Avoid calling user.Current() on windows

glog_file.go

@@ -26,7 +26,6 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"os/user"
 	"path/filepath"
 	"runtime"
 	"strings"
@@ -68,9 +67,8 @@ func init() {
 		host = shortHostname(h)
 	}
 
-	current, err := user.Current()
-	if err == nil {
-		userName = current.Username
+	if u := lookupUser(); u != "" {
+		userName = u
 	}
 	// Sanitize userName since it is used to construct file paths.
 	userName = strings.Map(func(r rune) rune {

glog_file_nonwindows.go

@@ -0,0 +1,12 @@
+//go:build !windows
+
+package glog
+
+import "os/user"
+
+func lookupUser() string {
+	if current, err := user.Current(); err == nil {
+		return current.Username
+	}
+	return ""
+}

glog_file_windows.go

@@ -0,0 +1,30 @@
+//go:build windows
+
+package glog
+
+import (
+	"syscall"
+)
+
+// This follows the logic in the standard library's user.Current() function, except
+// that it leaves out the potentially expensive calls required to look up the user's
+// display name in Active Directory.
+func lookupUser() string {
+	token, err := syscall.OpenCurrentProcessToken()
+	if err != nil {
+		return ""
+	}
+	defer token.Close()
+	tokenUser, err := token.GetTokenUser()
+	if err != nil {
+		return ""
+	}
+	username, _, accountType, err := tokenUser.User.Sid.LookupAccount("")
+	if err != nil {
+		return ""
+	}
+	if accountType != syscall.SidTypeUser {
+		return ""
+	}
+	return username
+}