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/api/api.go

@@ -0,0 +1,64 @@
+// 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 api
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+type CurrentStateInternalAPI interface {
+	QueryCurrentState(ctx context.Context, req *QueryCurrentStateRequest, res *QueryCurrentStateResponse) error
+}
+
+type QueryCurrentStateRequest struct {
+	RoomID      string
+	StateTuples []gomatrixserverlib.StateKeyTuple
+}
+
+type QueryCurrentStateResponse struct {
+	StateEvents map[gomatrixserverlib.StateKeyTuple]gomatrixserverlib.HeaderedEvent
+}
+
+// MarshalJSON stringifies the StateKeyTuple keys so they can be sent over the wire in HTTP API mode.
+func (r *QueryCurrentStateResponse) MarshalJSON() ([]byte, error) {
+	se := make(map[string]gomatrixserverlib.HeaderedEvent, len(r.StateEvents))
+	for k, v := range r.StateEvents {
+		// use 0x1F (unit separator) as the delimiter between type/state key,
+		se[fmt.Sprintf("%s\x1F%s", k.EventType, k.StateKey)] = v
+	}
+	return json.Marshal(se)
+}
+
+func (r *QueryCurrentStateResponse) UnmarshalJSON(data []byte) error {
+	res := make(map[string]gomatrixserverlib.HeaderedEvent)
+	err := json.Unmarshal(data, &res)
+	if err != nil {
+		return err
+	}
+	r.StateEvents = make(map[gomatrixserverlib.StateKeyTuple]gomatrixserverlib.HeaderedEvent, len(res))
+	for k, v := range res {
+		fields := strings.Split(k, "\x1F")
+		r.StateEvents[gomatrixserverlib.StateKeyTuple{
+			EventType: fields[0],
+			StateKey:  fields[1],
+		}] = v
+	}
+	return nil
+}