Failed to stream on for some special test

[sanbrother]

Step 2: Describe your environment

• v4l2loopback version: 0x0006020d

    commit: 5bb9bed4e2a34761dc5dc188faec7315ae3930fe

• kernel version: Linux MagicBookPro 6.2.0-26-generic #26~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Thu Jul 13 16:27:29 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

• Distribution (+version): _____

    No LSB modules are available.
    Distributor ID:   Ubuntu
    Description:  Ubuntu 22.04.3 LTS
    Release:  22.04
    Codename: jammy

Step 3: Describe the problem:

Steps to reproduce:

1. Reload the module sudo modprobe -r v4l2loopback && sudo modprobe v4l2loopback video_nr=10,11 max_buffers=8
2. Poll some event

   # V4L2_EVENT_PRI_CLIENT_USAGE => 283115521
   v4l2-ctl --poll-for-event=283115521 -d /dev/video10

3. Run test app

   ./producer -d /dev/video10

Observed Results:

VIDIOC_STREAMON error 22, Invalid argument

Relevant Code:

    if (dev->image) {
        dprintk("allocating buffers again: %ld %ld\n",
            dev->buffer_size * dev->buffers_number, dev->imagesize);
        /* FIXME: prevent double allocation more intelligently! */
        if (dev->buffer_size * dev->buffers_number == dev->imagesize)
            return 0;

        /* if there is only one writer, no problem should occur */
        if (dev->open_count.counter == 1)
            free_buffers(dev);
        else
            return -EINVAL;
    }

I guess, v4l2-ctl makes counter == 1, and then the producer increases it to 2. So, EINVAL is returned.