Use OS-specific libs when computing client User-Agent.

**What this PR does / why we need it**:

The User-Agent reported by clients (e.g. kubectl) in request
headers should include the name of the client executable
but not the full path to that executable.

This PR changes how this name is determined by using the
operating-system specific package "path/filepath" (meant for
working with file system paths) instead of the "path" package
(meant for URL paths).

This fixes a problem on the Windows OS in the case where, if the
user has not set their PATH to point to the location of their
client executable, the User-Agent unnecessarily includes the
full path.

Fixes: #44419

staging/src/k8s.io/client-go/rest/BUILD

@@ -22,6 +22,7 @@ go_test(
     tags = ["automanaged"],
     deps = [
         "//vendor/github.com/google/gofuzz:go_default_library",
+        "//vendor/github.com/stretchr/testify/assert:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/api/equality:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/api/errors:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/apis/meta/v1:go_default_library",

staging/src/k8s.io/client-go/rest/config.go

@@ -22,7 +22,7 @@ import (
 	"net"
 	"net/http"
 	"os"
-	"path"
+	"path/filepath"
 	gruntime "runtime"
 	"strings"
 	"time"
@@ -255,19 +255,51 @@ func SetKubernetesDefaults(config *Config) error {
 	return nil
 }
 
-// DefaultKubernetesUserAgent returns the default user agent that clients can use.
+// adjustCommit returns sufficient significant figures of the commit's git hash.
+func adjustCommit(c string) string {
+	if len(c) == 0 {
+		return "unknown"
+	}
+	if len(c) > 7 {
+		return c[:7]
+	}
+	return c
+}
+
+// adjustVersion strips "alpha", "beta", etc. from version in form
+// major.minor.patch-[alpha|beta|etc].
+func adjustVersion(v string) string {
+	if len(v) == 0 {
+		return "unknown"
+	}
+	seg := strings.SplitN(v, "-", 2)
+	return seg[0]
+}
+
+// adjustCommand returns the last component of the
+// OS-specific command path for use in User-Agent.
+func adjustCommand(p string) string {
+	// Unlikely, but better than returning "".
+	if len(p) == 0 {
+		return "unknown"
+	}
+	return filepath.Base(p)
+}
+
+// buildUserAgent builds a User-Agent string from given args.
+func buildUserAgent(command, version, os, arch, commit string) string {
+	return fmt.Sprintf(
+		"%s/%s (%s/%s) kubernetes/%s", command, version, os, arch, commit)
+}
+
+// DefaultKubernetesUserAgent returns a User-Agent string built from static global vars.
 func DefaultKubernetesUserAgent() string {
-	commit := version.Get().GitCommit
-	if len(commit) > 7 {
-		commit = commit[:7]
-	}
-	if len(commit) == 0 {
-		commit = "unknown"
-	}
-	version := version.Get().GitVersion
-	seg := strings.SplitN(version, "-", 2)
-	version = seg[0]
-	return fmt.Sprintf("%s/%s (%s/%s) kubernetes/%s", path.Base(os.Args[0]), version, gruntime.GOOS, gruntime.GOARCH, commit)
+	return buildUserAgent(
+		adjustCommand(os.Args[0]),
+		adjustVersion(version.Get().GitVersion),
+		gruntime.GOOS,
+		gruntime.GOARCH,
+		adjustCommit(version.Get().GitCommit))
 }
 
 // InClusterConfig returns a config object which uses the service account

staging/src/k8s.io/client-go/rest/config_test.go

@@ -19,6 +19,7 @@ package rest
 import (
 	"io"
 	"net/http"
+	"path/filepath"
 	"reflect"
 	"strings"
 	"testing"
@@ -32,6 +33,7 @@ import (
 	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
 	"k8s.io/client-go/util/flowcontrol"
 
+	"github.com/stretchr/testify/assert"
 	_ "k8s.io/client-go/pkg/api/install"
 )
 
@@ -100,6 +102,42 @@ func TestSetKubernetesDefaultsUserAgent(t *testing.T) {
 	}
 }
 
+func TestAdjustVersion(t *testing.T) {
+	assert := assert.New(t)
+	assert.Equal("1.2.3", adjustVersion("1.2.3-alpha4"))
+	assert.Equal("1.2.3", adjustVersion("1.2.3-alpha"))
+	assert.Equal("1.2.3", adjustVersion("1.2.3"))
+	assert.Equal("unknown", adjustVersion(""))
+}
+
+func TestAdjustCommit(t *testing.T) {
+	assert := assert.New(t)
+	assert.Equal("1234567", adjustCommit("1234567890"))
+	assert.Equal("123456", adjustCommit("123456"))
+	assert.Equal("unknown", adjustCommit(""))
+}
+
+func TestAdjustCommand(t *testing.T) {
+	assert := assert.New(t)
+	assert.Equal("beans", adjustCommand(filepath.Join("home", "bob", "Downloads", "beans")))
+	assert.Equal("beans", adjustCommand(filepath.Join(".", "beans")))
+	assert.Equal("beans", adjustCommand("beans"))
+	assert.Equal("unknown", adjustCommand(""))
+}
+
+func TestBuildUserAgent(t *testing.T) {
+	assert.New(t).Equal(
+		"lynx/nicest (beos/itanium) kubernetes/baaaaaaaaad",
+		buildUserAgent(
+			"lynx", "nicest",
+			"beos", "itanium", "baaaaaaaaad"))
+}
+
+// This function untestable since it doesn't accept arguments.
+func TestDefaultKubernetesUserAgent(t *testing.T) {
+	assert.New(t).Contains(DefaultKubernetesUserAgent(), "kubernetes")
+}
+
 func TestRESTClientRequires(t *testing.T) {
 	if _, err := RESTClientFor(&Config{Host: "127.0.0.1", ContentConfig: ContentConfig{NegotiatedSerializer: api.Codecs}}); err == nil {
 		t.Errorf("unexpected non-error")