Preserve field order when updating kustomization.yaml file

This commit causes argocd-image-updater to preserve the order of fields
in the kustomization.yaml file when updating the image(s). Doing so will
minimize git diffs in the case that other tools also edit the
kustomization.yaml file.

Fixes #502

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))
+			}
+		})
+	}
+}