fix: add validation for the each values

Signed-off-by: Juan Pablo Garcia Ripa <sarabadu@gmail.com>

.changeset/fair-rocks-dream.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-scaffolder-backend': patch
+---
+
+add validation for `each` values

plugins/scaffolder-backend/src/scaffolder/tasks/NunjucksWorkflowRunner.test.ts

@@ -988,6 +988,27 @@ describe('NunjucksWorkflowRunner', () => {
       );
       expect(fakeActionHandler).not.toHaveBeenCalled();
     });
+
+    it('should validate each parameter renders to a valid value', async () => {
+      const task = createMockTaskWithSpec({
+        apiVersion: 'scaffolder.backstage.io/v1beta3',
+        steps: [
+          {
+            id: 'test',
+            name: 'name',
+            each: '${{parameters.data}}',
+            action: 'jest-validated-action',
+            input: { foo: '${{each.value}}' },
+          },
+        ],
+        output: {},
+        parameters: {},
+      });
+      await expect(runner.execute(task)).rejects.toThrow(
+        'Invalid each value passed to action jest-validated-action, "${{parameters.data}}" cannot be resolved to a value',
+      );
+      expect(fakeActionHandler).not.toHaveBeenCalled();
+    });
   });
 
   describe('secrets', () => {

plugins/scaffolder-backend/src/scaffolder/tasks/NunjucksWorkflowRunner.ts

@@ -304,13 +304,21 @@ export class NunjucksWorkflowRunner implements WorkflowRunner {
           return;
         }
       }
+
+      const resolvedEach =
+        step.each && this.render(step.each, context, renderTemplate);
+
+      if (step.each && !resolvedEach) {
+        throw new InputError(
+          `Invalid each value passed to action ${action.id}, "${step.each}" cannot be resolved to a value`,
+        );
+      }
+
       const iterations = (
-        step.each
-          ? Object.entries(this.render(step.each, context, renderTemplate)).map(
-              ([key, value]) => ({
-                each: { key, value },
-              }),
-            )
+        resolvedEach
+          ? Object.entries(resolvedEach).map(([key, value]) => ({
+              each: { key, value },
+            }))
           : [{}]
       ).map(i => ({
         ...i,