Harden configuration validation

internal/adapters/storage/factory.go

@@ -14,16 +14,15 @@ func NewObjectStoreFromConfig(ctx context.Context, cfg *config.Config) (ObjectSt
 		return nil, fmt.Errorf("pipeline config is required")
 	}
 
-	if strings.EqualFold(strings.TrimSpace(cfg.Pipeline.Storage.Backend), "s3") {
+	switch strings.ToLower(strings.TrimSpace(cfg.Pipeline.Storage.Backend)) {
+	case config.StorageBackendS3:
 		if cfg.Pipeline.Storage.S3 == nil {
 			return nil, fmt.Errorf("pipeline.storage.s3 is required when pipeline.storage.backend is s3")
 		}
 		return NewS3BackendFromConfig(ctx, *cfg.Pipeline.Storage.S3)
+	case "", config.StorageBackendLocal:
+		return nil, fmt.Errorf("no remote object store backend is configured")
+	default:
+		return nil, fmt.Errorf("unsupported pipeline.storage.backend %q", cfg.Pipeline.Storage.Backend)
 	}
-
-	if cfg.Pipeline.Storage.S3 != nil && strings.TrimSpace(cfg.Pipeline.Storage.S3.Bucket) != "" {
-		return NewS3BackendFromConfig(ctx, *cfg.Pipeline.Storage.S3)
-	}
-
-	return nil, fmt.Errorf("no remote object store backend is configured")
 }

internal/adapters/storage/factory_test.go

@@ -54,3 +54,35 @@ func TestNewObjectStoreFromConfigNoRemoteBackendConfigured(t *testing.T) {
 		t.Fatalf("NewObjectStoreFromConfig() error = %v, want no-backend error", err)
 	}
 }
+
+func TestNewObjectStoreFromConfigDoesNotInferS3FromProviderFields(t *testing.T) {
+	called := false
+	original := newS3Client
+	t.Cleanup(func() { newS3Client = original })
+	newS3Client = func(_ context.Context, _ s3ClientOptions) (s3API, error) {
+		called = true
+		return &fakeS3API{}, nil
+	}
+
+	_, err := NewObjectStoreFromConfig(context.Background(), &config.Config{
+		Pipeline: &config.PipelineConfig{Storage: config.StorageConfig{
+			Backend: config.StorageBackendLocal,
+			S3:      &config.StorageS3Config{Bucket: "my-archive"},
+		}},
+	})
+	if err == nil || !strings.Contains(err.Error(), "no remote object store backend is configured") {
+		t.Fatalf("NewObjectStoreFromConfig() error = %v, want no-backend error", err)
+	}
+	if called {
+		t.Fatal("NewObjectStoreFromConfig() constructed S3 from incidental provider fields")
+	}
+}
+
+func TestNewObjectStoreFromConfigRejectsUnknownBackend(t *testing.T) {
+	_, err := NewObjectStoreFromConfig(context.Background(), &config.Config{
+		Pipeline: &config.PipelineConfig{Storage: config.StorageConfig{Backend: "s33"}},
+	})
+	if err == nil || !strings.Contains(err.Error(), "unsupported pipeline.storage.backend") {
+		t.Fatalf("NewObjectStoreFromConfig() error = %v, want unsupported-backend error", err)
+	}
+}