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/inthttp/client.go

@@ -0,0 +1,62 @@
+// 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 inthttp
+
+import (
+	"context"
+	"errors"
+	"net/http"
+
+	"github.com/matrix-org/dendrite/currentstateserver/api"
+	"github.com/matrix-org/dendrite/internal/httputil"
+	"github.com/opentracing/opentracing-go"
+)
+
+// HTTP paths for the internal HTTP APIs
+const (
+	QueryCurrentStatePath = "/currentstateserver/queryCurrentState"
+)
+
+// NewCurrentStateAPIClient creates a CurrentStateInternalAPI implemented by talking to a HTTP POST API.
+// If httpClient is nil an error is returned
+func NewCurrentStateAPIClient(
+	apiURL string,
+	httpClient *http.Client,
+) (api.CurrentStateInternalAPI, error) {
+	if httpClient == nil {
+		return nil, errors.New("NewCurrentStateAPIClient: httpClient is <nil>")
+	}
+	return &httpCurrentStateInternalAPI{
+		apiURL:     apiURL,
+		httpClient: httpClient,
+	}, nil
+}
+
+type httpCurrentStateInternalAPI struct {
+	apiURL     string
+	httpClient *http.Client
+}
+
+func (h *httpCurrentStateInternalAPI) QueryCurrentState(
+	ctx context.Context,
+	request *api.QueryCurrentStateRequest,
+	response *api.QueryCurrentStateResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "QueryCurrentState")
+	defer span.Finish()
+
+	apiURL := h.apiURL + QueryCurrentStatePath
+	return httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}