fix: signal disconnect loop on server start

this fixes the disconnect loop when the server has just started by
waiting for the connection to open before subsribing to events.

also includes other improvements:
- rename types to interfaces
- allow to input single receiver for signal
- use discovery api to check for upgrade path instead hard coded one

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

plugins/signals-react/api-report.md

@@ -7,18 +7,23 @@ import { ApiRef } from '@backstage/core-plugin-api';
 import { JsonObject } from '@backstage/types';
 
 // @public (undocumented)
-export type SignalApi = {
+export interface SignalApi {
+  // (undocumented)
   subscribe(
     channel: string,
     onMessage: (message: JsonObject) => void,
-  ): {
-    unsubscribe: () => void;
-  };
-};
+  ): SignalSubscriber;
+}
 
 // @public (undocumented)
 export const signalApiRef: ApiRef<SignalApi>;
 
+// @public (undocumented)
+export interface SignalSubscriber {
+  // (undocumented)
+  unsubscribe(): void;
+}
+
 // @public (undocumented)
 export const useSignal: (channel: string) => {
   lastSignal: JsonObject | null;

plugins/signals-react/src/api/SignalApi.ts

@@ -22,9 +22,14 @@ export const signalApiRef = createApiRef<SignalApi>({
 });
 
 /** @public */
-export type SignalApi = {
+export interface SignalSubscriber {
+  unsubscribe(): void;
+}
+
+/** @public */
+export interface SignalApi {
   subscribe(
     channel: string,
     onMessage: (message: JsonObject) => void,
-  ): { unsubscribe: () => void };
-};
+  ): SignalSubscriber;
+}