backend-plugin-api: backwards compat for service ref multiton field

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/backend-plugin-api/api-report.md

@@ -703,7 +703,7 @@ export type ServiceRef<
 > = {
   id: string;
   scope: TScope;
-  multiton: TInstances extends 'multiton' ? true : false;
+  multiton?: TInstances extends 'multiton' ? true : false;
   T: TService;
   $$type: '@backstage/ServiceRef';
 };

packages/backend-plugin-api/src/services/system/types.test.ts

@@ -324,6 +324,24 @@ describe('createServiceFactory', () => {
     metaFactory();
   });
 
+  it('should support old service refs without a multiton field', () => {
+    const oldPluginDep = pluginDep as Omit<typeof pluginDep, 'multiton'>; // Old refs don't have a multiton field
+    const metaFactory = createServiceFactory({
+      service: ref,
+      deps: {
+        plugin: oldPluginDep,
+      },
+      async factory({ plugin }) {
+        const plugin1: boolean = plugin;
+        // @ts-expect-error
+        const plugin2: number = plugin;
+        unused(plugin1, plugin2);
+        return 'x';
+      },
+    });
+    expect(metaFactory).toEqual(expect.any(Function));
+  });
+
   it('should only allow objects as options', () => {
     // @ts-expect-error
     const metaFactory = createServiceFactory((_opts: string) => ({

packages/backend-plugin-api/src/services/system/types.ts

@@ -39,7 +39,7 @@ export type ServiceRef<
    */
   scope: TScope;
 
-  multiton: TInstances extends 'multiton' ? true : false;
+  multiton?: TInstances extends 'multiton' ? true : false;
 
   /**
    * Utility for getting the type of the service, using `typeof serviceRef.T`.
@@ -189,7 +189,7 @@ type ServiceRefsToInstances<
 > = {
   [key in keyof T as T[key]['scope'] extends TScope
     ? key
-    : never]: T[key]['multiton'] extends true
+    : never]: T[key]['multiton'] extends true | undefined
     ? Array<T[key]['T']>
     : T[key]['T'];
 };