Fix control-pipe client race: retry CreateFile on ERROR_PIPE_BUSY

WaitNamedPipe can report an instance that a concurrent client (the
monitor polls status every second) grabs before our CreateFile runs;
the single-attempt Ask then failed with 'no answer from the service'.
Retry the wait+open until a 5s overall deadline.

internal/control/control_windows.go

@@ -8,6 +8,7 @@ import (
 	"log/slog"
 	"os"
 	"syscall"
+	"time"
 	"unsafe"
 
 	"golang.org/x/sys/windows"
@@ -99,6 +100,11 @@ func Serve(ctx context.Context, h Handler, log *slog.Logger) error {
 // called" result, which means the connection is established.
 var errnoPipeConnected = syscall.Errno(535) // ERROR_PIPE_CONNECTED
 
+// errnoPipeBusy is CreateFile's "all pipe instances are busy" result. It
+// loses the race against another client that grabbed the instance
+// WaitNamedPipe just reported — the caller must wait and retry.
+var errnoPipeBusy = syscall.Errno(231) // ERROR_PIPE_BUSY
+
 // pipeConn adapts a pipe handle to io.ReadWriteCloser. Close flushes first
 // (FlushFileBuffers blocks until the client has read the reply) and then
 // disconnects — closing the bare handle right after writing can discard
@@ -126,21 +132,36 @@ func (c *pipeConn) ForceClose() error {
 }
 
 // Ask sends one command to the running service and returns its reply.
+//
+// The server keeps exactly one listening instance per connection, so
+// concurrent clients race for it: WaitNamedPipe can report an instance
+// that another client grabs before our CreateFile runs (ERROR_PIPE_BUSY).
+// Retry on that — with the monitor polling status every second, a single
+// attempt loses that race regularly.
 func Ask(cmd string) (string, error) {
 	name, err := windows.UTF16PtrFromString(pipePath)
 	if err != nil {
 		return "", err
 	}
-	if err := waitNamedPipe(name, 2000); err != nil {
+	deadline := time.Now().Add(5 * time.Second)
-		return "", ErrUnavailable
+	for {
+		if err := waitNamedPipe(name, 2000); err != nil {
+			return "", ErrUnavailable
+		}
+		handle, err := windows.CreateFile(name,
+			windows.GENERIC_READ|windows.GENERIC_WRITE, 0, nil,
+			windows.OPEN_EXISTING, 0, 0)
+		if err == errnoPipeBusy {
+			if time.Now().After(deadline) {
+				return "", ErrUnavailable
+			}
+			continue
+		}
+		if err != nil {
+			return "", ErrUnavailable
+		}
+		f := os.NewFile(uintptr(handle), pipePath)
+		defer f.Close()
+		return readReply(f, cmd)
 	}
-	handle, err := windows.CreateFile(name,
-		windows.GENERIC_READ|windows.GENERIC_WRITE, 0, nil,
-		windows.OPEN_EXISTING, 0, 0)
-	if err != nil {
-		return "", ErrUnavailable
-	}
-	f := os.NewFile(uintptr(handle), pipePath)
-	defer f.Close()
-	return readReply(f, cmd)
 }