Add .well-known/matrix/client to clientapi (#2551)

Signed-off-by: Jonathan Bartlett <jonathan@jonnobrow.co.uk>

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

clientapi/clientapi.go

@@ -59,6 +59,7 @@ func AddPublicRoutes(
 
 	routing.Setup(
 		base.PublicClientAPIMux,
+		base.PublicWellKnownAPIMux,
 		base.SynapseAdminMux,
 		base.DendriteAdminMux,
 		cfg, rsAPI, asAPI,

clientapi/routing/routing.go

@@ -48,7 +48,7 @@ import (
 // applied:
 // nolint: gocyclo
 func Setup(
-	publicAPIMux, synapseAdminRouter, dendriteAdminRouter *mux.Router,
+	publicAPIMux, wkMux, synapseAdminRouter, dendriteAdminRouter *mux.Router,
 	cfg *config.ClientAPI,
 	rsAPI roomserverAPI.ClientRoomserverAPI,
 	asAPI appserviceAPI.AppServiceInternalAPI,
@@ -74,6 +74,26 @@ func Setup(
 		unstableFeatures["org.matrix."+msc] = true
 	}
 
+	if cfg.Matrix.WellKnownClientName != "" {
+		logrus.Infof("Setting m.homeserver base_url as %s at /.well-known/matrix/client", cfg.Matrix.WellKnownClientName)
+		wkMux.Handle("/client", httputil.MakeExternalAPI("wellknown", func(r *http.Request) util.JSONResponse {
+			return util.JSONResponse{
+				Code: http.StatusOK,
+				JSON: struct {
+					HomeserverName struct {
+						BaseUrl string `json:"base_url"`
+					} `json:"m.homeserver"`
+				}{
+					HomeserverName: struct {
+						BaseUrl string `json:"base_url"`
+					}{
+						BaseUrl: cfg.Matrix.WellKnownClientName,
+					},
+				},
+			}
+		})).Methods(http.MethodGet, http.MethodOptions)
+	}
+
 	publicAPIMux.Handle("/versions",
 		httputil.MakeExternalAPI("versions", func(req *http.Request) util.JSONResponse {
 			return util.JSONResponse{