Fix update of PostgreSQL Flex instance flavor (#494)

* Fix update of PostgreSQL Flex instance flavor

* Fix linter

stackit/internal/services/postgresflex/instance/resource.go

@@ -131,7 +131,7 @@ func (r *instanceResource) Configure(ctx context.Context, req resource.Configure
 }
 
 // Schema defines the schema for the resource.
-func (r *instanceResource) Schema(_ context.Context, _ resource.SchemaRequest, resp *resource.SchemaResponse) {
+func (r *instanceResource) Schema(_ context.Context, req resource.SchemaRequest, resp *resource.SchemaResponse) {
 	descriptions := map[string]string{
 		"main":        "Postgres Flex instance resource schema. Must have a `region` specified in the provider configuration.",
 		"id":          "Terraform's internal resource ID. It is structured as \"`project_id`,`instance_id`\".",
@@ -199,13 +199,13 @@ func (r *instanceResource) Schema(_ context.Context, _ resource.SchemaRequest, r
 					"id": schema.StringAttribute{
 						Computed: true,
 						PlanModifiers: []planmodifier.String{
-							stringplanmodifier.UseStateForUnknown(),
+							UseStateForUnknownIfFlavorUnchanged(req),
 						},
 					},
 					"description": schema.StringAttribute{
 						Computed: true,
 						PlanModifiers: []planmodifier.String{
-							stringplanmodifier.UseStateForUnknown(),
+							UseStateForUnknownIfFlavorUnchanged(req),
 						},
 					},
 					"cpu": schema.Int64Attribute{

stackit/internal/services/postgresflex/instance/use_state_for_unknown_if_flavor_unchanged_modifier.go

@@ -0,0 +1,85 @@
+package postgresflex
+
+import (
+	"context"
+
+	"github.com/hashicorp/terraform-plugin-framework/resource"
+	"github.com/hashicorp/terraform-plugin-framework/resource/schema/planmodifier"
+	"github.com/hashicorp/terraform-plugin-framework/types/basetypes"
+)
+
+type useStateForUnknownIfFlavorUnchangedModifier struct {
+	Req resource.SchemaRequest
+}
+
+// UseStateForUnknownIfFlavorUnchanged returns a plan modifier similar to UseStateForUnknown
+// if the RAM and CPU values are not changed in the plan. Otherwise, the plan modifier does nothing.
+func UseStateForUnknownIfFlavorUnchanged(req resource.SchemaRequest) planmodifier.String {
+	return useStateForUnknownIfFlavorUnchangedModifier{
+		Req: req,
+	}
+}
+
+func (m useStateForUnknownIfFlavorUnchangedModifier) Description(context.Context) string {
+	return "UseStateForUnknownIfFlavorUnchanged returns a plan modifier similar to UseStateForUnknown if the RAM and CPU values are not changed in the plan. Otherwise, the plan modifier does nothing."
+}
+
+func (m useStateForUnknownIfFlavorUnchangedModifier) MarkdownDescription(ctx context.Context) string {
+	return m.Description(ctx)
+}
+
+func (m useStateForUnknownIfFlavorUnchangedModifier) PlanModifyString(ctx context.Context, req planmodifier.StringRequest, resp *planmodifier.StringResponse) { // nolint:gocritic // function signature required by Terraform
+	// Do nothing if there is no state value.
+	if req.StateValue.IsNull() {
+		return
+	}
+
+	// Do nothing if there is a known planned value.
+	if !req.PlanValue.IsUnknown() {
+		return
+	}
+
+	// Do nothing if there is an unknown configuration value, otherwise interpolation gets messed up.
+	if req.ConfigValue.IsUnknown() {
+		return
+	}
+
+	// The above checks are taken from the UseStateForUnknown plan modifier implementation
+	// (https://github.com/hashicorp/terraform-plugin-framework/blob/main/resource/schema/stringplanmodifier/use_state_for_unknown.go#L38)
+
+	var stateModel Model
+	diags := req.State.Get(ctx, &stateModel)
+	resp.Diagnostics.Append(diags...)
+	if resp.Diagnostics.HasError() {
+		return
+	}
+
+	var stateFlavor = &flavorModel{}
+	if !(stateModel.Flavor.IsNull() || stateModel.Flavor.IsUnknown()) {
+		diags = stateModel.Flavor.As(ctx, stateFlavor, basetypes.ObjectAsOptions{})
+		resp.Diagnostics.Append(diags...)
+		if resp.Diagnostics.HasError() {
+			return
+		}
+	}
+
+	var planModel Model
+	diags = req.Plan.Get(ctx, &planModel)
+	resp.Diagnostics.Append(diags...)
+	if resp.Diagnostics.HasError() {
+		return
+	}
+
+	var planFlavor = &flavorModel{}
+	if !(planModel.Flavor.IsNull() || planModel.Flavor.IsUnknown()) {
+		diags = planModel.Flavor.As(ctx, planFlavor, basetypes.ObjectAsOptions{})
+		resp.Diagnostics.Append(diags...)
+		if resp.Diagnostics.HasError() {
+			return
+		}
+	}
+
+	if planFlavor.CPU == stateFlavor.CPU && planFlavor.RAM == stateFlavor.RAM {
+		resp.PlanValue = req.StateValue
+	}
+}