Unified Storage: Don't read before create (#102906)

* Unified Storage: Don't read before create

* test: use the existing test infra

* fix: support pq

We use pgx, but it seems to be wrapped in a pq driver shim, causing the errors to be remapped to pq's type. Weird
situation.

* feat: support CDK backend

* revert: there is a postgres_tests block

* fix(CDK): only check existence on ADDED updates

* fix(CDK): use ReadResource to deal with deleted files

pkg/storage/unified/resource/server.go

@@ -476,22 +476,14 @@ func (s *server) Create(ctx context.Context, req *CreateRequest) (*CreateRespons
 		return rsp, nil
 	}
 
-	found := s.backend.ReadResource(ctx, &ReadRequest{Key: req.Key})
-	if found != nil && len(found.Value) > 0 {
-		rsp.Error = &ErrorResult{
-			Code:    http.StatusConflict,
-			Reason:  string(metav1.StatusReasonAlreadyExists),
-			Message: "key already exists", // TODO?? soft delete replace?
-		}
-		return rsp, nil
-	}
-
 	event, e := s.newEvent(ctx, user, req.Key, req.Value, nil)
 	if e != nil {
 		rsp.Error = e
 		return rsp, nil
 	}
 
+	// If the resource already exists, the create will return an already exists error that is remapped appropriately by AsErrorResult.
+	// This also benefits from ACID behaviours on our databases, so we avoid race conditions.
 	var err error
 	rsp.ResourceVersion, err = s.backend.WriteEvent(ctx, *event)
 	if err != nil {