backend-app-api: nicer error messaging when service deps are missing

Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/tiny-rocks-flash.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-app-api': patch
+---
+
+Improved error messaging when failing to instantiate services.

packages/backend-app-api/src/wiring/ServiceRegistry.test.ts

@@ -220,4 +220,24 @@ describe('ServiceRegistry', () => {
     expect(innerFactory).toHaveBeenCalledWith('catalog');
     expect(innerFactory).toHaveBeenCalledWith('scaffolder');
   });
+
+  it('should throw if dependencies are not available', async () => {
+    const myFactory = createServiceFactory({
+      service: ref1,
+      deps: { dep: ref2 },
+      async factory({ dep }) {
+        return async pluginId => {
+          const d = await dep(pluginId);
+          return { x: d.x, pluginId };
+        };
+      },
+    });
+
+    const registry = new ServiceRegistry([myFactory]);
+    const factory = registry.get(ref1)!;
+
+    await expect(factory('catalog')).rejects.toThrow(
+      "Failed to instantiate service '1' for 'catalog'. The following dependent services are missing: '2'",
+    );
+  });
 });

packages/backend-app-api/src/wiring/ServiceRegistry.ts

@@ -56,12 +56,24 @@ export class ServiceRegistry {
 
       let implementation = this.#implementations.get(factory);
       if (!implementation) {
-        const factoryDeps = Object.fromEntries(
-          Object.entries(factory.deps).map(([name, serviceRef]) => [
-            name,
-            this.get(serviceRef)!, // TODO: throw
-          ]),
-        );
+        const missingRefs = new Array<ServiceRef<unknown>>();
+        const factoryDeps: { [name in string]: FactoryFunc<unknown> } = {};
+
+        for (const [name, serviceRef] of Object.entries(factory.deps)) {
+          const target = this.get(serviceRef);
+          if (!target) {
+            missingRefs.push(serviceRef);
+          } else {
+            factoryDeps[name] = target;
+          }
+        }
+
+        if (missingRefs.length) {
+          const missing = missingRefs.map(r => `'${r.id}'`).join(', ');
+          throw new Error(
+            `Failed to instantiate service '${ref.id}' for '${pluginId}'. The following dependent services are missing: ${missing}`,
+          );
+        }
 
         implementation = {
           factoryFunc: factory.factory(factoryDeps),