From 1f384c56443fc553ca6e3f72db009a1a524d1815 Mon Sep 17 00:00:00 2001
From: Patrik Oldsberg <poldsberg@gmail.com>
Date: Fri, 2 Sep 2022 11:24:17 +0200
Subject: [PATCH] backend-app-api: nicer error messaging when service deps are
 missing
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Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
---
 .changeset/tiny-rocks-flash.md                |  5 ++++
 .../src/wiring/ServiceRegistry.test.ts        | 20 ++++++++++++++++
 .../src/wiring/ServiceRegistry.ts             | 24 ++++++++++++++-----
 3 files changed, 43 insertions(+), 6 deletions(-)
 create mode 100644 .changeset/tiny-rocks-flash.md

diff --git a/.changeset/tiny-rocks-flash.md b/.changeset/tiny-rocks-flash.md
new file mode 100644
index 0000000000..32b467a6af
--- /dev/null
+++ b/.changeset/tiny-rocks-flash.md
@@ -0,0 +1,5 @@
+---
+'@backstage/backend-app-api': patch
+---
+
+Improved error messaging when failing to instantiate services.
diff --git a/packages/backend-app-api/src/wiring/ServiceRegistry.test.ts b/packages/backend-app-api/src/wiring/ServiceRegistry.test.ts
index d5b688c0de..94ec77eea8 100644
--- a/packages/backend-app-api/src/wiring/ServiceRegistry.test.ts
+++ b/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'",
+    );
+  });
 });
diff --git a/packages/backend-app-api/src/wiring/ServiceRegistry.ts b/packages/backend-app-api/src/wiring/ServiceRegistry.ts
index 9df092db42..f8d99bffd8 100644
--- a/packages/backend-app-api/src/wiring/ServiceRegistry.ts
+++ b/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),