Using classic clobbers /dev/pts on ubuntu-core 16 systems

[zyga]

tl;dr; installation and usage of classic changes the permission of /dev/pts as follows:

-7 4 2:3 / /dev/pts rw,nosuid,noexec,relatime shared:6 - devpts devpts rw,gid=5,mode=620,ptmxmode=000
+7 4 2:3 / /dev/pts rw,nosuid,noexec,relatime shared:6 - devpts devpts rw,mode=666,ptmxmode=666

This can be reproduced by running spread -shell-before google:ubuntu-core-16-64:tests/main/ubuntu-core-classic and then following with those commands:

State before

google:ubuntu-core-16-64 .../tests/main/ubuntu-core-classic# mountinfo-tool /dev/pts
22 21 0:14 / /dev/pts rw,nosuid,noexec,relatime shared:49 - devpts devpts rw,gid=5,mode=620,ptmxmode=000

Installation & startup of classic

google:ubuntu-core-16-64 .../tests/main/ubuntu-core-classic# sudo snap install --devmode --beta classic
classic (beta) 16.04 from Canonical✓ installed

google:ubuntu-core-16-64 .../tests/main/ubuntu-core-classic# mountinfo-tool /dev/pts
22 21 0:14 / /dev/pts rw,nosuid,noexec,relatime shared:49 - devpts devpts rw,gid=5,mode=620,ptmxmode=000

google:ubuntu-core-16-64 .../tests/main/ubuntu-core-classic# classic
Creating classic environment
(redacted for brevity)
(classic)test@jul251846-303208:~$ exit
logout

State after

At this point /dev/pts is changed, even after you remove the snap.

google:ubuntu-core-16-64 .../tests/main/ubuntu-core-classic# mountinfo-tool /dev/pts
22 21 0:14 / /dev/pts rw,nosuid,noexec,relatime shared:49 - devpts devpts rw,mode=666,ptmxmode=666