mangoapp: Select unique SysV message queue

[ColinKinloch]

This allows multiple mangoapp instances to run on the same system.

Currently gamescope and mangoapp communicate using a System V message queue with the key 0xffffffff. This is because ftok("mangoapp", 65) returns -1 when it fails to find mangoapp in the current working directory.
This means messages from any instance of gamescope will be received by any instance of mangoapp.

A simple fix for this would be to use ftok(".", 65) which would generate a key from the inode of the working directory and the number 65, however this would break backwards compatibility and cause collisions for gamescope instances sharing a working directory.

Instead I decided to use a POSIX message queues with the descriptive name /gamescope-mangoapp-{gamescope pid} which gets shared with mangoapp using the environment variable MANGOAPP_MQ_NAME.

This branch avoids collisions by using the current directory and incrementing the proj_id field until getmsg successfully creates a new queue.

This implementation also leaves the old message queues intact which should silently fail when they fill up.

Works with flightlessmango/MangoHud#1502

before	after

[matte-schwartz]

I was able to find a case where #1653 would rapidly switch the reported engine name between showing "Gamescope" and "Vulkan" if you launched gamescope --mangoapp -- glxgears and gamescope --mangoapp -- vkcube alongside one another on Cosmic desktop without your PR.

Adding your PR to both gamescope and MangoHud on top of what I already had solved this issue, and made each window lock to the correct engine information

[misyltoad]

Is this backwards compatible with old mangoapp and vice-versa? It looks it, but I won't lie I am not super up on mq code.

[ColinKinloch]

Yes.

gamescope passes the same message to both queues so whichever version of mangoapp will continue receiving data. Eventually the unused queue will fill up but both are non blocking so this will return EAGAIN errors and will waste 16384 bytes in the case of the sysv queue.

From mangoapps perspective if MANGOAPP_MQ_NAME is set it will only consume messages from the POSIX queue, so will continue to work with old gamescope versions.

I hope everything I've said is correct, but this is my first time interacting with message queues.

[ColinKinloch]

I run steam in a flatpak and it seems sandboxing message queues isn't simple.

I'm going to come up with a better solution 😩.

[ColinKinloch]

I've updated this to attempt to find a clear System V message queue key.

It continues to push messages to the old queue so is still backwards compatible.