[VDSM] loop device tests fail again on jenkins

[Nir Soffer]

We have random failures of loop device tests on jenkins (see example
bellow).

Barak commented that losetup -f /path does not work sometimes in the CI
but I don't recall what it the alternative way to get a loop device.

We need a reliable way to create a loop device for vdsm tests. What
is the recommended way to do this?

Until we have a reliable solution I'm going to mark this test as broken
on ovirt CI:
https://gerrit.ovirt.org/#/c/86241/

Please report if you see other tests fail with this error - typically:

Error: Command ['losetup', '--find', '--show', '/tmp/tmp17Wqri/file']
failed with rc=1 out='' err='losetup: /tmp/tmp17Wqri/file: failed to
set up loop device: No such file or directory\n'


http://jenkins.ovirt.org/job/vdsm_master_check-patch-el7-x86_64/20862/consoleFull

*00:05:48.766* ======================================================================*00:05:48.766*
ERROR: test_attach_detach_manually
(loopback_test.TestDevice)*00:05:48.767*
----------------------------------------------------------------------*00:05:48.767*
Traceback (most recent call last):*00:05:48.767*   File
"/home/jenkins/workspace/vdsm_master_check-patch-el7-x86_64/vdsm/tests/testValidation.py",
line 191, in wrapper*00:05:48.768*     return f(*args,
**kwargs)*00:05:48.768*   File
"/home/jenkins/workspace/vdsm_master_check-patch-el7-x86_64/vdsm/tests/loopback_test.py",
line 56, in test_attach_detach_manually*00:05:48.768*
device.attach()*00:05:48.768*   File
"/home/jenkins/workspace/vdsm_master_check-patch-el7-x86_64/vdsm/tests/loopback.py",
line 56, in attach*00:05:48.769*     raise cmdutils.Error(cmd, rc,
out, err)*00:05:48.769* Error: Command ['losetup', '--find', '--show',
'/tmp/tmp17Wqri/file'] failed with rc=1 out='' err='losetup:
/tmp/tmp17Wqri/file: failed to set up loop device: No such file or
directory\n'*00:05:48.770* -------------------- >> begin captured
logging << --------------------*00:05:48.770* 2018-01-11 13:42:45,676
DEBUG (MainThread) [root] /usr/bin/taskset --cpu-list 0-1 losetup
--find --show /tmp/tmp17Wqri/file (cwd None)
(commands:65)*00:05:48.771* 2018-01-11 13:42:45,733 DEBUG (MainThread)
[root] FAILED: <err> = 'losetup: /tmp/tmp17Wqri/file: failed to set up
loop device: No such file or directory\n'; <rc> = 1
(commands:86)*00:05:48.772* --------------------- >> end captured
logging << ---------------------
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