qubes-dom0-update doesn't handle low disk space conditions well

[strugee]

Qubes OS version

R4.0

Affected component(s) or functionality

qubes-dom0-update

Brief summary

When the system has very low disk space, qubes-dom0-update fails with an error message that just says "failed with exit code 1" instead of something more helpful (like "No space left on device").

To Reproduce

1. Make sure your disk utilization is above the percentage reserved by ext4. You may change this value on your filesystem using sudo tune2fs -m <percentage> /dev/mapper/qubes_dom0-root, where <percentage> is the % of your filesystem that will be reserved for the superuser only; df -h / should report 0 in the "available" column
2. Run qubes-dom0-update with updates pending

Expected behavior

Either the update succeeds or the update fails and tells me that I'm out of disk space.

Actual behavior

The update fails with this output:

[alex@dom0 ~]$ sudo qubes-dom0-update
Using sys-firewall-disp as UpdateVM to download updates for Dom0; this may take some time...
--> Running transaction check
---> Package kernel-latest.x86_64 1000:5.1.2-1.pvops.qubes will be installed
---> Package qubes-gpg-split-dom0.x86_64 0:2.0.38-1.fc25 will be installed
---> Package qubes-manager.noarch 0:4.0.34-1.fc25 will be installed
---> Package qubes-mgmt-salt-dom0-qvm.noarch 0:4.0.8-1.fc25 will be installed
---> Package qubes-usb-proxy-dom0.noarch 0:1.0.21-1.fc25 will be installed
--> Finished Dependency Resolution
/var/lib/qubes/dom0-updates/packages/kernel-latest-5.1.2-1.pvops.qubes.x86_64.rpm already exists and appears to be complete
/var/lib/qubes/dom0-updates/packages/qubes-gpg-split-dom0-2.0.38-1.fc25.x86_64.rpm already exists and appears to be complete
/var/lib/qubes/dom0-updates/packages/qubes-manager-4.0.34-1.fc25.noarch.rpm already exists and appears to be complete
/var/lib/qubes/dom0-updates/packages/qubes-mgmt-salt-dom0-qvm-4.0.8-1.fc25.noarch.rpm already exists and appears to be complete
/var/lib/qubes/dom0-updates/packages/qubes-usb-proxy-dom0-1.0.21-1.fc25.noarch.rpm already exists and appears to be complete
find: '/var/lib/qubes/dom0-updates/var/cache': No such file or directory
EOF
'/usr/lib/qubes/qrexec-client-vm dom0 qubes.ReceiveUpdates /usr/lib/qubes/qfile-agent /var/lib/qubes/dom0-updates/packages/*.rpm' failed with exit code 1!

Additional context

ext4 reserves a certain percentage of disk space for the superuser; see this Stack Exchange question for an explanation of why. This can come out to tens of gigabytes on large partitions. For example, in my case my Qubes partition is 590GB which comes out to ~29.5 GB of disk space reserved for the superuser only.

The reason I said the expected behavior might include the update succeeding is because in this case, there actually is plenty of free space, but it seems that some process invoked by qubes-dom0-update is running as a non-superuser. If it ran as the superuser, it would have permission to write to the reserved space in the filesystem. I do think it's probably a better idea to signal an ENOSPC error, though; that route seems like it's playing with fire.

Relevant documentation you've consulted

None

Related, non-duplicate issues

https://github.com/QubesOS/qubes-issues/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3Aopen+qubes-dom0-update+free+space

[andrewdavidwong]

Somewhat related: #3299

[github-actions]

This issue is being closed because:

• This issue is on the "Release 4.0 updates" milestone.
• Qubes OS 4.0 reached EOL (end-of-life) over one year ago.
• There has not been any activity on this issue in over one year.

If anyone believes that this issue should be reopened and reassigned to an active milestone, please leave a brief comment.
(For example, if a bug still affects Qubes OS 4.1, then the comment "Affects 4.1" will suffice.)