[PULP-1045] Cached repomd.xml data fails due to primary.xml, etc being removed

[bmbouter]

To reproduce:

1. Publish really frequently
2. Have dnf clients installing constantly along the way
3. Some dnf clients will have errors in between fetching repomd.xml

The issue is there's a race condition, when dnf fetches the repomd.xml they will then get 404 when seconds later a new publish replaces the metadata files.

[dralley]

Correct. The only way to resolve that race condition would be to keep some old metadata around. But, that's a bit tricky because currently any publication is de-novo and pure, there's not currently any precedent for inheriting metadata from older publications. Also options can change from one publication to the next, and still you can have this problem when swapping distributions from one publication to another, so it would not be a 100% fix either.

But also, normally that race condition window is pretty small. It's not between publishes, it's between the distribution switching over from one to another. So the window is more like milliseconds than seconds.

[mdellweg]

There is (i think) a complete solution in pulp_deb:
All but the top release metadata files get a version with a hash in the name:
https://github.com/pulp/pulp_deb/blob/main/pulp_deb/app/tasks/publishing.py#L446
And the distributions fall back to serve from earlier publications for a certain amount of time:
https://github.com/pulp/pulp_deb/blob/main/pulp_deb/app/models/publication.py#L135Obviously this only works, because the client tools understand the by-hash metadata.

[daviddavis]

There is (i think) a complete solution in pulp_deb

Agreed, I added this feature to pulp_deb and I think it would work for pulp_rpm. The repomd.xml file points to files that have a digest in their filenames similar to the apt-by-hash scheme in pulp_deb. So you just need to continue to serve the old repo metadata files like primary.xml in case clients get an older version of repomd.xml. One option might be to move DistributedPublication into pulpcore.