fix: preserve indent if possible

It's possible to preserve the indent in the original yaml when
we Unmarshal to a yaml.Node, and the structure of the original
yaml allows us to guess the indent.

The two easiest cases are when we have a nested block-style mapping,
or a sequence inside a mapping and the sequence is indented. For
those, the node tells us the parsed column (starting at 1) and we
use that indent.

However: if we parse to some other data structure (as we do with
Kustomize) or the original yaml has some structure where the indentation
is unclear, eg:

    root:
    - unguessable indent
    - deeper:
        guessable: but requires deeper traversal

... then we just guess '2'.

This should generate _fewer_ useless whitespace changes, but cannot
prevent them all.

Signed-off-by: Brian Ewins <bewins@nerdwallet.com>

Author: bewinsnw

pkg/argocd/update.go

@@ -423,6 +423,7 @@ func marshalWithIndent(in interface{}, indent int) (out []byte, err error) {
 	var b bytes.Buffer
 	encoder := yaml.NewEncoder(&b)
 	defer encoder.Close()
+	// note: yaml.v3 will only respect indents from 1 to 9 inclusive.
 	encoder.SetIndent(indent)
 	if err = encoder.Encode(in); err != nil {
 		return nil, err
@@ -433,6 +434,30 @@ func marshalWithIndent(in interface{}, indent int) (out []byte, err error) {
 	return b.Bytes(), nil
 }
 
+func guessIndent(root *yaml.Node) int {
+	node := root
+	if root.Kind == yaml.DocumentNode {
+		if len(node.Content) == 0 {
+			return 2
+		}
+		node = root.Content[0]
+	}
+	// anything other than a map at the root makes guessing difficult
+	if node.Kind != yaml.MappingNode || len(node.Content) == 0 {
+		return 2
+	}
+	// first level map entries that are themselves mappings or sequences,
+	// in block style, and are indented, allow guessing the preferred indent.
+	for i, child := range node.Content {
+		if i%2 == 1 && child.Column > 1 && child.Column < 10 && child.Style != yaml.FlowStyle {
+			if child.Kind == yaml.MappingNode || child.Kind == yaml.SequenceNode {
+				return child.Column - 1
+			}
+		}
+	}
+	return 2
+}
+
 // marshalParamsOverride marshals the parameter overrides of a given application
 // into YAML bytes
 func marshalParamsOverride(app *v1alpha1.Application, originalData []byte) ([]byte, error) {
@@ -479,6 +504,7 @@ func marshalParamsOverride(app *v1alpha1.Application, originalData []byte) ([]by
 			if err != nil {
 				return nil, err
 			}
+			indent := guessIndent(&helmNewValues)
 
 			for _, c := range images {
 				if c.ImageAlias == "" {
@@ -520,7 +546,7 @@ func marshalParamsOverride(app *v1alpha1.Application, originalData []byte) ([]by
 				}
 			}
 
-			override, err = marshalWithIndent(&helmNewValues, 2)
+			override, err = marshalWithIndent(&helmNewValues, indent)
 		} else {
 			var params helmOverride
 			newParams := helmOverride{

pkg/argocd/update_test.go

@@ -2439,6 +2439,68 @@ image:
 		assert.Equal(t, strings.TrimSpace(expected), strings.TrimSpace(string(output)))
 	})
 
+	t.Run("Indentation is guessed from nested mappings", func(t *testing.T) {
+		expected := `
+unguessable:
+   - unguessable
+image:
+   attributes:
+      tag: v2.0.0
+`
+
+		inputData := []byte(`
+unguessable:
+- unguessable
+image:
+   attributes:
+      tag: v1.0.0
+`)
+		input := yaml.Node{}
+		err := yaml.Unmarshal(inputData, &input)
+		require.NoError(t, err)
+		indent := guessIndent(&input)
+
+		key := "image.attributes.tag"
+		value := "v2.0.0"
+
+		err = setHelmValue(&input, key, value)
+		require.NoError(t, err)
+
+		output, err := marshalWithIndent(&input, indent)
+		require.NoError(t, err)
+		assert.Equal(t, strings.TrimSpace(expected), strings.TrimSpace(string(output)))
+	})
+
+	t.Run("Indentation is guessed from indented lists", func(t *testing.T) {
+		expected := `
+unguessable: [unguessable]
+guessable:
+   - guessable
+image:
+   attributes:
+      tag: v2.0.0
+`
+
+		inputData := []byte(`
+unguessable: [unguessable]
+guessable:
+   - guessable
+`)
+		input := yaml.Node{}
+		err := yaml.Unmarshal(inputData, &input)
+		require.NoError(t, err)
+		indent := guessIndent(&input)
+
+		key := "image.attributes.tag"
+		value := "v2.0.0"
+
+		err = setHelmValue(&input, key, value)
+		require.NoError(t, err)
+
+		output, err := marshalWithIndent(&input, indent)
+		require.NoError(t, err)
+		assert.Equal(t, strings.TrimSpace(expected), strings.TrimSpace(string(output)))
+	})
 }
 
 func Test_GetWriteBackConfig(t *testing.T) {