readonly`. We
could rename all our code but I think this is better for a few reasons:

- Legacy apps that have been using multiple databases for a long time
  can have an eaiser time switching.
- If we later find this to cause more issues than it's worth we can
  easily deprecate.
- We won't force old apps to rewrite the resolver middleware just to use
  a different handler.

Adding the writing_role/reading_role required that I move the code that
creates the first handler for writing to the railtie. If I didn't move
this the core class would assign the handler before I was able to assign
a new one in my configuration and I'd end up with 3 handlers instead of
2.

activerecord/lib/active_record/core.rb

@@ -125,6 +125,10 @@ def self.configurations
 
       mattr_accessor :connection_handlers, instance_accessor: false, default: {}
 
+      mattr_accessor :writing_role, instance_accessor: false, default: :writing
+
+      mattr_accessor :reading_role, instance_accessor: false, default: :reading
+
       class_attribute :default_connection_handler, instance_writer: false
 
       self.filter_attributes = []
@@ -138,7 +142,6 @@ def self.connection_handler=(handler)
       end
 
       self.default_connection_handler = ConnectionAdapters::ConnectionHandler.new
-      self.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
     end
 
     module ClassMethods

activerecord/lib/active_record/middleware/database_selector/resolver.rb

@@ -45,7 +45,7 @@ def write(&blk)
 
           def read_from_primary(&blk)
             ActiveRecord::Base.connection.while_preventing_writes do
-              ActiveRecord::Base.connected_to(role: :writing) do
+              ActiveRecord::Base.connected_to(role: ActiveRecord::Base.writing_role) do
                 instrumenter.instrument("database_selector.active_record.read_from_primary") do
                   yield
                 end
@@ -54,7 +54,7 @@ def read_from_primary(&blk)
           end
 
           def read_from_replica(&blk)
-            ActiveRecord::Base.connected_to(role: :reading) do
+            ActiveRecord::Base.connected_to(role: ActiveRecord::Base.reading_role) do
               instrumenter.instrument("database_selector.active_record.read_from_replica") do
                 yield
               end
@@ -62,7 +62,7 @@ def read_from_replica(&blk)
           end
 
           def write_to_primary(&blk)
-            ActiveRecord::Base.connected_to(role: :writing) do
+            ActiveRecord::Base.connected_to(role: ActiveRecord::Base.writing_role) do
               instrumenter.instrument("database_selector.active_record.wrote_to_primary") do
                 yield
               ensure

activerecord/lib/active_record/railtie.rb

@@ -197,6 +197,7 @@ class Railtie < Rails::Railtie # :nodoc:
     # and then establishes the connection.
     initializer "active_record.initialize_database" do
       ActiveSupport.on_load(:active_record) do
+        self.connection_handlers = { writing_role => ActiveRecord::Base.default_connection_handler }
         self.configurations = Rails.application.config.database_configuration
         establish_connection
       end

activerecord/test/cases/connection_adapters/connection_handler_test.rb

@@ -382,6 +382,11 @@ def test_remove_connection_should_not_remove_parent
           assert_not_nil ActiveRecord::Base.connection
           assert_same klass2.connection, ActiveRecord::Base.connection
         end
+
+        def test_default_handlers_are_writing_and_reading
+          assert_equal :writing, ActiveRecord::Base.writing_role
+          assert_equal :reading, ActiveRecord::Base.reading_role
+        end
       end
     end
   end

activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb

@@ -126,6 +126,30 @@ def test_switching_connections_via_handler
           ENV["RAILS_ENV"] = previous_env
         end
 
+        def test_establish_connection_using_3_levels_config_with_non_default_handlers
+          previous_env, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], "default_env"
+
+          config = {
+            "default_env" => {
+              "readonly" => { "adapter" => "sqlite3", "database" => "db/readonly.sqlite3" },
+              "primary"  => { "adapter" => "sqlite3", "database" => "db/primary.sqlite3" }
+            }
+          }
+          @prev_configs, ActiveRecord::Base.configurations = ActiveRecord::Base.configurations, config
+
+          ActiveRecord::Base.connects_to(database: { default: :primary, readonly: :readonly })
+
+          assert_not_nil pool = ActiveRecord::Base.connection_handlers[:default].retrieve_connection_pool("primary")
+          assert_equal "db/primary.sqlite3", pool.spec.config[:database]
+
+          assert_not_nil pool = ActiveRecord::Base.connection_handlers[:readonly].retrieve_connection_pool("primary")
+          assert_equal "db/readonly.sqlite3", pool.spec.config[:database]
+        ensure
+          ActiveRecord::Base.configurations = @prev_configs
+          ActiveRecord::Base.establish_connection(:arunit)
+          ENV["RAILS_ENV"] = previous_env
+        end
+
         def test_switching_connections_with_database_url
           previous_env, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], "default_env"
           previous_url, ENV["DATABASE_URL"] = ENV["DATABASE_URL"], "postgres://localhost/foo"
@@ -344,6 +368,24 @@ def test_calling_connected_to_on_a_non_existent_handler_raises
 
         assert_equal "No connection pool with 'primary' found for the 'reading' role.", error.message
       end
+
+      def test_default_handlers_are_writing_and_reading
+        assert_equal :writing, ActiveRecord::Base.writing_role
+        assert_equal :reading, ActiveRecord::Base.reading_role
+      end
+
+      def test_an_application_can_change_the_default_handlers
+        old_writing = ActiveRecord::Base.writing_role
+        old_reading = ActiveRecord::Base.reading_role
+        ActiveRecord::Base.writing_role = :default
+        ActiveRecord::Base.reading_role = :readonly
+
+        assert_equal :default, ActiveRecord::Base.writing_role
+        assert_equal :readonly, ActiveRecord::Base.reading_role
+      ensure
+        ActiveRecord::Base.writing_role = old_writing
+        ActiveRecord::Base.reading_role = old_reading
+      end
     end
   end
 end

activerecord/test/support/connection.rb

@@ -21,6 +21,7 @@ def self.connection_config
   def self.connect
     puts "Using #{connection_name}"
     ActiveRecord::Base.logger = ActiveSupport::Logger.new("debug.log", 0, 100 * 1024 * 1024)
+    ActiveRecord::Base.connection_handlers = { ActiveRecord::Base.writing_role => ActiveRecord::Base.default_connection_handler }
     ActiveRecord::Base.configurations = connection_config
     ActiveRecord::Base.establish_connection :arunit
     ARUnit2Model.establish_connection :arunit2