Race condition in map_partitions method in Disk class

[M. Vefa Bicakci]

Hello,

I have been experimenting with gitian, which uses vmbuilder to build virtual machines. I noticed that vmbuilder uses kpartx to map partitions from a disk image to loopback block devices in /dev/mapper, and then creates ext4 filesystems on the partitions.

In my set-up, however, mapping the partitions to loopback block devices seems to take some time, which introduces a race condition, as vmbuilder expects that the loopback devices are available right after the execution of kpartx. Please see the debugging log output below.

If I add a "time.sleep(3)" call to the end of the map_partitions method in the Disk class in disk.py, then this problem disappears.

Would it be possible to either sleep for a few seconds and/or poll the existence of the loopback block devices, so as to remove this race condition?

Thank you!

=== 8< ===
2014-08-31 01:33 DEBUG : ['kpartx', '-av', '/tmp/tmpwgateA']
2014-08-31 01:33 DEBUG : add map loop2p1 (253:14): 0 19997984 linear /dev/loop2 63
2014-08-31 01:33 DEBUG : add map loop2p2 (253:15): 0 1996800 linear /dev/loop2 20000768
2014-08-31 01:33 INFO : Creating file systems
2014-08-31 01:33 DEBUG : ['mkfs.ext4', '-F', '/dev/mapper/loop2p1']
2014-08-31 01:33 INFO : mke2fs 1.42.12 (29-Aug-2014)
2014-08-31 01:33 INFO : The file /dev/mapper/loop2p1 does not exist and no size was specified.
2014-08-31 01:33 INFO : Cleaning up
2014-08-31 01:33 DEBUG : ['kpartx', '-d', '/tmp/tmpwgateA']
2014-08-31 01:33 DEBUG : loop deleted : /dev/loop2
2014-08-31 01:33 DEBUG : ['kpartx', '-d', '/tmp/tmpwgateA']
2014-08-31 01:33 DEBUG : Removing partition /dev/mapper/loop2p1
2014-08-31 01:33 DEBUG : ['dmsetup', 'info', 'loop2pp1']
2014-08-31 01:33 DEBUG : Device does not exist.
2014-08-31 01:33 DEBUG : Command failed
2014-08-31 01:33 DEBUG : Removing partition /dev/mapper/loop2p2
2014-08-31 01:33 DEBUG : ['dmsetup', 'info', 'loop2pp2']
2014-08-31 01:33 DEBUG : Device does not exist.
2014-08-31 01:33 DEBUG : Command failed
2014-08-31 01:33 ERROR : Process (['mkfs.ext4', '-F', '/dev/mapper/loop2p1']) returned 1. stdout: , stderr: mke2fs 1.42.12 (29-Aug-2014)
The file /dev/mapper/loop2p1 does not exist and no size was specified.
=== >8 ===

[M. Vefa Bicakci (mvb)]

(Sorry for changing the type to "Public Security". I did this by accident and then reverted my accidental change.)

[Serge Hallyn (serge-hallyn)]

Hi - sleep 3 did fix it for me, but is racy. Changing the kpartx to add '-s' (sync) also fixed it fo rme, so I'm adding that to the bzr tree. It will be in x-series when I post the new release there.

[W. J. van der Laan (laanwj)]

Still happens on Wily.
Serge Hallyn's fix to add '-s' worked.

[Cristian Klein (cristiklein)]

The '-s' workaround helped me too.

Is there any update on this bug? Has anybody notified the vmbuilder developers so they can fix the source code?