pkg/resource: add AdditiveMergePatchApplyOption

Signed-off-by: Dr. Stefan Schimanski <stefan.schimanski@upbound.io>

go.mod

@@ -5,6 +5,7 @@ go 1.20
 require (
 	dario.cat/mergo v1.0.0
 	github.com/bufbuild/buf v1.26.1
+	github.com/evanphx/json-patch v5.6.0+incompatible
 	github.com/go-logr/logr v1.2.4
 	github.com/google/go-cmp v0.5.9
 	github.com/spf13/afero v1.9.5

go.sum

@@ -96,6 +96,7 @@ github.com/envoyproxy/go-control-plane v0.9.7/go.mod h1:cwu0lG7PUMfa9snN8LXBig5y
 github.com/envoyproxy/go-control-plane v0.9.9-0.20201210154907-fd9021fe5dad/go.mod h1:cXg6YxExXjJnVBQHBLXeUAgxn2UodCpnH306RInaBQk=
 github.com/envoyproxy/protoc-gen-validate v0.1.0/go.mod h1:iSmxcyjqTsJpI2R4NaDN7+kN2VEUnK/pcBlmesArF7c=
 github.com/evanphx/json-patch v5.6.0+incompatible h1:jBYDEEiFBPxA0v50tFdvOzQQTCvpL6mnFh5mB2/l16U=
+github.com/evanphx/json-patch v5.6.0+incompatible/go.mod h1:50XU6AFN0ol/bzJsmQLiYLvXMP4fmwYFNcr97nuDLSk=
 github.com/evanphx/json-patch/v5 v5.6.0 h1:b91NhWfaz02IuVxO9faSllyAtNXHMPkC5J8sJCLunww=
 github.com/evanphx/json-patch/v5 v5.6.0/go.mod h1:G79N1coSVB93tBe7j6PhzjmR3/2VvlbKOFpnXhI9Bw4=
 github.com/fatih/color v1.15.0 h1:kOqh6YHBtK8aywxGerMG2Eq3H6Qgoqeo13Bk2Mv/nBs=

pkg/resource/api.go

@@ -17,10 +17,14 @@ limitations under the License.
 package resource
 
 import (
+	"bytes"
 	"context"
 
+	jsonpatch "github.com/evanphx/json-patch"
 	kerrors "k8s.io/apimachinery/pkg/api/errors"
+	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/apimachinery/pkg/runtime/serializer/json"
 	"k8s.io/apimachinery/pkg/types"
 	"sigs.k8s.io/controller-runtime/pkg/client"
 
@@ -54,7 +58,7 @@ func NewAPIPatchingApplicator(c client.Client) *APIPatchingApplicator {
 // Apply changes to the supplied object. The object will be created if it does
 // not exist, or patched if it does. If the object does exist, it will only be
 // patched if the passed object has the same or an empty resource version.
-func (a *APIPatchingApplicator) Apply(ctx context.Context, obj client.Object, ao ...ApplyOption) error { //nolint:gocyclo // the logic here is crucial and deserves to stay in one method
+func (a *APIPatchingApplicator) Apply(ctx context.Context, obj client.Object, ao ...ApplyOption) error {
 	if obj.GetName() == "" && obj.GetGenerateName() != "" {
 		return a.client.Create(ctx, obj)
 	}
@@ -102,6 +106,50 @@ func groupResource(c client.Client, o client.Object) (schema.GroupResource, erro
 	return m.Resource.GroupResource(), nil
 }
 
+var emptyScheme = runtime.NewScheme() // no need to recognize any types
+var jsonSerializer = json.NewSerializerWithOptions(json.DefaultMetaFactory, emptyScheme, emptyScheme, json.SerializerOptions{})
+
+// AdditiveMergePatchApplyOption returns an ApplyOption that makes
+// the Apply additive in the sense of a merge patch without null values. This is
+// the old behavior of the APIPatchingApplicator.
+//
+// This only works with a desired object of type *unstructured.Unstructured.
+//
+// Deprecated: replace with Server Side Apply.
+func AdditiveMergePatchApplyOption(_ context.Context, current, desired runtime.Object) error {
+	// set GVK uniformly to the desired object to make serializer happy
+	currentGVK, desiredGVK := current.GetObjectKind().GroupVersionKind(), desired.GetObjectKind().GroupVersionKind()
+	if !desiredGVK.Empty() && currentGVK != desiredGVK {
+		return errors.Errorf("cannot apply %v to %v", desired.GetObjectKind().GroupVersionKind(), current.GetObjectKind().GroupVersionKind())
+	}
+	desired.GetObjectKind().SetGroupVersionKind(currentGVK)
+
+	// merge `desired` additively with `current`
+	var currentBytes, desiredBytes bytes.Buffer
+	if err := jsonSerializer.Encode(current, &currentBytes); err != nil {
+		return errors.Wrapf(err, "cannot marshal current %s", HumanReadableReference(nil, current))
+	}
+	if err := jsonSerializer.Encode(desired, &desiredBytes); err != nil {
+		return errors.Wrapf(err, "cannot marshal desired %s", HumanReadableReference(nil, desired))
+	}
+	mergedBytes, err := jsonpatch.MergePatch(currentBytes.Bytes(), desiredBytes.Bytes())
+	if err != nil {
+		return errors.Wrapf(err, "cannot merge patch to %s", HumanReadableReference(nil, desired))
+	}
+
+	// write merged object back to `desired`
+	if _, _, err := jsonSerializer.Decode(mergedBytes, nil, desired); err != nil {
+		return errors.Wrapf(err, "cannot unmarshal merged patch to %s", HumanReadableReference(nil, desired))
+	}
+
+	// restore empty GVK for typed objects
+	if _, isUnstructured := desired.(runtime.Unstructured); !isUnstructured {
+		desired.GetObjectKind().SetGroupVersionKind(schema.GroupVersionKind{})
+	}
+
+	return nil
+}
+
 // An APIUpdatingApplicator applies changes to an object by either creating or
 // updating it in a Kubernetes API server.
 type APIUpdatingApplicator struct {