Add a new component: currentstateserver (#1171)

* Add a new component: currentstateserver

- Add a skeleton for it, with databases and a single query method.
- Add integration tests for it.
- Add listen/address fields in the config (breaking as this will force people to specify this to validate)

Not currently hooked up to anything yet.

* Unbreak config tests

* Add current_state to sample config

* comments

currentstateserver/internal/api.go

@@ -0,0 +1,41 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package internal
+
+import (
+	"context"
+
+	"github.com/matrix-org/dendrite/currentstateserver/api"
+	"github.com/matrix-org/dendrite/currentstateserver/storage"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+type CurrentStateInternalAPI struct {
+	DB storage.Database
+}
+
+func (a *CurrentStateInternalAPI) QueryCurrentState(ctx context.Context, req *api.QueryCurrentStateRequest, res *api.QueryCurrentStateResponse) error {
+	res.StateEvents = make(map[gomatrixserverlib.StateKeyTuple]gomatrixserverlib.HeaderedEvent)
+	for _, tuple := range req.StateTuples {
+		ev, err := a.DB.GetStateEvent(ctx, req.RoomID, tuple.EventType, tuple.StateKey)
+		if err != nil {
+			return err
+		}
+		if ev != nil {
+			res.StateEvents[tuple] = *ev
+		}
+	}
+	return nil
+}