Add whoami endpoint support

Signed-off-by: Anant Prakash <anantprakashjsr@gmail.com>

src/github.com/matrix-org/dendrite/clientapi/routing/routing.go

@@ -379,6 +379,12 @@ func Setup(
 		}),
 	).Methods(http.MethodPut, http.MethodOptions)
 
+	r0mux.Handle("/account/whoami",
+		common.MakeAuthAPI("whoami", accountDB, deviceDB, cfg.Derived.ApplicationServices, func(req *http.Request, user string, device *authtypes.Device) util.JSONResponse {
+			return Whoami(req, user)
+		}),
+	).Methods(http.MethodGet, http.MethodOptions)
+
 	// Stub implementations for sytest
 	r0mux.Handle("/events",
 		common.MakeExternalAPI("events", func(req *http.Request) util.JSONResponse {

src/github.com/matrix-org/dendrite/clientapi/routing/whoami.go

@@ -0,0 +1,46 @@
+// 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/jsonerror"
+	"github.com/matrix-org/util"
+)
+
+// whoamiResponse represents an response for a `whoami` request
+type whoamiResponse struct {
+	UserID string `json:"user_id"`
+}
+
+// Whoami implements `/account/whoami` which enables client to query owner of the HTTP request.
+// https://matrix.org/docs/spec/client_server/r0.3.0.html#get-matrix-client-r0-account-whoami
+func Whoami(req *http.Request, userID string) util.JSONResponse {
+
+	if req.Method == http.MethodGet {
+		util.GetLogger(req.Context()).WithField("user", userID).Info("Processing whoami request")
+
+		return util.JSONResponse{
+			Code: http.StatusOK,
+			JSON: whoamiResponse{
+				UserID: userID,
+			},
+		}
+	}
+
+	return util.JSONResponse{
+		Code: http.StatusMethodNotAllowed,
+		JSON: jsonerror.NotFound("Bad method"),
+	}
+}