Should support ServerSideApply=false sync-option

[sathieu]

Currently, SSA is opt-in either at application-level or at resource-level. However, to workaround https://github.com/argoproj/argo-cd/issues/11143, I need to use ServerSideApply=false for a specific resource while SSA is enabled for all other resources.

Currently it does app-enabled || resource-enabled.

https://github.com/argoproj/gitops-engine/blob/98ccd3d43fd9f4dbc3f6610b9701fbf56be53ded/pkg/diff/diff.go#L91

https://github.com/argoproj/gitops-engine/blob/98ccd3d43fd9f4dbc3f6610b9701fbf56be53ded/pkg/diff/diff.go#L98-L99

[sathieu]

Quick and dirty patch:

diff --git a/pkg/diff/diff.go b/pkg/diff/diff.go
--- pkg/diff/diff.go
+++ pkg/diff/diff.go
@@ -87,17 +87,21 @@
 	// dependency with the kube package that blocks the usage of constants
 	// from common package. common package needs to be refactored and exclude
 	// dependency from kube.
 	syncOptAnnotation := "argocd.argoproj.io/sync-options"
-	ssaAnnotation := "ServerSideApply=true"
+	ssaEnabledAnnotation := "ServerSideApply=true"
+	ssaDisabledAnnotation := "ServerSideApply=false"
 
 	// structuredMergeDiff is mainly used as a feature flag to enable
 	// calculating diffs using the structured-merge-diff library
 	// used in k8s while performing server-side applies. It checks the
 	// given diff Option or if the desired state resource has the
 	// Server-Side apply sync option annotation enabled.
 	structuredMergeDiff := o.structuredMergeDiff ||
-		(config != nil && resource.HasAnnotationOption(config, syncOptAnnotation, ssaAnnotation))
+		(config != nil && resource.HasAnnotationOption(config, syncOptAnnotation, ssaEnabledAnnotation))
+	if structuredMergeDiff && config != nil && resource.HasAnnotationOption(config, syncOptAnnotation, ssaDisabledAnnotation) {
+		structuredMergeDiff = false
+	}
 	if structuredMergeDiff {
 		r, err := StructuredMergeDiff(config, live, o.gvkParser, o.manager)
 		if err != nil {
 			return nil, fmt.Errorf("error calculating structured merge diff: %w", err)