fix(auth): harden diagnostic cleanup races

tools/tht/internal/compose/process_windows.go

@@ -6,15 +6,20 @@ import (
 	"context"
 	"io"
 	"os/exec"
+	"path/filepath"
 	"strconv"
 	"syscall"
 	"time"
+
+	"golang.org/x/sys/windows"
 )
 
 const finalTerminationBound = 2 * time.Second
 const processTreeTerminationBound = 2 * time.Second
 const createNewProcessGroup = 0x00000200
 
+var windowsSystemDirectory = windows.GetSystemDirectory
+
 func configureProcess(command *exec.Cmd) {
 	command.SysProcAttr = &syscall.SysProcAttr{CreationFlags: createNewProcessGroup}
 }
@@ -23,23 +28,22 @@ func terminateProcess(command *exec.Cmd, done <-chan error) error {
 	if command.Process == nil {
 		return nil
 	}
-	treeErr := terminateWindowsProcessTree(command.Process.Pid)
-	_ = command.Process.Kill()
-	select {
-	case <-done:
-		if treeErr != nil {
-			return ErrProcessReap
-		}
-		return nil
-	case <-time.After(finalTerminationBound):
-		return ErrProcessReap
-	}
+	return terminateWindowsProcess(
+		done,
+		func() error { return terminateWindowsProcessTree(command.Process.Pid) },
+		command.Process.Kill,
+		finalTerminationBound,
+	)
 }
 
 func terminateWindowsProcessTree(pid int) error {
 	ctx, cancel := context.WithTimeout(context.Background(), processTreeTerminationBound)
 	defer cancel()
-	command := windowsTreeKillCommand(ctx, pid)
+	taskkillPath, err := systemTaskkillPath()
+	if err != nil {
+		return ErrProcessReap
+	}
+	command := windowsTreeKillCommand(ctx, taskkillPath, pid)
 	command.Stdout = io.Discard
 	command.Stderr = io.Discard
 	if err := command.Run(); err != nil {
@@ -51,6 +55,23 @@ func terminateWindowsProcessTree(pid int) error {
 	return nil
 }
 
-func windowsTreeKillCommand(ctx context.Context, pid int) *exec.Cmd {
-	return exec.CommandContext(ctx, "taskkill.exe", "/PID", strconv.Itoa(pid), "/T", "/F")
+// systemTaskkillPath resolves taskkill through Windows' protected system-directory API, never PATH.
+func systemTaskkillPath() (string, error) {
+	systemDirectory, err := windowsSystemDirectory()
+	if err != nil {
+		return "", ErrProcessReap
+	}
+	systemDirectory = filepath.Clean(systemDirectory)
+	if systemDirectory == "." || !filepath.IsAbs(systemDirectory) {
+		return "", ErrProcessReap
+	}
+	taskkillPath := filepath.Join(systemDirectory, "taskkill.exe")
+	if !filepath.IsAbs(taskkillPath) || filepath.Dir(taskkillPath) != systemDirectory {
+		return "", ErrProcessReap
+	}
+	return taskkillPath, nil
+}
+
+func windowsTreeKillCommand(ctx context.Context, taskkillPath string, pid int) *exec.Cmd {
+	return exec.CommandContext(ctx, taskkillPath, "/PID", strconv.Itoa(pid), "/T", "/F")
 }