🔫 darwin: set TCP_FASTOPEN_FORCE_ENABLE

This disables the absolutely brutal TFO backoff mechanism.

tfo_darwin.go

@@ -8,13 +8,44 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+const TCP_FASTOPEN_FORCE_ENABLE = 0x218
+
+// setTFOForceEnable disables the absolutely brutal TFO backoff mechanism.
+func setTFOForceEnable(fd uintptr) error {
+	return unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, TCP_FASTOPEN_FORCE_ENABLE, 1)
+}
+
 func SetTFOListener(fd uintptr) error {
 	return unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_FASTOPEN, 1)
 }
 
 func (lc *ListenConfig) listenTFO(ctx context.Context, network, address string) (net.Listener, error) {
-	// darwin requires setting TCP_FASTOPEN after bind() and listen() calls.
-	ln, err := lc.ListenConfig.Listen(ctx, network, address)
+	// When setting TCP_FASTOPEN_FORCE_ENABLE, the socket must be in the TCPS_CLOSED state.
+	// This means setting it before listen().
+	//
+	// However, setting TCP_FASTOPEN requires being in the TCPS_LISTEN state,
+	// which means setting it after listen().
+	llc := *lc
+	llc.Control = func(network, address string, c syscall.RawConn) (err error) {
+		if lc.Control != nil {
+			if err = lc.Control(network, address, c); err != nil {
+				return err
+			}
+		}
+
+		if cerr := c.Control(func(fd uintptr) {
+			err = setTFOForceEnable(fd)
+		}); cerr != nil {
+			return cerr
+		}
+
+		if err != nil {
+			return wrapSyscallError("setsockopt", err)
+		}
+		return nil
+	}
+
+	ln, err := llc.ListenConfig.Listen(ctx, network, address)
 	if err != nil {
 		return nil, err
 	}
@@ -25,25 +56,23 @@ func (lc *ListenConfig) listenTFO(ctx context.Context, network, address string)
 		return nil, err
 	}
 
-	var innerErr error
-
-	if err = rawConn.Control(func(fd uintptr) {
-		innerErr = SetTFOListener(fd)
-	}); err != nil {
+	if cerr := rawConn.Control(func(fd uintptr) {
+		err = SetTFOListener(fd)
+	}); cerr != nil {
 		ln.Close()
-		return nil, err
+		return nil, cerr
 	}
 
-	if innerErr != nil {
+	if err != nil {
 		ln.Close()
-		return nil, innerErr
+		return nil, wrapSyscallError("setsockopt", err)
 	}
 
 	return ln, nil
 }
 
 func SetTFODialer(fd uintptr) error {
-	return nil
+	return setTFOForceEnable(fd)
 }
 
 func socket(domain int) (fd int, err error) {