[Bug 1553097] Re: lxc-attach does not output stderr any more if stdout is redirected

[Christian Brauner]

Fix proposed (https://github.com/lxc/lxc/pull/873). Tests are included
as well.

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https://bugs.launchpad.net/bugs/1553097

Title:
  lxc-attach does not output stderr any more if stdout is redirected

Status in autopkgtest package in Ubuntu:
  Triaged
Status in lxc package in Ubuntu:
  Triaged

Bug description:
  With lxc 2.0.0~rc5-0ubuntu1 the autopkgtest LXC runner is still broken
  (even with bug 1551960 fixed) when stderr is a terminal:

  $ adt-run gzip --- lxc -s adt-wily
  [...]
  adt-run [10:15:45]: ERROR: erroneous package: got 10 lines of results from extract where 4 expected:
  gpgv: Signature made Mon 27 Oct 2014 09:48:46 AM CET using RSA key ID BD4CA59E
  gpgv: Can't check signature: public key not found
  dpkg-source: warning: failed to verify signature on ./gzip_1.6-4ubuntu1.dsc
  Get:1 http://archive.ubuntu.com/ubuntu/ wily/main gzip 1.6-4ubuntu1 (dsc) [1,907 B]
  Get:2 http://archive.ubuntu.com/ubuntu/ wily/main gzip 1.6-4ubuntu1 (tar) [1,075 kB]
  Get:3 http://archive.ubuntu.com/ubuntu/ wily/main gzip 1.6-4ubuntu1 (diff) [14.9 kB]
  /tmp/adt-virt-lxc.shared.xklzp9z7/downtmp/build.ehH/gzip-1.6
  gzip
  1.6-4ubuntu1
  1

  Thie first couple of lines are supposed to go to stderr, not stdout.
  This works if stderr is not a terminal, but a pipe, with e. g. "adt-
  run -l /dev/null gzip --- lxc -s adt-wily" (where stderr is sent to a
  pipe which gets tee'ed to both the log file and the real stderr).

  This is probably a regression in LXC, and it doesn't happen with older
  LXC nor LXD nor any other backend. But it might also be some
  intentional change which autopkgtest needs to be adjusted to, so for
  now I keep tasks for both projects.

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