Some standard updates break on classic, due to missing /etc/writable bind mount

[Alan Pope 🍺🐧🐱 🦄]

Install snappy on a raspberry pi, and enable classic mode with:-

sudo snappy enable-classic

Shell into the classic mode install and do updates:-

snappy shell classic

Now do your regular updates. When one for tzdata comes along (which happens relatively frequently), it breaks due to /etc/timezone being read-only (I think). This has the further effect of breaking snapcraft inside the classic shell when installing. As shown below.

(classic)ubuntu@localhost:~/qtox/snap$ snapcraft snap
Installing build dependencies: autopoint
[sudo] password for ubuntu:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
  autopoint
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
2 not fully installed or removed.
Need to get 406 kB of archives.
After this operation, 457 kB of additional disk space will be used.
Get:1 http://ports.ubuntu.com/ubuntu-ports xenial/main armhf autopoint all 0.19.7-2ubuntu3 [406 kB]
Fetched 406 kB in 0s (1537 kB/s)
E: Can not write log (Is /dev/pts mounted?) - posix_openpt (2: No such file or directory)
Selecting previously unselected package autopoint.
(Reading database ... 40965 files and directories currently installed.)
Preparing to unpack .../autopoint_0.19.7-2ubuntu3_all.deb ...
Unpacking autopoint (0.19.7-2ubuntu3) ...
Processing triggers for man-db (2.7.5-1) ...
Setting up tzdata (2016b-0ubuntu1) ...
/var/lib/dpkg/info/tzdata.config: 379: /var/lib/dpkg/info/tzdata.config: cannot create /etc/timezone: Directory nonexistent
dpkg: error processing package tzdata (--configure):
 subprocess installed post-installation script returned error exit status 2
dpkg: dependency problems prevent configuration of ubuntu-minimal:
 ubuntu-minimal depends on tzdata; however:
  Package tzdata is not configured yet.

dpkg: error processing package ubuntu-minimal (--configure):
 dependency problems - leaving unconfigured
Setting up autopoint (0.19.7-2ubuntu3) ...
Errors were encountered while processing:
 tzdata
 ubuntu-minimal

[Alexander Sack (asac)]

filed partly dupe (*also has sudo problem which is RO mounted) with maybe a bit more triaging info: https://bugs.launchpad.net/snappy/+bug/1564316

[Oliver Grawert (ogra)]

this is a different bug, not a duplicate.
the files /etc/timezone, /etc/localtime and /etc/hostname are links to files in /etc/writable/.
the classic shell does not bind mount /etc/writable to etc/writable/ inside the classic env, so the three files are dangling symlinks.

ubuntu@localhost:~$ sudo mkdir -p /writable/classic/etc/writable
ubuntu@localhost:~$ sudo mount --bind /etc/writable /writable/classic/etc/writable

will fix this issue.