Race condition in fuse mount on Ubuntu 26.04

[tpasternak-engflow]

Repro steps:

1. Use Ubuntu 26.04
2. Run for i in $(seq 1 100); do docker run --rm --runtime=sysbox-runc debian/latest /bin/true& done

Observed results:

docker: Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: failed to pre-register with sysbox-fs: failed to pre-register container with sysbox-fs: rpc error: code = Internal desc = Initialization error for container-id

journalctl -u sysbox-fs.service contains lines:

mount helper error: fusermount3: file descriptor 3 is not a socket, can't send fuse fd

Affected versions:

Confirmed on 0.7.0 and 0.7.1

Early investigation:

It seems that something weird happen between these lines:

	fds, err := syscall.Socketpair(syscall.AF_UNIX, syscall.SOCK_STREAM, 0)
	if err != nil {
		return nil, fmt.Errorf("socketpair error: %v", err)
	}


	writeFile := os.NewFile(uintptr(fds[0]), "fusermount-child-writes")
	defer writeFile.Close()


	readFile := os.NewFile(uintptr(fds[1]), "fusermount-parent-reads")
	defer readFile.Close()


	cmd := exec.Command(
		"fusermount3",
		"-o", conf.getOptions(),
		"--",
		dir,
	)
	cmd.Env = append(os.Environ(), "_FUSE_COMMFD=3")

Most likely, syscall.Socketpair returns an incorrect fd, or it initializes it lazily