Add start_stream to /messages (#1648)

syncapi/routing/messages.go

@@ -51,6 +51,7 @@ type messagesReq struct {
 
 type messagesResp struct {
 	Start       string                          `json:"start"`
+	StartStream string                          `json:"start_stream,omitempty"` // NOTSPEC: so clients can hit /messages then immediately /sync with a latest sync token
 	End         string                          `json:"end"`
 	Chunk       []gomatrixserverlib.ClientEvent `json:"chunk"`
 }
@@ -87,7 +88,8 @@ func OnIncomingMessagesRequest(
 	// Pagination tokens.
 	var fromStream *types.StreamingToken
 	fromQuery := req.URL.Query().Get("from")
-	if fromQuery == "" {
+	emptyFromSupplied := fromQuery == ""
+	if emptyFromSupplied {
 		// NOTSPEC: We will pretend they used the latest sync token if no ?from= was provided.
 		// We do this to allow clients to get messages without having to call `/sync` e.g Cerulean
 		currPos := srp.Notifier.CurrentPosition()
@@ -195,14 +197,19 @@ func OnIncomingMessagesRequest(
 		"return_end":   end.String(),
 	}).Info("Responding")
 
-	// Respond with the events.
-	return util.JSONResponse{
-		Code: http.StatusOK,
-		JSON: messagesResp{
+	res := messagesResp{
 		Chunk: clientEvents,
 		Start: start.String(),
 		End:   end.String(),
-		},
+	}
+	if emptyFromSupplied {
+		res.StartStream = fromStream.String()
+	}
+
+	// Respond with the events.
+	return util.JSONResponse{
+		Code: http.StatusOK,
+		JSON: res,
 	}
 }