a very very WIP first cut of peeking via MSC2753.

doesn't yet compile or work.
needs to actually add the peeking block into the sync response.
checking in now before it gets any bigger, and to gather any initial feedback on the vague shape of it.

clientapi/routing/joinroom.go

@@ -41,7 +41,7 @@ func JoinRoomByIDOrAlias(
 	}
 	joinRes := roomserverAPI.PerformJoinResponse{}
 
-	// If content was provided in the request then incude that
+	// If content was provided in the request then include that
 	// in the request. It'll get used as a part of the membership
 	// event content.
 	_ = httputil.UnmarshalJSONRequest(req, &joinReq.Content)

clientapi/routing/peekroom.go

@@ -0,0 +1,71 @@
+// Copyright 2020 New Vector Ltd
+//
+// 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 routing
+
+import (
+	"net/http"
+
+	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
+	"github.com/matrix-org/dendrite/clientapi/httputil"
+	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/userapi/api"
+	"github.com/matrix-org/dendrite/userapi/storage/accounts"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+)
+
+func PeekRoomByIDOrAlias(
+	req *http.Request,
+	device *api.Device,
+	rsAPI roomserverAPI.RoomserverInternalAPI,
+	accountDB accounts.Database,
+	roomIDOrAlias string,
+) util.JSONResponse {
+	// if this is a remote roomIDOrAlias, we have to ask the roomserver (or federation sender?) to
+	// to call /peek and /state on the remote server.
+	// TODO: in future we could skip this if we know we're already participating in the room,
+	// but this is fiddly in case we stop participating in the room.
+
+	// then we create a local peek.
+	peekReq := roomserverAPI.PerformPeekRequest{
+		RoomIDOrAlias: roomIDOrAlias,
+		UserID:        device.UserID,
+		DeviceID:	   device.ID,
+	}
+	peekRes := roomserverAPI.PerformPeekResponse{}
+
+	// Ask the roomserver to perform the join.
+	rsAPI.PerformPeek(req.Context(), &peekReq, &peekRes)
+	if peekRes.Error != nil {
+		return peekRes.Error.JSONResponse()
+	}
+
+	// if this user is already joined to the room, we let them peek anyway
+	// (given they might be about to part the room, and it makes things less fiddly)
+
+
+	// Peeking stops if none of the devices who started peeking have been
+	// /syncing for a while, or if everyone who was peeking calls /leave
+	// (or /unpeek with a server_name param? or DELETE /peek?)
+	// on the peeked room.
+
+	return util.JSONResponse{
+		Code: http.StatusOK,
+		// TODO: Put the response struct somewhere internal.
+		JSON: struct {
+			RoomID string `json:"room_id"`
+		}{joinRes.RoomID},
+	}
+}

clientapi/routing/routing.go

@@ -103,6 +103,17 @@ func Setup(
 			)
 		}),
 	).Methods(http.MethodPost, http.MethodOptions)
+	r0mux.Handle("/peek/{roomIDOrAlias}",
+		httputil.MakeAuthAPI(gomatrixserverlib.Join, userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {
+			vars, err := httputil.URLDecodeMapValues(mux.Vars(req))
+			if err != nil {
+				return util.ErrorResponse(err)
+			}
+			return PeekRoomByIDOrAlias(
+				req, device, rsAPI, accountDB, vars["roomIDOrAlias"],
+			)
+		}),
+	).Methods(http.MethodPost, http.MethodOptions)
 	r0mux.Handle("/joined_rooms",
 		httputil.MakeAuthAPI("joined_rooms", userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {
 			return GetJoinedRooms(req, device, stateAPI)