Move plugin EnvForTest manager into new package

Move the EnvForTest manager into an independent package that is not
imported by any non-test code. Previously this code was directly
embedded in the plugins package resulting in testing flags being exposed
in the main kustomize binary.

pkg/plugins/test/envfortest.go

@@ -0,0 +1,111 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package test
+
+import (
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"sigs.k8s.io/kustomize/v3/pkg/pgmconfig"
+	"sigs.k8s.io/kustomize/v3/pkg/plugins"
+)
+
+// EnvForTest manages the plugin test environment.
+// It sets/resets XDG_CONFIG_HOME, makes/removes a temp objRoot.
+type EnvForTest struct {
+	t        *testing.T
+	compiler *plugins.Compiler
+	workDir  string
+	oldXdg   string
+	wasSet   bool
+}
+
+func NewEnvForTest(t *testing.T) *EnvForTest {
+	return &EnvForTest{t: t}
+}
+
+func (x *EnvForTest) Set() *EnvForTest {
+	x.createWorkDir()
+	x.compiler = x.makeCompiler()
+	x.setEnv()
+	return x
+}
+
+func (x *EnvForTest) Reset() {
+	x.resetEnv()
+	x.removeWorkDir()
+}
+
+func (x *EnvForTest) BuildGoPlugin(g, v, k string) {
+	err := x.compiler.Compile(g, v, k)
+	if err != nil {
+		x.t.Errorf("compile failed: %v", err)
+	}
+}
+
+func (x *EnvForTest) BuildExecPlugin(g, v, k string) {
+	lowK := strings.ToLower(k)
+	obj := filepath.Join(x.compiler.ObjRoot(), g, v, lowK, k)
+	src := filepath.Join(x.compiler.SrcRoot(), g, v, lowK, k)
+	if err := os.MkdirAll(filepath.Dir(obj), 0755); err != nil {
+		x.t.Errorf("error making directory: %s", filepath.Dir(obj))
+	}
+	cmd := exec.Command("cp", src, obj)
+	cmd.Env = os.Environ()
+	if err := cmd.Run(); err != nil {
+		x.t.Errorf("error copying %s to %s: %v", src, obj, err)
+	}
+}
+
+func (x *EnvForTest) makeCompiler() *plugins.Compiler {
+	// The plugin loader wants to find object code under
+	//    $XDG_CONFIG_HOME/kustomize/plugins
+	// and the compiler writes object code to
+	//    $objRoot
+	// so set things up accordingly.
+	objRoot := filepath.Join(
+		x.workDir, pgmconfig.ProgramName, pgmconfig.PluginRoot)
+	err := os.MkdirAll(objRoot, os.ModePerm)
+	if err != nil {
+		x.t.Error(err)
+	}
+	srcRoot, err := plugins.DefaultSrcRoot()
+	if err != nil {
+		x.t.Error(err)
+	}
+	return plugins.NewCompiler(srcRoot, objRoot)
+}
+
+func (x *EnvForTest) createWorkDir() {
+	var err error
+	x.workDir, err = ioutil.TempDir("", "kustomize-plugin-tests")
+	if err != nil {
+		x.t.Errorf("failed to make work dir: %v", err)
+	}
+}
+
+func (x *EnvForTest) removeWorkDir() {
+	err := os.RemoveAll(x.workDir)
+	if err != nil {
+		x.t.Errorf(
+			"removing work dir: %s %v", x.workDir, err)
+	}
+}
+
+func (x *EnvForTest) setEnv() {
+	x.oldXdg, x.wasSet = os.LookupEnv(pgmconfig.XDG_CONFIG_HOME)
+	os.Setenv(pgmconfig.XDG_CONFIG_HOME, x.workDir)
+}
+
+func (x *EnvForTest) resetEnv() {
+	if x.wasSet {
+		os.Setenv(pgmconfig.XDG_CONFIG_HOME, x.oldXdg)
+	} else {
+		os.Unsetenv(pgmconfig.XDG_CONFIG_HOME)
+	}
+}