listergen: add lister expansions

Signed-off-by: Steve Kuznetsov <skuznets@redhat.com>

Author: stevekuznetsov

pkg/generators/listergen/listergen.go

@@ -104,6 +104,20 @@ func (g Generator) Generate(ctx *genall.GenerationContext) error {
 				}, outputFile); err != nil {
 					return err
 				}
+
+				outputFile = filepath.Join(listerDir, strings.ToLower(kind.String())+"_expansion.go")
+				logger = logger.WithValues(
+					"path", outputFile,
+				)
+				logger.Info("generating lister expansion")
+
+				if err := util.InitializeGeneratedCode(ctx, headerText, &listergen.Expansions{
+					Group:                 groupInfo,
+					Kind:                  kind,
+					UseUpstreamInterfaces: g.SingleClusterListerPackagePath != "",
+				}, outputFile); err != nil {
+					return err
+				}
 			}
 		}
 	}

pkg/internal/listergen/expansions.go

@@ -0,0 +1,57 @@
+package listergen
+
+import (
+	"io"
+	"text/template"
+
+	clientgentypes "k8s.io/code-generator/cmd/client-gen/types"
+
+	"github.com/kcp-dev/code-generator/pkg/parser"
+)
+
+type Expansions struct {
+	// Group is:
+	// - the name of the API group, e.g. "authorization",
+	// - the version and package path of the API, e.g. "v1" and "k8s.io/api/rbac/v1"
+	Group clientgentypes.GroupVersionInfo
+	// Kind is the kind for which we are generating listers, e.g. "ClusterRole"
+	Kind parser.Kind
+
+	// UseUpstreamInterfaces determines if we're generating against existing single-cluster lister interfaces or not.
+	UseUpstreamInterfaces bool
+}
+
+func (l *Expansions) WriteContent(w io.Writer) error {
+	templ, err := template.New("expansions").Funcs(templateFuncs).Parse(expansions)
+	if err != nil {
+		return err
+	}
+
+	m := map[string]interface{}{
+		"group":                 l.Group,
+		"kind":                  &l.Kind,
+		"useUpstreamInterfaces": l.UseUpstreamInterfaces,
+	}
+	return templ.Execute(w, m)
+}
+
+var expansions = `
+//go:build !ignore_autogenerated
+// +build !ignore_autogenerated
+
+// Code generated by kcp code-generator. DO NOT EDIT.
+
+package {{.group.Version.PackageName}}
+
+// {{.kind.String}}ClusterListerExpansion allows custom methods to be added to {{.kind.String}}ClusterLister.
+type {{.kind.String}}ClusterListerExpansion interface {}
+
+{{ if not .useUpstreamInterfaces -}}
+// {{.kind.String}}ListerExpansion allows custom methods to be added to {{.kind.String}}Lister.
+type {{.kind.String}}ListerExpansion interface {}
+{{ if  .kind.IsNamespaced -}}
+// {{.kind.String}}NamespaceListerExpansion allows custom methods to be added to {{.kind.String}}NamespaceLister.
+type {{.kind.String}}NamespaceListerExpansion interface {}
+{{ end -}}
+{{ end -}}
+`

pkg/internal/listergen/lister.go

@@ -75,7 +75,8 @@ type {{.kind.String}}ClusterLister interface {
 	Cluster(cluster logicalcluster.Name) {{.kind.String}}Lister
 {{else -}}
 	Cluster(cluster logicalcluster.Name){{.group.PackageAlias}}listers.{{.kind.String}}Lister
-{{end -}}	
+{{end -}}
+	{{.kind.String}}ClusterListerExpansion
 }
 
 type {{.kind.String | lowerFirst }}ClusterLister struct {
@@ -123,6 +124,7 @@ type {{.kind.String}}Lister interface {
 	// {{.kind.Plural}} returns a lister that can list and get {{.kind.Plural}} in one workspace and namespace.
 	{{.kind.Plural}}(namespace string) {{.kind.String}}NamespaceLister
 {{end -}}
+	{{.kind.String}}ListerExpansion
 }
 {{end -}}
 
@@ -177,7 +179,8 @@ type {{.kind.String}}NamespaceLister interface {
 	// Get retrieves the {{.kind.String}} from the indexer for a given workspace, namespace and name.
 	// Objects returned here must be treated as read-only.
 	Get(name string) (*{{.group.PackageAlias}}.{{.kind.String}}, error)
-	}
+	{{.kind.String}}NamespaceListerExpansion
+}
 {{end -}}
 
 {{ if not .useUpstreamInterfaces -}}

pkg/util/util.go

@@ -18,6 +18,7 @@ package util
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -42,7 +43,17 @@ type Generator interface {
 	WriteContent(w io.Writer) error
 }
 
+// InitializeGeneratedCode ensures that intoPath exists; if the file is not yet present it is initialized with the output of the generator.
+func InitializeGeneratedCode(ctx *genall.GenerationContext, header string, generator Generator, intoPath string) error {
+	return writeGeneratedCode(ctx, header, generator, intoPath, false)
+}
+
+// WriteGeneratedCode ensures that intoPath contains only the output of the generator.
 func WriteGeneratedCode(ctx *genall.GenerationContext, header string, generator Generator, intoPath string) error {
+	return writeGeneratedCode(ctx, header, generator, intoPath, true)
+}
+
+func writeGeneratedCode(ctx *genall.GenerationContext, header string, generator Generator, intoPath string, overwrite bool) error {
 	data := &bytes.Buffer{}
 	if n, err := data.WriteString(header); err != nil {
 		return err
@@ -58,10 +69,20 @@ func WriteGeneratedCode(ctx *genall.GenerationContext, header string, generator
 	if !ok {
 		return fmt.Errorf("+output:dir is required, not %T", ctx.OutputRule)
 	}
-	if err := os.MkdirAll(filepath.Dir(filepath.Join(string(output), intoPath)), os.ModePerm); err != nil {
+	fullPath := filepath.Join(string(output), intoPath)
+	if err := os.MkdirAll(filepath.Dir(fullPath), os.ModePerm); err != nil {
 		return fmt.Errorf("couldn't create directory: %w", err)
 	}
 
+	if _, err := os.Stat(fullPath); err == nil {
+		if !overwrite {
+			klog.Background().WithValues("path", intoPath).Info("not overwriting file")
+			return nil
+		}
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return err
+	}
+
 	outputFile, err := ctx.Open(nil, intoPath)
 	if err != nil {
 		return fmt.Errorf("failed to open: %w", err)