Preserve field order when updating kustomization.yaml file

pkg/argocd/git.go

@@ -12,6 +12,7 @@ import (
 
 	"sigs.k8s.io/kustomize/api/konfig"
 	"sigs.k8s.io/kustomize/api/types"
+	"sigs.k8s.io/kustomize/kyaml/order"
 	kyaml "sigs.k8s.io/kustomize/kyaml/yaml"
 
 	"github.com/argoproj-labs/argocd-image-updater/pkg/image"
@@ -314,14 +315,43 @@ func writeKustomization(app *v1alpha1.Application, wbc *WriteBackConfig, gitC gi
 	if err != nil {
 		return err, false
 	}
-	err = kyaml.UpdateFile(filterFunc, kustFile)
-	if err != nil {
+
+	if err = updateKustomizeFile(filterFunc, kustFile); err != nil {
 		return err, false
 	}
 
 	return nil, false
 }
 
+// updateKustomizeFile reads the kustomization file at path, applies the filter to it, and writes the result back
+// to the file. This is the same behavior as kyaml.UpdateFile, but it preserves the original order
+// of YAML fields to minimize git diffs.
+func updateKustomizeFile(filter kyaml.Filter, path string) error {
+	// Read the yaml
+	y, err := kyaml.ReadFile(path)
+	if err != nil {
+		return err
+	}
+
+	// Update the yaml
+	yCpy := y.Copy()
+	if err := yCpy.PipeE(filter); err != nil {
+		return err
+	}
+
+	// Preserve the original order of fields
+	if err := order.SyncOrder(y, yCpy); err != nil {
+		return err
+	}
+
+	// Write the yaml
+	if err := kyaml.WriteFile(yCpy, path); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 func imagesFilter(images v1alpha1.KustomizeImages) (kyaml.Filter, error) {
 	var overrides []kyaml.Filter
 	for _, img := range images {

pkg/argocd/git_test.go

@@ -1,6 +1,7 @@
 package argocd
 
 import (
+	"os"
 	"testing"
 	"text/template"
 	"time"
@@ -215,3 +216,74 @@ images:
 		})
 	}
 }
+
+func Test_updateKustomizeFile(t *testing.T) {
+	makeTmpKustomization := func(t *testing.T, content []byte) string {
+		f, err := os.CreateTemp("", "kustomization-*.yaml")
+		if err != nil {
+			t.Fatal(err)
+		}
+		_, err = f.Write(content)
+		if err != nil {
+			t.Fatal(err)
+		}
+		f.Close()
+		t.Cleanup(func() {
+			os.Remove(f.Name())
+		})
+		return f.Name()
+	}
+
+	filter, err := imagesFilter(v1alpha1.KustomizeImages{"foo@sha23456"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tests := []struct {
+		name        string
+		content     []byte
+		wantContent []byte
+		filter      kyaml.Filter
+		wantErr     bool
+	}{
+		{
+			name: "sorted",
+			content: []byte(`images:
+- digest: sha12345
+  name: foo
+`),
+			wantContent: []byte(`images:
+- digest: sha23456
+  name: foo
+`),
+			filter: filter,
+		},
+		{
+			name: "not-sorted",
+			content: []byte(`images:
+- name: foo
+  digest: sha12345
+`),
+			wantContent: []byte(`images:
+- name: foo
+  digest: sha23456
+`),
+			filter: filter,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			path := makeTmpKustomization(t, tt.content)
+			err := updateKustomizeFile(tt.filter, path)
+			if tt.wantErr {
+				assert.Error(t, err)
+			} else {
+				got, err := os.ReadFile(path)
+				if err != nil {
+					t.Fatal(err)
+				}
+				assert.Equal(t, string(tt.wantContent), string(got))
+			}
+		})
+	}
+}