Add webhook validation for empty database and rabbitmq

[raukadah]

databaseInstance and rabbitMqClusterName are required fields. If an user specify databaseInstance and rabbitMqClusterName field as empty string. The webhook should fail it saying as there cannot be empty.

This pr adds the validations for the same.

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[abays]

Another option is to make these fields pointers and then use the kubebuilder required validation for these. Though I also see some places where databaseInstance and rabbitMqClusterName use kubebuilder validation without using a pointer string [1][2], which seems like it wouldn't work. That is because the defaulting webhooks will set the field to the empty string, and then the kubebuilder validation will be "tricked" into thinking the user explicitly set a value in this manner -- this is a classic problem. Using a pointer prevents this because the zero-value of nil remains intact throughout the webhook processing and thus will properly be treated as a empty value during subsequent kubebuilder validation.

[1] https://github.com/openstack-k8s-operators/keystone-operator/blob/16c3ed8e549f8591dd17c94fcd165620c8a5444f/api/v1beta1/keystoneapi_types.go#L60-L64
[2] https://github.com/openstack-k8s-operators/keystone-operator/blob/16c3ed8e549f8591dd17c94fcd165620c8a5444f/api/v1beta1/keystoneapi_types.go#L177-L181

[raukadah]

Another option is to make these fields pointers and then use the kubebuilder required validation for these. Though I also see some places where databaseInstance and rabbitMqClusterName use kubebuilder validation without using a pointer string [1][2], which seems like it wouldn't work. That is because the defaulting webhooks will set the field to the empty string, and then the kubebuilder validation will be "tricked" into thinking the user explicitly set a value in this manner -- this is a classic problem. Using a pointer prevents this because the zero-value of nil remains intact throughout the webhook processing and thus will properly be treated as a empty value during subsequent kubebuilder validation.

[1] https://github.com/openstack-k8s-operators/keystone-operator/blob/16c3ed8e549f8591dd17c94fcd165620c8a5444f/api/v1beta1/keystoneapi_types.go#L60-L64 [2] https://github.com/openstack-k8s-operators/keystone-operator/blob/16c3ed8e549f8591dd17c94fcd165620c8a5444f/api/v1beta1/keystoneapi_types.go#L177-L181

Thank you for the suggestion @abays , I have added them in existing patch. But while running tests, I am hitting following error:

chandankumar@raukadahlaptop:~/projects/watcher-operator$ make test
test -f go.work || GOTOOLCHAIN=go1.21.0 go work init
go work use .
go work use ./api
go work sync
test -s /home/chandankumar/projects/watcher-operator/bin/controller-gen && /home/chandankumar/projects/watcher-operator/bin/controller-gen --version | grep -q v0.14.0 || \
GOBIN=/home/chandankumar/projects/watcher-operator/bin go install sigs.k8s.io/controller-tools/cmd/[email protected]
/home/chandankumar/projects/watcher-operator/bin/controller-gen rbac:roleName=manager-role crd webhook paths="./..." output:crd:artifacts:config=config/crd/bases && \
 	rm -f api/bases/* && cp -a config/crd/bases api/
/home/chandankumar/projects/watcher-operator/bin/controller-gen object:headerFile="hack/boilerplate.go.txt" paths="./..."
go fmt ./...
go vet ./...
# github.com/openstack-k8s-operators/watcher-operator/tests/functional
vet: tests/functional/watcher_controller_test.go:199:6: cannot use GetWatcher(watcherTest.Instance).Spec.DatabaseInstance (variable of type *string) as string value in argument to mariadb.CreateDBService
make: *** [Makefile:127: vet] Error 1

I might have missed something in this change. Need your suggestion here, thank you!