klog_file: workaround stdlib.user not supporting Nano Server

user.Current() can panic on Windows if the OS is missing
netapi32.dll. This library is stripped from Windows Nano Server.

To workaround this, use environment variables on all versions
on Windows.

klog_file.go

@@ -24,6 +24,7 @@ import (
 	"os"
 	"os/user"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"sync"
 	"time"
@@ -55,13 +56,31 @@ func init() {
 		host = shortHostname(h)
 	}
 
-	current, err := user.Current()
-	if err == nil {
-		userName = current.Username
-	}
+	// On Windows, the Go 'user' package requires netapi32.dll.
+	// This affects Windows Nano Server:
+	//   https://github.com/golang/go/issues/21867
+	// Fallback to using environment variables.
+	if runtime.GOOS == "windows" {
+		u := os.Getenv("USERNAME")
+		if len(u) == 0 {
+			return
+		}
+		// Sanitize the USERNAME since it may contain filepath separators.
+		u = strings.Replace(u, `\`, "_", -1)
 
-	// Sanitize userName since it may contain filepath separators on Windows.
-	userName = strings.Replace(userName, `\`, "_", -1)
+		// user.Current().Username normally produces something like 'USERDOMAIN\USERNAME'
+		d := os.Getenv("USERDOMAIN")
+		if len(d) != 0 {
+			userName = d + "_" + u
+		} else {
+			userName = u
+		}
+	} else {
+		current, err := user.Current()
+		if err == nil {
+			userName = current.Username
+		}
+	}
 }
 
 // shortHostname returns its argument, truncating at the first period.