Add queue support [Security Issue]

[xkill]

Hi,

I think that having a queue for multiple conversions.

If a server is doing multiple conversions, even if it all of them are with low priority, it hangs.
I suggest to have a queue, allowing only one (or multiple, settings could be changed) process at the same time.
The queue can be changed using the priorities, so when a high priority process is added to the queue, it should be at the top of the queue.

The users should be able to see the queue status. Not other users queue, but the position on the list. For example if the queue is:

1. user1 -> video1 -> processing....
2. user1 -> video2
3. user2 -> video3
4. user3 -> video4
5. user1 -> video5

The user1, can see the queue as:

1. user1 -> video1 -> position 1 -> processing....
2. user1 -> video2 -> position 2
3. user1 -> video5 -> position 5

The user2, only can see the queue as:
3. user2 -> video3 -> position 3

The users should also be able to cancel the queued jobs and, potentially cancel the current process.

NOTE: It is a security issue as the users can generate a DoS because of several concurrent processes.

• CVSSv2: 6.8 AV:N/AC:L/Au:S/C:N/I:N/A:C
• CVSSv3: 7.7 AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H

EDIT: The denial of service can be done because two factors:

• CPU consuming
• RAM consuming (multiple instances of ffmpeg consume a lot of RAM)

CVE-2019-18214

[NicoleG25]

@PaulLereverend was this issue ever addressed?
Please note that CVE-2019-18214 was assigned to this issue.