ondemand does not work for WebEx

[mulbc]

Great project here! I tried to use it for WebEx, but the ondemand feature doesn't work for the in-browser tool. It always registers just one consumer and thus pauses the webcam.
When I unpause it using Ctrl+C, it immediately re-pauses again.

Checking out the same thing with Google Meet, it works fine (same browser, same everything)

I checked the code and maybe listenting to inodes isn't the best approach? Maybe using something like fuser $V4L2LOOPBACK_PATH (which lists the processes accessing the device) might work better?
This might also help with #167

Environment:
Browser Chrome
OS Fedora 36
v4l2loopback

[fangfufu]

Do you have suggestion of using anything other than inotify?

[mulbc]

yes, as I said, we could use lsof/fuser capabilities to check the open files by processes. That way we can count the PIDs that have our V4L2LOOPBACK device open. We can directly use this as the count of consumers in the existing code.
Unfortunately there do not appear to be libraries for lsof/fuser, so we would need to use a syscall for this. Would you be ok with that?

[mulbc]

We could also look into what WebEx does differently to Google Meet in regards to inotify, but I'm unsure how I would debug that

[mulbc]

So I was able to capture the inotify messages for WebEx and Google Meet

WebEx:

/dev/video2 CLOSE_WRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN             <-- This is the double OPEN which should add the consumer to fake.py
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
      --> Leaving meeting, camera access closes
/dev/video2 CLOSE_WRITE,CLOSE 

Google Meet:

  One OPEN/CLOSE before we even join the meeting (while on meet.google.com)
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
--> Joining the meeting
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN      --> Double OPEN to mark the consumer in fake.py
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 CLOSE_WRITE,CLOSE --> Leaving the meeting, double CLOSE
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 
/dev/video2 OPEN 
/dev/video2 CLOSE_NOWRITE,CLOSE 

In the end this might be a timing issue with the inotify messages coming in too quickly, thus we do not catch all of them?

Command to capture the messages is

inotifywait --inotify -m /dev/video2 | grep -v MODIFY

[fangfufu]

If you provide a patch, I am happy to review it and merge it in. :)

[mulbc]

Just FYI - I did rewrite the code so that the inotify part is in a separate thread, but for some reason, it still doesn't reliably detect webcam usage (that's why you didn't get a PR yet)
I will see if/how I can do that better with fuser now

[fangfufu]

What's fuser?

[mulbc]

It's a linux cli tool to list processes using a certain file:
https://man7.org/linux/man-pages/man1/fuser.1.html

[gchamon]

Using google chrome and gather.town I was unable to find a reliable pattern to make the ondemand work:

Each number is me toggling the fake camera (by configuring a different device in the app).

I guess this isn't a reliable method overall to detect consumers, maybe.

---

EDIT
sometimes it works as expected:

Still I guess this makes for the argument that this method is indeed unreliable.

---

EDIT2

Maybe something could be done with lsof?

We would have to make sure to filter out all system applications.

[machineghost]

Just two cents from a random user: making a command you can't CTRL+C, and have to close the entire terminal just to terminate, is very frustrating.

[fangfufu]

@machineghost , you can use CTRL + \ to terminate. That sends SIGKILL.

[machineghost]

I've run Linux for over a decade and never had to kill a command that way. I think it's fair to assume a decent percentage of other users won't magically know to do CTRL + \.

[fangfufu]

Well if you have better suggestions on which signal I should intercept to support changing background while the webcam is open, do let me know. :)

[machineghost]

CTRL+ C is pretty much the universal "cancel" signal. Failing that, CTRL + D is the fairly universal "disconnect" signal.