Allow REQBUF to change number of buffers

[stephematician]

According to the V4L API (https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/vidioc-reqbufs.html):

Applications can call ioctl VIDIOC_REQBUFS again to change the number of buffers. Note that if
any buffers are still mapped or exported via DMABUF, then ioctl VIDIOC_REQBUFS can only succeed
if the V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS capability is set. Otherwise ioctl VIDIOC_REQBUFS
will return the EBUSY error code. If V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS is set, then these
buffers are orphaned and will be freed when they are unmapped or when the exported DMABUF fds
are closed. A count value of zero frees or orphans all buffers, after aborting or finishing any
DMA in progress, an implicit VIDIOC_STREAMOFF.

So - if there hasn't been any call to mmap (or munmap has been called on all buffers), shouldn't it be possible to call REQBUF to change the number of buffers?

[stephematician]

Is it as simple as this?

diff --git a/v4l2loopback.c b/v4l2loopback.c
index 25cb1be..4bba805 100644
--- a/v4l2loopback.c
+++ b/v4l2loopback.c
@@ -1959,6 +1959,7 @@ static int vidioc_streamoff(struct file *file, void *fh,
        case V4L2_BUF_TYPE_VIDEO_OUTPUT:
                if (dev->ready_for_capture > 0)
                        dev->ready_for_capture--;
+               dev->ready_for_output = 1;
                return 0;
        case V4L2_BUF_TYPE_VIDEO_CAPTURE:
                if (opener->type == READER) {

This is assuming that only one process should be able to streamoff when writing memory mapped I/O.