os/user: make Current work without cgo

Fixes #14626

Change-Id: I91c40407dc35355e5c5046f24111a126f99260d9
Reviewed-on: https://go-review.googlesource.com/20192Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/os/user/lookup_stubs.go

@@ -6,7 +6,13 @@
 
 package user
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+	"os"
+	"runtime"
+	"strconv"
+)
 
 func init() {
 	userImplemented = false
@@ -14,7 +20,30 @@ func init() {
 }
 
 func current() (*User, error) {
-	return nil, errors.New("user: Current requires cgo")
+	u := &User{
+		Uid:      currentUID(),
+		Gid:      currentGID(),
+		Username: os.Getenv("USER"),
+		Name:     "", // ignored
+		HomeDir:  os.Getenv("HOME"),
+	}
+	if runtime.GOOS == "nacl" {
+		if u.Uid == "" {
+			u.Uid = "1"
+		}
+		if u.Username == "" {
+			u.Username = "nacl"
+		}
+		if u.HomeDir == "" {
+			u.HomeDir = "/home/nacl"
+		}
+	}
+	// cgo isn't available, but if we found the minimum information
+	// without it, use it:
+	if u.Uid != "" && u.Username != "" && u.HomeDir != "" {
+		return u, nil
+	}
+	return nil, fmt.Errorf("user: Current not implemented on %s/%s", runtime.GOOS, runtime.GOARCH)
 }
 
 func lookupUser(username string) (*User, error) {
@@ -36,3 +65,20 @@ func lookupGroupId(string) (*Group, error) {
 func listGroups(*User) ([]string, error) {
 	return nil, errors.New("user: GroupIds requires cgo")
 }
+
+func currentUID() string {
+	if id := os.Getuid(); id >= 0 {
+		return strconv.Itoa(id)
+	}
+	// Note: Windows returns -1, but this file isn't used on
+	// Windows anyway, so this empty return path shouldn't be
+	// used.
+	return ""
+}
+
+func currentGID() string {
+	if id := os.Getgid(); id >= 0 {
+		return strconv.Itoa(id)
+	}
+	return ""
+}

src/os/user/user_test.go

@@ -16,8 +16,6 @@ func checkUser(t *testing.T) {
 }
 
 func TestCurrent(t *testing.T) {
-	checkUser(t)
-
 	u, err := Current()
 	if err != nil {
 		t.Fatalf("Current: %v", err)