+++ This bug was initially created as a clone of Bug #1335250 +++

Description of problem:

lvm.listVgNames() method is invoked, even when yum is configured to not make snapshots.

Version-Release number of selected component (if applicable):

Installed Packages
Name        : yum
Arch        : noarch
Version     : 3.4.3
Release     : 132.el7
Size        : 5.5 M
Repo        : installed
From repo   : RHEL-7.2
Summary     : RPM package installer/updater/manager

How reproducible:

Always, in a sense. That is, it can be observed that _FSsnap constructor builds a list of vgs.

Steps to Reproduce:
1. I noticed this when encountering another bug.
2. I observed a stack trace:

Running transaction
Traceback (most recent call last):
  File "/usr/bin/yum", line 29, in <module>
    yummain.user_main(sys.argv[1:], exit_code=True)
  File "/usr/share/yum-cli/yummain.py", line 365, in user_main
    errcode = main(args)
  File "/usr/share/yum-cli/yummain.py", line 271, in main
    return_code = base.doTransaction()
  File "/usr/share/yum-cli/cli.py", line 773, in doTransaction
    resultobject = self.runTransaction(cb=cb)
  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 1736, in runTransaction
    if self.fssnap.available and ((self.conf.fssnap_automatic_pre or
  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 1126, in <lambda>
    fssnap = property(fget=lambda self: self._getFSsnap(),
  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 1062, in _getFSsnap
    devices=devices)
  File "/usr/lib/python2.7/site-packages/yum/fssnapshots.py", line 158, in __init__
    self._vgnames = _list_vg_names() if self.available else []
  File "/usr/lib/python2.7/site-packages/yum/fssnapshots.py", line 56, in _list_vg_names
    names = lvm.listVgNames()
lvm.LibLVMError: (-1, 'Request to list VGs in lvmetad failed after device filter mismatch.')

3. I looked at yum configuration:

fssnap_abort_on_errors = any
fssnap_automatic_keep = 1
fssnap_automatic_post = False
fssnap_automatic_pre = False
fssnap_devices = !*/swap,
   !*/lv_swap
fssnap_percentage = 100

and it seemed to me that ultimately, with this configuration, no snapshot would
be taken, so there was no point in calculating the vgnames in the first place.


Actual results:

A stack trace due to computing vg names, even when configuration options indicate that lvm will not be used.

Expected results:

No computing of vg names if the value is going to turn out to be useless anyway because is configured not to take snapshots.

Additional info:

I guess removing the calculating from the _FSsnap constructor would be safest,
but using short-circuiting to check the configuration options before calling the construction would also probably work.

I note that the if condition above

  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 1736, in runTransaction
    if self.fssnap.available and ((self.conf.fssnap_automatic_pre or

which is:

        if (self.conf.fssnap_automatic_pre or self.conf.fssnap_automatic_post) and not self.fssnap.available:

puts the self.fssnap.available clause after the configuration clauses, otherwise, I think the exception would have been raise there.