[Xen-API] [Project Kronos] Possible bug in pbd-unplug for lvm based SRs?

[Errol Neal <eneal@xxxxxxxxxxxxxxxxx>]

Hi. When unplugging an pbd, seems like dmsetup remove is run which runs 
successfully, however LVHDSR.detach reports a failure:

LVHDSR.detach: failed to remove the symlink for file 
/dev/mapper/VG_XenStorage--36659c63--3de9--2180--3697--dda90a0de303-MGT. Error: 
[Errno 2] No such file or directory: 
'/dev/VG_XenStorage-36659c63-3de9-2180-3697-dda90a0de303/MGT'

And I get an error:

root@squeeze:/usr/lib/xen-common/xapi/sm# xe pbd-unplug 
uuid=f4f71420-44f4-8b2d-6b84-301cbb13c2b9
The server failed to handle your request, due to an internal error.  The given 
message may give details useful for debugging the problem.
message: Failure("Storage_access failed with: SR_BACKEND_FAILURE: [ non-zero 
exit; ; /usr/lib/xen-common/xapi/sm/util.py:17: DeprecationWarning: The popen2 
module is deprecated.  Use the subprocess module.\n  import os, re, sys, 
popen2, subprocess\nTraceback (most recent call last):\n  File 
\"/usr/lib/xen-common/xapi/sm/LVMSR\", line 1959, in <module>\n    
SRCommand.run(LVHDSR, DRIVER_INFO)\n  File 
\"/usr/lib/xen-common/xapi/sm/SRCommand.py\", line 252, in run\n    ret = 
cmd.run(sr)\n  File \"/usr/lib/xen-common/xapi/sm/SRCommand.py\", line 94, in 
run\n    return self._run_locked(sr)\n  File 
\"/usr/lib/xen-common/xapi/sm/SRCommand.py\", line 131, in _run_locked\n    
return self._run(sr, target)\n  File 
\"/usr/lib/xen-common/xapi/sm/SRCommand.py\", line 232, in _run\n    return 
sr.detach(self.params['sr_uuid'])\n  File 
\"/usr/lib/xen-common/xapi/sm/LVMSR\", line 579, in detach\n    raise 
Exception(\"SR detach failed, please refer to the log \" \\\nException: SR 
detach failed, please refer to the log for details.\n ]")

Since lvutil.py runs dmsetup already, the symlink for the MGT volume disappears 
and when LVHDSR.detach runs, there isn't anything to unlink. Adding a condition 
to check for the existence of the symlink in LVHDSR.py is a workaround for me. 
Just so I don't have to run it twice (xe pbd-unplug)

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