Bug#842299: autopkgtest-virt-qemu: attaching base image makes btrfs very confused

[Simon McVittie]

Package: autopkgtest
Version: 4.1
Severity: normal
Tags: patch

btrfs gets *really* confused if you attach multiple copies of the
same filesystem, because it tracks filesystems by UUID, not by
device node (part of its built-in RAID-equivalent):
<https://btrfs.wiki.kernel.org/index.php/Gotchas#Block-level_copies_of_devices>

In my experiments with a btrfs-based virtual machine, /proc/self/mounts
etc. thought the mounted partitions and subvolumes all came from
/dev/vdb, even though they clearly weren't because only /dev/vda could
have been mounted read/write.

For least-astonishment, the default should perhaps be to *not* provide
the base image as a block device (and tests that need to exercise
nested virtualization can specifically ask for it), but that would be
a behaviour change so I haven't done it in the attached patch.

-- System Information:
Debian Release: stretch/sid
  APT prefers unstable-debug
  APT policy: (500, 'unstable-debug'), (500, 'unstable'), (500, 'testing'), (500, 'stable'), (1, 'experimental-debug'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.7.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.utf8, LC_CTYPE=en_GB.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages autopkgtest depends on:
ii  apt-utils       1.3.1
ii  libdpkg-perl    1.18.10
ii  procps          2:3.3.12-2
ii  python3         3.5.1-4
ii  python3-debian  0.1.29

Versions of packages autopkgtest recommends:
ii  autodep8  0.8

Versions of packages autopkgtest suggests:
pn  lxc          <none>
pn  lxd-client   <none>
ii  qemu-system  1:2.7+dfsg-1
ii  qemu-utils   1:2.7+dfsg-1
ii  schroot      1.6.10-2+b1

-- no debconf information
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