feat: inherit field for release template inheritance (#606)

* feat: `inherit` field for release template inheritance

Ref https://github.com/helmfile/helmfile/issues/435#issuecomment-1364749414

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

* Fix wording

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

* Comment on releaseWithInheritedTemplate

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

* Update Release Template doc with the new `inherit` feature

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

* Fix a typo in code comment

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

pkg/state/state_exec_tmpl.go

@@ -1,8 +1,12 @@
 package state
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
+	"strings"
+
+	"github.com/imdario/mergo"
 
 	"github.com/helmfile/helmfile/pkg/tmpl"
 	"github.com/helmfile/helmfile/pkg/yaml"
@@ -89,7 +93,15 @@ func (st *HelmState) ExecuteTemplates() (*HelmState, error) {
 	vals := st.Values()
 
 	for i, rt := range st.Releases {
-		release := rt
+		release, err := st.releaseWithInheritedTemplate(&rt, nil)
+		if err != nil {
+			var cyclicInheritanceErr CyclicReleaseTemplateInheritanceError
+			if errors.As(err, &cyclicInheritanceErr) {
+				return nil, fmt.Errorf("unable to load release %q with template: %w", rt.Name, cyclicInheritanceErr)
+			}
+			return nil, err
+		}
+
 		if release.KubeContext == "" {
 			release.KubeContext = r.HelmDefaults.KubeContext
 		}
@@ -107,7 +119,7 @@ func (st *HelmState) ExecuteTemplates() (*HelmState, error) {
 		}
 
 		successFlag := false
-		for it, prev := 0, &release; it < 6; it++ {
+		for it, prev := 0, release; it < 6; it++ {
 			tmplData := st.createReleaseTemplateData(prev, vals)
 			renderer := tmpl.NewFileRenderer(st.fs, st.basePath, tmplData)
 			r, err := release.ExecuteTemplateExpressions(renderer)
@@ -132,3 +144,81 @@ func (st *HelmState) ExecuteTemplates() (*HelmState, error) {
 
 	return &r, nil
 }
+
+type CyclicReleaseTemplateInheritanceError struct {
+	Message string
+}
+
+func (e CyclicReleaseTemplateInheritanceError) Error() string {
+	return e.Message
+}
+
+// releaseWithInheritedTemplate generates a new ReleaseSpec from a ReleaseSpec, by recursively inheriting
+// release templates referenced by the spec's `inherit` field.
+// The third parameter retains the current state of the recursive call, to detect a cyclic dependency a.k.a
+// a cyclic relese template inheritance.
+// This functions fails with a CyclicReleaseTemplateInheritanceError if it finds a cyclic inheritance.
+func (st *HelmState) releaseWithInheritedTemplate(r *ReleaseSpec, inheritancePath []string) (*ReleaseSpec, error) {
+	templateName := r.Inherit.Template
+	if templateName == "" {
+		return r, nil
+	}
+
+	path := append([]string{}, inheritancePath...)
+	path = append(path, templateName)
+
+	var cycleFound bool
+	for _, t := range inheritancePath {
+		if t == templateName {
+			cycleFound = true
+			break
+		}
+	}
+
+	if cycleFound {
+		return nil, CyclicReleaseTemplateInheritanceError{Message: fmt.Sprintf("cyclic inheritance detected: %s", strings.Join(path, "->"))}
+	}
+
+	template, defined := st.Templates[templateName]
+	if !defined {
+		return nil, fmt.Errorf("release %q tried to inherit inexistent release template %q", r.Name, templateName)
+	}
+
+	src, err := st.releaseWithInheritedTemplate(&template.ReleaseSpec, path)
+	if err != nil {
+		return nil, fmt.Errorf("unable to load release template %q: %w", templateName, err)
+	}
+
+	for _, k := range r.Inherit.Except {
+		switch k {
+		case "labels":
+			src.Labels = map[string]string{}
+		case "values":
+			src.Values = nil
+		case "valuesTemplate":
+			src.ValuesTemplate = nil
+		case "setTemplate":
+			src.SetValuesTemplate = nil
+		case "set":
+			src.SetValues = nil
+		default:
+			return nil, fmt.Errorf("%q is not allowed under `inherit`. Allowed values are \"set\", \"setTemplate\", \"values\", \"valuesTemplate\", and \"labels\"", k)
+		}
+
+		st.logger.Debugf("excluded field %q when inheriting template %q to release %q", k, templateName, r.Name)
+	}
+
+	var merged ReleaseSpec
+
+	if err := mergo.Merge(&merged, src, mergo.WithAppendSlice, mergo.WithSliceDeepCopy); err != nil {
+		return nil, fmt.Errorf("unable to inherit release template %q: %w", templateName, err)
+	}
+
+	if err := mergo.Merge(&merged, r, mergo.WithAppendSlice, mergo.WithSliceDeepCopy); err != nil {
+		return nil, fmt.Errorf("unable to load release %q: %w", r.Name, err)
+	}
+
+	merged.Inherit = Inherit{}
+
+	return &merged, nil
+}