Basic API for connection switching

This PR adds the ability to 1) connect to multiple databases in a model,
and 2) switch between those connections using a block.

To connect a model to a set of databases for writing and reading use
the following API. This API supercedes `establish_connection`. The
`writing` and `reading` keys represent handler / role names and
`animals` and `animals_replica` represents the database key to look up
the configuration hash from.

```
class AnimalsBase < ApplicationRecord
  connects_to database: { writing: :animals, reading: :animals_replica }
end
```

Inside the application - outside the model declaration - we can switch
connections with a block call to `connected_to`.

If we want to connect to a db that isn't default (ie readonly_slow) we
can connect like this:

Outside the model we may want to connect to a new database (one that is
not in the default writing/reading set) - for example a slow replica for
making slow queries. To do this we have the `connected_to` method that
takes a `database` hash that matches the signature of `connects_to`. The
`connected_to` method also takes a block.

```
AcitveRecord::Base.connected_to(database: { slow_readonly: :primary_replica_slow }) do
  ModelInPrimary.do_something_thats_slow
end
```

For models that are already loaded and connections that are already
connected, `connected_to` doesn't need to pass in a `database` because
you may want to run queries against multiple databases using a specific
role/handler.

In this case `connected_to` can take a `role` and use that to swap on
the connection passed. This simplies queries - and matches how we do it
in GitHub. Once you're connected to the database you don't need to
re-connect, we assume the connection is in the pool and simply pass the
handler we'd like to swap on.

```
ActiveRecord::Base.connected_to(role: :reading) do
  Dog.read_something_from_dog
  ModelInPrimary.do_something_from_model_in_primary
end
```

1 files changed, 193 insertions, 0 deletions

diff --git a/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb b/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
new file mode 100644
index 0000000000..d4e8cbee81
--- /dev/null
+++ b/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
@@ -0,0 +1,193 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "models/person"
+
+module ActiveRecord
+  module ConnectionAdapters
+    class ConnectionHandlersMultiDbTest < ActiveRecord::TestCase
+      self.use_transactional_tests = false
+
+      fixtures :people
+
+      def setup
+        @handlers = { writing: ConnectionHandler.new, reading: ConnectionHandler.new }
+        @rw_handler = @handlers[:writing]
+        @ro_handler = @handlers[:reading]
+        @spec_name = "primary"
+        @rw_pool = @handlers[:writing].establish_connection(ActiveRecord::Base.configurations["arunit"])
+        @ro_pool = @handlers[:reading].establish_connection(ActiveRecord::Base.configurations["arunit"])
+      end
+
+      def teardown
+        ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
+      end
+
+      class MultiConnectionTestModel < ActiveRecord::Base
+      end
+
+      def test_multiple_connection_handlers_works_in_a_threaded_environment
+        tf_writing = Tempfile.open "test_writing"
+        tf_reading = Tempfile.open "test_reading"
+
+        MultiConnectionTestModel.connects_to database: { writing: { database: tf_writing.path, adapter: "sqlite3" }, reading: { database: tf_reading.path, adapter: "sqlite3" } }
+
+        MultiConnectionTestModel.connection.execute("CREATE TABLE `test_1` (connection_role VARCHAR (255))")
+        MultiConnectionTestModel.connection.execute("INSERT INTO test_1 VALUES ('writing')")
+
+        ActiveRecord::Base.connected_to(role: :reading) do
+          MultiConnectionTestModel.connection.execute("CREATE TABLE `test_1` (connection_role VARCHAR (255))")
+          MultiConnectionTestModel.connection.execute("INSERT INTO test_1 VALUES ('reading')")
+        end
+
+        read_latch = Concurrent::CountDownLatch.new
+        write_latch = Concurrent::CountDownLatch.new
+
+        MultiConnectionTestModel.connection
+
+        thread = Thread.new do
+          MultiConnectionTestModel.connection
+
+          write_latch.wait
+          assert_equal "writing", MultiConnectionTestModel.connection.select_value("SELECT connection_role from test_1")
+          read_latch.count_down
+        end
+
+        ActiveRecord::Base.connected_to(role: :reading) do
+          write_latch.count_down
+          assert_equal "reading", MultiConnectionTestModel.connection.select_value("SELECT connection_role from test_1")
+          read_latch.wait
+        end
+
+        thread.join
+      ensure
+        tf_reading.close
+        tf_reading.unlink
+        tf_writing.close
+        tf_writing.unlink
+      end
+
+      unless in_memory_db?
+        def test_establish_connection_using_3_levels_config
+          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: { writing: :primary, reading: :readonly })
+
+          assert_not_nil pool = ActiveRecord::Base.connection_handlers[:writing].retrieve_connection_pool("primary")
+          assert_equal "db/primary.sqlite3", pool.spec.config[:database]
+
+          assert_not_nil pool = ActiveRecord::Base.connection_handlers[:reading].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_via_handler
+          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: { writing: :primary, reading: :readonly })
+
+          ActiveRecord::Base.connected_to(role: :reading) do
+            @ro_handler = ActiveRecord::Base.connection_handler
+            assert_equal ActiveRecord::Base.connection_handler, ActiveRecord::Base.connection_handlers[:reading]
+          end
+
+          ActiveRecord::Base.connected_to(role: :writing) do
+            assert_equal ActiveRecord::Base.connection_handler, ActiveRecord::Base.connection_handlers[:writing]
+            assert_not_equal @ro_handler, ActiveRecord::Base.connection_handler
+          end
+        ensure
+          ActiveRecord::Base.configurations = @prev_configs
+          ActiveRecord::Base.establish_connection(:arunit)
+          ENV["RAILS_ENV"] = previous_env
+        end
+
+        def test_connects_to_with_single_configuration
+          config = {
+            "development" => { "adapter" => "sqlite3", "database" => "db/primary.sqlite3" },
+          }
+          @prev_configs, ActiveRecord::Base.configurations = ActiveRecord::Base.configurations, config
+
+          ActiveRecord::Base.connects_to database: { writing: :development }
+
+          assert_equal 1, ActiveRecord::Base.connection_handlers.size
+          assert_equal ActiveRecord::Base.connection_handler, ActiveRecord::Base.connection_handlers[:writing]
+        ensure
+          ActiveRecord::Base.configurations = @prev_configs
+          ActiveRecord::Base.establish_connection(:arunit)
+        end
+
+        def test_connects_to_using_top_level_key_in_two_level_config
+          config = {
+            "development" => { "adapter" => "sqlite3", "database" => "db/primary.sqlite3" },
+            "development_readonly" => { "adapter" => "sqlite3", "database" => "db/readonly.sqlite3" }
+          }
+          @prev_configs, ActiveRecord::Base.configurations = ActiveRecord::Base.configurations, config
+
+          ActiveRecord::Base.connects_to database: { writing: :development, reading: :development_readonly }
+
+          assert_not_nil pool = ActiveRecord::Base.connection_handlers[:reading].retrieve_connection_pool("primary")
+          assert_equal "db/readonly.sqlite3", pool.spec.config[:database]
+        ensure
+          ActiveRecord::Base.configurations = @prev_configs
+          ActiveRecord::Base.establish_connection(:arunit)
+        end
+      end
+
+      def test_connection_pools
+        assert_equal([@rw_pool], @handlers[:writing].connection_pools)
+        assert_equal([@ro_pool], @handlers[:reading].connection_pools)
+      end
+
+      def test_retrieve_connection
+        assert @rw_handler.retrieve_connection(@spec_name)
+        assert @ro_handler.retrieve_connection(@spec_name)
+      end
+
+      def test_active_connections?
+        assert_not_predicate @rw_handler, :active_connections?
+        assert_not_predicate @ro_handler, :active_connections?
+
+        assert @rw_handler.retrieve_connection(@spec_name)
+        assert @ro_handler.retrieve_connection(@spec_name)
+
+        assert_predicate @rw_handler, :active_connections?
+        assert_predicate @ro_handler, :active_connections?
+
+        @rw_handler.clear_active_connections!
+        assert_not_predicate @rw_handler, :active_connections?
+
+        @ro_handler.clear_active_connections!
+        assert_not_predicate @ro_handler, :active_connections?
+      end
+
+      def test_retrieve_connection_pool
+        assert_not_nil @rw_handler.retrieve_connection_pool(@spec_name)
+        assert_not_nil @ro_handler.retrieve_connection_pool(@spec_name)
+      end
+
+      def test_retrieve_connection_pool_with_invalid_id
+        assert_nil @rw_handler.retrieve_connection_pool("foo")
+        assert_nil @ro_handler.retrieve_connection_pool("foo")
+      end
+    end
+  end
+end