Add background mode support to localagent and implement instance check

lib/instance/detach.go

@@ -0,0 +1,49 @@
+package instance
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+func Detach() error {
+	if AlreadyRunning() {
+		return fmt.Errorf("agent already running")
+	}
+	exe, err := os.Executable()
+	if err != nil {
+		return err
+	}
+	nul, err := os.OpenFile("NUL", os.O_RDWR, 0)
+	if err != nil {
+		return err
+	}
+	defer nul.Close()
+
+	cmd := exec.Command(exe, stripBackground(os.Args[1:])...)
+	cmd.Stdin = nul
+	cmd.Stdout = nul
+	cmd.Stderr = nul
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		HideWindow:    true,
+		CreationFlags: windows.CREATE_NO_WINDOW | windows.CREATE_NEW_PROCESS_GROUP,
+	}
+	return cmd.Start()
+}
+
+func stripBackground(args []string) []string {
+	out := make([]string, 0, len(args))
+	for _, arg := range args {
+		name := strings.TrimLeft(arg, "-/")
+		lower := strings.ToLower(name)
+		if lower == "background" || strings.HasPrefix(lower, "background=") {
+			continue
+		}
+		out = append(out, arg)
+	}
+	return out
+}