Generic-based internal HTTP API (#2626)

* Generic-based internal HTTP API (tested out on a few endpoints in the federation API)

* Add `PerformInvite`

* More tweaks

* Fix metric name

* Fix LookupStateIDs

* Lots of changes to clients

* Some serverside stuff

* Some error handling

* Use paths as metric names

* Revert "Use paths as metric names"

This reverts commit a9323a6a343f5ce6461a2e5bd570fe06465f1b15.

* Namespace metric names

* Remove duplicate entry

* Remove another duplicate entry

* Tweak error handling

* Some more tweaks

* Update error behaviour

* Some more error tweaking

* Fix API path for `PerformDeleteKeys`

* Fix another path

* Tweak federation client proxying

* Fix another path

* Don't return typed nils

* Some more tweaks, not that it makes any difference

* Tweak federation client proxying

* Maybe fix the key backup test

internal/httputil/internalapi.go

@@ -0,0 +1,93 @@
+// Copyright 2022 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 httputil
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"reflect"
+
+	"github.com/matrix-org/util"
+	opentracing "github.com/opentracing/opentracing-go"
+)
+
+type InternalAPIError struct {
+	Type    string
+	Message string
+}
+
+func (e InternalAPIError) Error() string {
+	return fmt.Sprintf("internal API returned %q error: %s", e.Type, e.Message)
+}
+
+func MakeInternalRPCAPI[reqtype, restype any](metricsName string, f func(context.Context, *reqtype, *restype) error) http.Handler {
+	return MakeInternalAPI(metricsName, func(req *http.Request) util.JSONResponse {
+		var request reqtype
+		var response restype
+		if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+			return util.MessageResponse(http.StatusBadRequest, err.Error())
+		}
+		if err := f(req.Context(), &request, &response); err != nil {
+			return util.JSONResponse{
+				Code: http.StatusInternalServerError,
+				JSON: &InternalAPIError{
+					Type:    reflect.TypeOf(err).String(),
+					Message: fmt.Sprintf("%s", err),
+				},
+			}
+		}
+		return util.JSONResponse{
+			Code: http.StatusOK,
+			JSON: &response,
+		}
+	})
+}
+
+func MakeInternalProxyAPI[reqtype, restype any](metricsName string, f func(context.Context, *reqtype) (*restype, error)) http.Handler {
+	return MakeInternalAPI(metricsName, func(req *http.Request) util.JSONResponse {
+		var request reqtype
+		if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+			return util.MessageResponse(http.StatusBadRequest, err.Error())
+		}
+		response, err := f(req.Context(), &request)
+		if err != nil {
+			return util.JSONResponse{
+				Code: http.StatusInternalServerError,
+				JSON: err,
+			}
+		}
+		return util.JSONResponse{
+			Code: http.StatusOK,
+			JSON: response,
+		}
+	})
+}
+
+func CallInternalRPCAPI[reqtype, restype any](name, url string, client *http.Client, ctx context.Context, request *reqtype, response *restype) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, name)
+	defer span.Finish()
+
+	return PostJSON[reqtype, restype, InternalAPIError](ctx, span, client, url, request, response)
+}
+
+func CallInternalProxyAPI[reqtype, restype any, errtype error](name, url string, client *http.Client, ctx context.Context, request *reqtype) (restype, error) {
+	span, ctx := opentracing.StartSpanFromContext(ctx, name)
+	defer span.Finish()
+
+	var response restype
+	return response, PostJSON[reqtype, restype, errtype](ctx, span, client, url, request, &response)
+}