Bug 1300878 - [RFE] dnf makecache enabled on readonly-root systems wastes cpu
[RFE] dnf makecache enabled on readonly-root systems wastes cpu
Product: Fedora
Classification: Fedora
Component: dnf (Show other bugs)
Unspecified Unspecified
low Severity unspecified
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Assigned To: rpm-software-management
Fedora Extras Quality Assurance
: FutureFeature, Triaged
Depends On:
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Reported: 2016-01-21 19:33 EST by Oleg Drokin
Modified: 2016-12-20 13:03 EST (History)
5 users (show)

See Also:
Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Story Points: ---
Clone Of:
Last Closed: 2016-12-20 13:03:46 EST
Type: Bug
Regression: ---
Mount Type: ---
Documentation: ---
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oVirt Team: ---
RHEL 7.3 requirements from Atomic Host:
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External Trackers
Tracker ID Priority Status Summary Last Updated
Red Hat Bugzilla 1187111 None None None 2016-01-21 19:33 EST

  None (edit)
Description Oleg Drokin 2016-01-21 19:33:37 EST
Description of problem:

It looks like dnf makecache timer that is eanbled by default for some reason cannot understand that it's running on a readonly root system (i.e. nfsroot)
and fires anyway, so 10 minutes after system start we end up with dnf makecache process that hogs 100% of cpu.
32441 ?        RNs   14:24 /usr/bin/python -OO /usr/bin/dnf -v makecache timer

strace shows that it's looping trying to perform:
open("/var/cache/dnf/metadata_lock.pid", O_WRONLY|O_CREAT|O_EXCL, 0644) = -1 EEXIST (File exists)
open("/var/cache/dnf/metadata_lock.pid", O_RDONLY) = -1 ENOENT (No such file or directory)

The file does not exist:
# ls -la /var/cache/dnf/metadata_lock.pid
ls: cannot access /var/cache/dnf/metadata_lock.pid: No such file or directory

The /var/cache/dnf is not one of the default targets to be mounted in tmpfs (and for a good reason).

I see that the timer in question already contains a condition against running when started from livecd image:

Now, if this could be replaced with a check of /etc/sysconfig/readonly-root and not run when READONLY is set to yes - that would be a lot more proper solution.
Or at least a check that /var/cache/dnf is actually writeable.

Version-Release number of selected component (if applicable):
present in all version of dnf starting from FC21

How reproducible:

100% reproducible

Steps to Reproduce:
If you are not up to the task of doing nfs readonly-root install or the like, you can simulate it by:

1. mount none /var/cache/dnf -t tmpfs
2. mount / /media --bind
3. cp -a /media/var/cache/dnf/* /var/cache/dnf/
4. umount /media

5. strace dnf makecache

Note it's important to do the copying of a real existing dnf cache as otherwise it's smart enough to fail seeing as it cannot create it if the /var/cache/dnf is empty (which it is not on a real readonly nfsroot install)

Actual results:

dnf enters infinite loop hogging 100% of one cpu.

Expected results:

should notice that it cannot do anything.
Even better if dnf makecache was not run by default in such a case.

Additional info:
This all got introduced with the mandatory on-by-default dnf makecache timer as part of dnf package, that sparked quite a heated discussion in bug 1187111.
Comment 1 Oleg Drokin 2016-01-21 19:38:51 EST
There's one more step in the reproducer, btw, that I forgot to include:

3.5 mount /var/cache/dnf -o remount,ro

Otherwise the /var/cache/dnf is not readonly and you won't see the problem.
Comment 2 Honza Silhan 2016-01-25 08:11:33 EST
we can add the readonly checker everytime we load cache directory.
Comment 3 Fedora Admin XMLRPC Client 2016-07-08 05:36:03 EDT
This package has changed ownership in the Fedora Package Database.  Reassigning to the new owner of this component.
Comment 4 Fedora End Of Life 2016-11-24 10:06:12 EST
This message is a reminder that Fedora 23 is nearing its end of life.
Approximately 4 (four) weeks from now Fedora will stop maintaining
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Comment 5 Fedora End Of Life 2016-12-20 13:03:46 EST
Fedora 23 changed to end-of-life (EOL) status on 2016-12-20. Fedora 23 is
no longer maintained, which means that it will not receive any further
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