Fix control pipe races: treat ERROR_PIPE_CONNECTED as success, flush+disconnect before close so replies are never discarded

internal/control/control_windows.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"log/slog"
 	"os"
+	"syscall"
 	"unsafe"
 
 	"golang.org/x/sys/windows"
@@ -78,19 +79,43 @@ func Serve(ctx context.Context, h Handler, log *slog.Logger) error {
 			}
 			go func() {
 				// Blocks until a client connects; on process exit the
-				// handle goes away with everything else.
-				if err := windows.ConnectNamedPipe(pipe, nil); err != nil {
+				// handle goes away with everything else. A client that
+				// raced us and connected between CreateNamedPipe and
+				// ConnectNamedPipe reports ERROR_PIPE_CONNECTED — that is
+				// a success, not a failure.
+				if err := windows.ConnectNamedPipe(pipe, nil); err != nil && err != errnoPipeConnected {
 					windows.CloseHandle(pipe)
 					return
 				}
-				f := os.NewFile(uintptr(pipe), pipePath)
-				serveConn(f, h) // closes f, and with it the pipe handle
+				serveConn(&pipeConn{f: os.NewFile(uintptr(pipe), pipePath), h: pipe}, h)
 			}()
 		}
 	}()
 	return nil
 }
 
+// errnoPipeConnected is ConnectNamedPipe's "the client connected before we
+// called" result, which means the connection is established.
+var errnoPipeConnected = syscall.Errno(535) // ERROR_PIPE_CONNECTED
+
+// 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
+// unread reply bytes, which clients see as an empty, failed request.
+type pipeConn struct {
+	f *os.File
+	h windows.Handle
+}
+
+func (c *pipeConn) Read(p []byte) (int, error)  { return c.f.Read(p) }
+func (c *pipeConn) Write(p []byte) (int, error) { return c.f.Write(p) }
+
+func (c *pipeConn) Close() error {
+	windows.FlushFileBuffers(c.h)    //nolint:errcheck // best effort
+	windows.DisconnectNamedPipe(c.h) //nolint:errcheck // best effort
+	return c.f.Close()
+}
+
 // Ask sends one command to the running service and returns its reply.
 func Ask(cmd string) (string, error) {
 	name, err := windows.UTF16PtrFromString(pipePath)