Mir does not start on Ubuntu Core 18/RPi 3

[AlanGriffiths]

This affects mir-kiosk, but is best documented using mir-test-tools:

Install mir-test-tools and run:

sudo snap run mir-test-tools.bash

Then run:

mir_demo_server

Expect: server starts
Actual: server hangs after message "mirserver: Starting"

This only happens the first time, so press Ctrl+C and try again

mir_demo_server

This time the server starts.

The server is hung somewhere in the virtual terminal setup - and if you switch to VT4 while it is hung it will continue to a normal startup.

[AlanGriffiths]

This seems to be system-boot/cmdline.txt in the core18 image. It reads:

dwc_otg.lpm_enable=0 console=serial0,115200 elevator=deadline rng_core.default_quality=700 vt.handoff=2 quiet splash

Changing that to:

dwc_otg.lpm_enable=0 console=serial0,115200 elevator=deadline rng_core.default_quality=700 quiet splash

Allows Mir to start correctly.

(Thanks ogra for telling me about this.)

[AlanGriffiths]

Some fun facts discovered while investigating:

1. attaching strace to a hung server causes it to start working. Looking at the control flow, and assuming strace doesn't miss something, we're probably hanging on ioctl(vt_fd.fd(), KDSETMODE, KD_GRAPHICS)
2. Extracting the VT from /proc/cmdline and using it as the default DOESN'T work. But...
3. snap set mir-kiosk vt=0 does work (after a reboot).

[AlanGriffiths]

No longer causing problems (workarounds implemented)