Merge pull request #31470 from caesarxuchao/regenerate-release-1-4

Automatic merge from submit-queue

Fix missing expansion files in clientset release 1.4

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**What this PR does / why we need it**:
Fix missing expansion files in clientset release 1.4;
Update the client-gen tool to automatically update generated_expansion.go;
Update the readme to document the best practice of using client-gen.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

https://github.com/kubernetes/kubernetes/issues/29698#issuecomment-241315798


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**Release note**:
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cc @lavalamp @krousey @mbohlool

cmd/libs/go2idl/client-gen/generators/client_generator.go

@@ -19,7 +19,6 @@ package generators
 
 import (
 	"fmt"
-	"os"
 	"path/filepath"
 	"strings"
 
@@ -107,15 +106,13 @@ func packageForGroup(gv unversioned.GroupVersion, typeList []*types.Type, packag
 			})
 
 			expansionFileName := "generated_expansion"
-			// To avoid overriding user's manual modification, only generate the expansion file if it doesn't exist.
-			if _, err := os.Stat(filepath.Join(srcTreePath, outputPackagePath, expansionFileName+".go")); os.IsNotExist(err) {
-				generators = append(generators, &genExpansion{
-					DefaultGen: generator.DefaultGen{
-						OptionalName: expansionFileName,
-					},
-					types: typeList,
-				})
-			}
+			generators = append(generators, &genExpansion{
+				groupPath: filepath.Join(srcTreePath, outputPackagePath),
+				DefaultGen: generator.DefaultGen{
+					OptionalName: expansionFileName,
+				},
+				types: typeList,
+			})
 
 			return generators
 		},

cmd/libs/go2idl/client-gen/generators/generator_for_expansion.go

@@ -18,6 +18,9 @@ package generators
 
 import (
 	"io"
+	"os"
+	"path/filepath"
+	"strings"
 
 	"k8s.io/kubernetes/cmd/libs/go2idl/generator"
 	"k8s.io/kubernetes/cmd/libs/go2idl/types"
@@ -26,6 +29,7 @@ import (
 // genExpansion produces a file for a group client, e.g. ExtensionsClient for the extension group.
 type genExpansion struct {
 	generator.DefaultGen
+	groupPath string
 	// types in a group
 	types []*types.Type
 }
@@ -38,7 +42,9 @@ func (g *genExpansion) Filter(c *generator.Context, t *types.Type) bool {
 func (g *genExpansion) GenerateType(c *generator.Context, t *types.Type, w io.Writer) error {
 	sw := generator.NewSnippetWriter(w, c, "$", "$")
 	for _, t := range g.types {
-		sw.Do(expansionInterfaceTemplate, t)
+		if _, err := os.Stat(filepath.Join(g.groupPath, strings.ToLower(t.Name.Name+"_expansion.go"))); os.IsNotExist(err) {
+			sw.Do(expansionInterfaceTemplate, t)
+		}
 	}
 	return sw.Error()
 }