chroot unmount failure? Re: Build failures without a build log

[Rebecca N. Palmer]

statsmodels/riscv64 has been retried (possibly several times) but 
continued to fail without a log.  During the attempt that finished today 
(on riscv64-qemu-lcy01-087, taking 16 hours) I found that the build page 
was showing the last few lines of the in-progress build log, and watched 
it finish there.

The below is the last I got before this page 
(https://launchpad.net/ubuntu/+source/statsmodels/0.11.1-5/+build/19843284) 
changed to "Failed" without a build log.

ild depends: as requested

+------------------------------------------------------------------------------+
| Summary |
+------------------------------------------------------------------------------+

Build Architecture: riscv64
Build Type: any
Build-Space: 429340
Build-Time: 57377
Distribution: groovy-proposed
Host Architecture: riscv64
Install-Time: 572
Job: statsmodels_0.11.1-5.dsc
Machine Architecture: riscv64
Package: statsmodels
Package-Time: 57985
Source-Version: 0.11.1-5
Space: 429340
Status: successful
Version: 0.11.1-5
--------------------------------------------------------------------------------
Finished at 2020-08-30T09:10:52Z
Build needed 16:06:25, 429340k disk space
RUN: /usr/share/launchpad-buildd/bin/in-target scan-for-processes 
--backend=chroot --series=groovy --arch=riscv64 PACKAGEBUILD-19843284
Scanning for processes to kill in build PACKAGEBUILD-19843284
RUN: /usr/share/launchpad-buildd/bin/in-target umount-chroot 
--backend=chroot --series=groovy --arch=riscv64 PACKAGEBUILD-19843284
Stopping target for build PACKAGEBUILD-19843284
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.
umount: 
/home/buildd/build-PACKAGEBUILD-19843284/chroot-autobuild/dev/shm: 
target is busy.