refactor: move readback preservation into codegen policy

Closes: #1289

Author: dmacvicar

internal/codegen/generator/ir.go

@@ -141,6 +141,11 @@ type FieldIR struct {
 	// PreserveUserIntent means this optional field should only be set if user specified it
 	PreserveUserIntent bool
 
+	// PreservePlannedValueOnReadbackOmit means this optional scalar field should
+	// preserve the planned value when libvirt omits or canonicalizes it on
+	// readback.
+	PreservePlannedValueOnReadbackOmit bool
+
 	// StringToBool indicates this string field should be boolean in TF
 	StringToBool *StringToBoolPattern
 

internal/codegen/policy/field_policy.go

@@ -19,6 +19,7 @@ func applyStructPolicies(s *generator.StructIR) {
 		applyTopLevelIdentityPolicy(s, field)
 		applyTopLevelImmutabilityPolicy(s, field)
 		applyReportedFieldPolicy(s, field)
+		applyReadbackPreservationPolicy(s, field)
 	}
 }
 
@@ -91,3 +92,16 @@ func applyReportedFieldPolicy(s *generator.StructIR, field *generator.FieldIR) {
 		}
 	}
 }
+
+func applyReadbackPreservationPolicy(s *generator.StructIR, field *generator.FieldIR) {
+	switch s.Name {
+	case "DomainCPU":
+		if field.TFName == "mode" {
+			field.PreservePlannedValueOnReadbackOmit = true
+		}
+	case "DomainGraphicSpice":
+		if field.TFName == "listen" {
+			field.PreservePlannedValueOnReadbackOmit = true
+		}
+	}
+}

internal/codegen/policy/field_policy_test.go

@@ -50,3 +50,29 @@ func TestApplyFieldPoliciesLeavesNestedIdentityUserManaged(t *testing.T) {
 		t.Fatalf("expected nested id to have no plan modifier, got %q", field.PlanModifier)
 	}
 }
+
+func TestApplyFieldPoliciesMarksReadbackPreservationOverrides(t *testing.T) {
+	structs := []*generator.StructIR{
+		{
+			Name: "DomainCPU",
+			Fields: []*generator.FieldIR{
+				{TFName: "mode"},
+			},
+		},
+		{
+			Name: "DomainGraphicSpice",
+			Fields: []*generator.FieldIR{
+				{TFName: "listen"},
+			},
+		},
+	}
+
+	ApplyFieldPolicies(structs)
+
+	if !structs[0].Fields[0].PreservePlannedValueOnReadbackOmit {
+		t.Fatal("expected DomainCPU.mode to preserve planned value on omitted readback")
+	}
+	if !structs[1].Fields[0].PreservePlannedValueOnReadbackOmit {
+		t.Fatal("expected DomainGraphicSpice.listen to preserve planned value on omitted readback")
+	}
+}

internal/codegen/templates/convert.go.tmpl

@@ -202,19 +202,20 @@ func {{ .Name }}FromXML(ctx context.Context, xml *libvirtxml.{{ .Name }}, plan *
 	} else {
 		model.Disks = types.ListNull(types.ObjectType{AttrTypes: DomainDiskAttributeTypes()})
 	}
-	{{- else if and (eq $structName "DomainCPU") (eq .GoName "Mode") }}
-	// Optional field: Mode
-	// Preserve explicit user mode to avoid libvirt normalization drift (e.g. host-model -> custom).
+	{{- else if .PreservePlannedValueOnReadbackOmit }}
+	// Optional field: {{ .GoName }}
+	// Preserve the planned value when libvirt omits or canonicalizes this field on
+	// readback, to avoid drift for user-managed optional scalars.
 	if plan == nil {
-		if xml.Mode != "" {
-			model.Mode = types.StringValue(xml.Mode)
+		if xml.{{ .GoName }} != "" {
+			model.{{ .GoName }} = types.StringValue(xml.{{ .GoName }})
 		} else {
-			model.Mode = types.StringNull()
+			model.{{ .GoName }} = types.StringNull()
 		}
-	} else if !plan.Mode.IsNull() && !plan.Mode.IsUnknown() {
-		model.Mode = plan.Mode
+	} else if !plan.{{ .GoName }}.IsNull() && !plan.{{ .GoName }}.IsUnknown() {
+		model.{{ .GoName }} = plan.{{ .GoName }}
 	} else {
-		model.Mode = types.StringNull()
+		model.{{ .GoName }} = types.StringNull()
 	}
 	{{- else }}
 	{{ template "fromXMLField" . }}