Add configuration option for sliding sync when hosting /.well-known/matrix/client (#3189)

Adds the `org.matrix.msc3575.proxy` field (used for configuring sliding
sync) to /.well-known/matrix/client when Dendrite is serving that
endpoint and `well_known_sliding_sync_proxy` has been configured.

ie. Config values of:
``` yaml
global:
    well_known_client_name: https://example.com
    well_known_sliding_sync_proxy: https://syncv3.example.com
```
results in a /.well-known/matrix/client of:
``` json
{
    "m.homeserver": {
        "base_url": "https://example.com"
    },
    "org.matrix.msc3575.proxy": {
        "url": "https://syncv3.example.com"
    }
}
```

If `well_known_sliding_sync_proxy` is not provided, the json provided by
/.well-known/matrix/client does not include the proxy field.
ie.
``` json
{
    "m.homeserver": {
        "base_url": "https://example.com"
    }
}
```

clientapi/routing/routing.go

@@ -44,6 +44,19 @@ import (
 	"github.com/matrix-org/dendrite/setup/jetstream"
 )
 
+type WellKnownClientHomeserver struct {
+	BaseUrl string `json:"base_url"`
+}
+
+type WellKnownSlidingSyncProxy struct {
+	Url string `json:"url"`
+}
+
+type WellKnownClientResponse struct {
+	Homeserver       WellKnownClientHomeserver  `json:"m.homeserver"`
+	SlidingSyncProxy *WellKnownSlidingSyncProxy `json:"org.matrix.msc3575.proxy,omitempty"`
+}
+
 // Setup registers HTTP handlers with the given ServeMux. It also supplies the given http.Client
 // to clients which need to make outbound HTTP requests.
 //
@@ -96,20 +109,22 @@ func Setup(
 
 	if cfg.Matrix.WellKnownClientName != "" {
 		logrus.Infof("Setting m.homeserver base_url as %s at /.well-known/matrix/client", cfg.Matrix.WellKnownClientName)
+		if cfg.Matrix.WellKnownSlidingSyncProxy != "" {
+			logrus.Infof("Setting org.matrix.msc3575.proxy url as %s at /.well-known/matrix/client", cfg.Matrix.WellKnownSlidingSyncProxy)
+		}
 		wkMux.Handle("/client", httputil.MakeExternalAPI("wellknown", func(r *http.Request) util.JSONResponse {
+			response := WellKnownClientResponse{
+				Homeserver: WellKnownClientHomeserver{cfg.Matrix.WellKnownClientName},
+			}
+			if cfg.Matrix.WellKnownSlidingSyncProxy != "" {
+				response.SlidingSyncProxy = &WellKnownSlidingSyncProxy{
+					Url: cfg.Matrix.WellKnownSlidingSyncProxy,
+				}
+			}
+
 			return util.JSONResponse{
 				Code: http.StatusOK,
-				JSON: struct {
+				JSON: response,
-					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)
 	}

setup/config/config_global.go

@@ -48,6 +48,10 @@ type Global struct {
 	// The server name to delegate client-server communications to, with optional port
 	WellKnownClientName string `yaml:"well_known_client_name"`
 
+	// The server name to delegate sliding sync communications to, with optional port.
+	// Requires `well_known_client_name` to also be configured.
+	WellKnownSlidingSyncProxy string `yaml:"well_known_sliding_sync_proxy"`
+
 	// Disables federation. Dendrite will not be able to make any outbound HTTP requests
 	// to other servers and the federation API will not be exposed.
 	DisableFederation bool `yaml:"disable_federation"`