Relaunch through UAC when (un)installing the service unprivileged

--install-service/--remove-service on Windows no longer fail with
'Access is denied' from a normal shell: the process re-runs itself via
ShellExecuteEx 'runas', waits for the elevated child and mirrors its
exit code. The child gets --elevated-child and pauses for a keypress so
its console output stays readable. Declining the prompt reports
'UAC prompt declined'.

internal/service/errors.go

@@ -0,0 +1,8 @@
+package service
+
+import "errors"
+
+// ErrUserCancelled is returned by RelaunchElevated when the user declines
+// the UAC prompt. Windows-only in practice; defined here so cross-platform
+// callers can compare against it.
+var ErrUserCancelled = errors.New("UAC prompt declined")

internal/service/service_linux.go

@@ -33,6 +33,15 @@ const (
 // IsService reports whether the process was started by systemd.
 func IsService() bool { return os.Getenv("INVOCATION_ID") != "" }
 
+// Elevated is always true on Linux: there is no UAC equivalent; privilege
+// errors surface from the failing operation with a "run as root" hint.
+func Elevated() bool { return true }
+
+// RelaunchElevated is a Windows-only concept (UAC).
+func RelaunchElevated([]string) (int, error) {
+	return 0, fmt.Errorf("elevated relaunch is only supported on Windows")
+}
+
 // Run executes run with SIGINT/SIGTERM cancellation (which is how systemctl
 // stop signals the process) and tells systemd when the shutdown begins.
 func Run(run func(ctx context.Context) error) error {

internal/service/service_other.go

@@ -31,5 +31,12 @@ func Run(run func(ctx context.Context) error) error {
 // Install is unsupported on non-Windows, non-Linux platforms.
 func Install(string, bool) error { return errUnsupported }
 
-// Remove is unsupported on non-Windows platforms.
+// Remove is unsupported on non-Windows, non-Linux platforms.
 func Remove() error { return errUnsupported }
+
+// Elevated is always true here: there is no UAC concept, and privilege
+// errors surface from the failing operation with a "run as root" hint.
+func Elevated() bool { return true }
+
+// RelaunchElevated is unsupported on non-Windows, non-Linux platforms.
+func RelaunchElevated([]string) (int, error) { return 0, errUnsupported }

internal/service/service_windows.go

@@ -9,14 +9,18 @@ package service
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"
+	"syscall"
 	"time"
+	"unsafe"
 
+	"golang.org/x/sys/windows"
 	"golang.org/x/sys/windows/svc"
 	"golang.org/x/sys/windows/svc/mgr"
 
@@ -242,6 +246,83 @@ func Remove() error {
 	return nil
 }
 
+var procShellExecuteExW = windows.NewLazySystemDLL("shell32.dll").NewProc("ShellExecuteExW")
+
+const seeMaskNoCloseProcess = 0x40
+
+// shellExecuteInfo mirrors SHELLEXECUTEINFOW (64-bit layout).
+type shellExecuteInfo struct {
+	cbSize       uint32
+	fMask        uint32
+	hwnd         uintptr
+	lpVerb       *uint16
+	lpFile       *uint16
+	lpParameters *uint16
+	lpDirectory  *uint16
+	nShow        int32
+	_            int32
+	hInstApp     uintptr
+	lpIDList     unsafe.Pointer
+	lpClass      *uint16
+	hkeyClass    uintptr
+	dwHotKey     uint32
+	_            uint32
+	hIcon        uintptr
+	hProcess     windows.Handle
+}
+
+// Elevated reports whether the current process token is UAC-elevated.
+func Elevated() bool {
+	var token windows.Token
+	if err := windows.OpenProcessToken(windows.CurrentProcess(), windows.TOKEN_QUERY, &token); err != nil {
+		return false
+	}
+	defer token.Close()
+	return token.IsElevated()
+}
+
+// RelaunchElevated re-runs the current executable elevated via the UAC
+// "runas" verb with the given arguments, waits for the child, and returns
+// its exit code. The child gets a fresh console window for its output.
+func RelaunchElevated(args []string) (int, error) {
+	exe, err := os.Executable()
+	if err != nil {
+		return 0, err
+	}
+	quoted := make([]string, len(args))
+	for i, a := range args {
+		quoted[i] = syscall.EscapeArg(a)
+	}
+	cwd, _ := os.Getwd()
+	verb, _ := windows.UTF16PtrFromString("runas")
+	exeP, _ := windows.UTF16PtrFromString(exe)
+	params, _ := windows.UTF16PtrFromString(strings.Join(quoted, " "))
+	dir, _ := windows.UTF16PtrFromString(cwd)
+	info := shellExecuteInfo{
+		fMask:        seeMaskNoCloseProcess,
+		lpVerb:       verb,
+		lpFile:       exeP,
+		lpParameters: params,
+		lpDirectory:  dir,
+		nShow:        windows.SW_NORMAL,
+	}
+	info.cbSize = uint32(unsafe.Sizeof(info))
+	r, _, callErr := procShellExecuteExW.Call(uintptr(unsafe.Pointer(&info)))
+	if r == 0 {
+		if errors.Is(callErr, syscall.Errno(1223)) { // ERROR_CANCELLED
+			return 0, ErrUserCancelled
+		}
+		return 0, fmt.Errorf("ShellExecuteEx: %w", callErr)
+	}
+	defer windows.CloseHandle(windows.Handle(info.hProcess))
+	windows.WaitForSingleObject(windows.Handle(info.hProcess), windows.INFINITE)
+	var code uint32
+	if err := windows.GetExitCodeProcess(windows.Handle(info.hProcess), &code); err != nil {
+		return 0, err
+	}
+	return int(code), nil
+}
+
 // grantAccess gives the virtual account the icacls permission set (e.g.
 // "(OI)(CI)(M)") on path.
 func grantAccess(path, perms string) error {