frontend-app-api: reject undeclared attachments

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

.changeset/five-hornets-provide.md

@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-app-api': patch
+---
+
+The app will now reject any extensions that attach to nonexistent inputs.

packages/frontend-app-api/src/wiring/createExtensionInstance.test.ts

@@ -250,6 +250,49 @@ describe('createExtensionInstance', () => {
     );
   });
 
+  it('should refuse to create an instance with undeclared inputs', () => {
+    expect(() =>
+      createExtensionInstance({
+        attachments: new Map([
+          [
+            'declared',
+            [
+              createExtensionInstance({
+                attachments: new Map(),
+                config: { output: 'many1' },
+                extension: simpleExtension,
+              }),
+            ],
+          ],
+          [
+            'undeclared',
+            [
+              createExtensionInstance({
+                attachments: new Map(),
+                config: { output: 'many1' },
+                extension: simpleExtension,
+              }),
+            ],
+          ],
+        ]),
+        config: undefined,
+        extension: createExtension({
+          id: 'core.test',
+          attachTo: { id: 'ignored', input: 'ignored' },
+          inputs: {
+            declared: createExtensionInput({
+              test: testDataRef,
+            }),
+          },
+          output: {},
+          factory() {},
+        }),
+      }),
+    ).toThrow(
+      "Failed to instantiate extension 'core.test', received undeclared input(s) 'undeclared'",
+    );
+  });
+
   it('should refuse to create an instance with multiple inputs for required singleton', () => {
     expect(() =>
       createExtensionInstance({

packages/frontend-app-api/src/wiring/createExtensionInstance.ts

@@ -60,6 +60,15 @@ function resolveInputs(
   inputMap: AnyExtensionInputMap,
   attachments: Map<string, ExtensionInstance[]>,
 ) {
+  const undeclaredAttachments = Array.from(attachments.keys()).filter(
+    name => inputMap[name] === undefined,
+  );
+  if (undeclaredAttachments.length > 0) {
+    throw new Error(
+      `received undeclared input(s) '${undeclaredAttachments.join("', '")}'`,
+    );
+  }
+
   return mapValues(inputMap, (input, inputName) => {
     const attachedInstances = attachments.get(inputName) ?? [];
     if (input.config.singleton) {