Problems switching VTs

Crashing X (or any display server) can happen if it tries to make drm calls when not owning DRM mastership. Each display server needs to have its own guarantees that it only calls drm functions after successfully taking DRM mastership.

X will crash if it cannot claim DRM master. Do we have an X log for this bit?

However, the USC log strongly suggests that USC has successfully dropped master.

The X log says "Fatal server error: EnterVT failed for screen 0". Nothing more specific.

Some more bits of info:

- Nothing bad happens if I manually (Ctrl+Alt+FN) switch between the VTs. But I'm guessing that's a deep kernel thing, a different code path.

- If I instead try to reproduce by using unity8-greeter (which uses Mir and USC just like a unity8 session does) and logging into an existing unity7 session -- i.e. the same reproduction steps just with a greeter instead of a session -- I can't reproduce. So the biggest difference here is that the USC/greeter session is torn down as part of the switch (I don't know if before or after the switch yet). With my original repro steps, USC stays around the whole time.

Next step is to use chvt to test whether removing LightDM from the mix helps.

I tested with chvt over ssh, didn't reproduce.

HOWEVER, this seems a touch harder to reproduce than I had thought originally. So my earlier lack of success reproducing in different cases carry less weight than I hoped.

However, with RAOF's help, I have a bit more of a verbose message from X. I also now see this error: "(WW) intel(0): drmDropMaster failed: Invalid argument"

But that message comes after, not before the EnterVT error. And seems to happen during a LeaveVT call. So that might simply be a response to the previous EnterVT failure.