Creating "slim" stream mirrors for modular RPM/DNF packages

Creating “slim” stream mirrors of modular RPM packages enables the SIMP ISO to support modularity while still mix/matching specific RPMs from various sources. For instance, it permits adding a few epel-modular packages without distributing epel-modular's entire collection of modules x streams x packages.

See https://simp-project.atlassian.net/wiki/pages/resumedraft.action?draftId=2193326084 for a summary of motivating challenges and requirements .

Process

The “slim” mirroring process must happen during/before the ISO build process.

1. At the same time each modular RPM is acquired, save its source repo’s modulemd metadata.

2. Use unique N:S:V:C:A combinations from the resulting modular RPMs to determine which “slim” module streams to reconstruct.

3. For each unique “slim” modular stream: generate modulemd metadata for all relevant RPMs

4. Combine all “slim” modules' modulemd data into a single data structure and write it to modules.yaml

5. Rebuild the modular repository using createrepo_c (or createrepo_mod) with the new modules.yaml file

Implementing slim modular repos

Modular RPM data/metadata to get/record/cache

At a minimum, a new field (only required for modular RPMs) that specifies the N:S (module:stream) for modular packages should be added to the build’s packages.yaml.

Problems that are probably solved

yumdownloader can’t see RPMs in modules/streams that aren’t enabled

1. TODO Add an optional field to packages.yaml entries to specify the N:S: for each modular RPM

2. TODO Identify and enable all unique N: from packages.yaml (fail if there are conflicting S:)

3. TODO dnf module enable each N:S: before beginning to use yumdownloader

4. TODO Individual yumdownloader runs can change repository mirrors, which may be out of sync with each other and have different modulemd data.

   1. TODO UNSOLVED? (When using the yumdownloader) the modulemd metadata must be fetched at the same time as the RPM is downloaded, in order to preserve the precise state of that RPM’s modular metadata.
      

A single RPM could be part of multiple streams in an upstream repository

Nothing in the modulemd data prevents this, so we need a way to determine the correct stream.

TODO This isn’t a problem for External packages, because we will already need to add a field to explicitly set N:S: to packages.yaml.

However: there is no way to hint streams in *pkglist.txt files for minimal BaseOS packages (unless we do something elaborate, like add comment keywords and a parser)

1. TODO Most BaseOS EL8 modules have a default stream; use the default stream if it exists

2. TODO If there is only a single stream, default to the only stream.
   This is hacky, but it will work for EL8.3—Base OS (i.e., AppStream) modules without a default stream are currently very rare, and at the moment all of them have a single stream:

   # dnf module --disablerepo=\* --enablerepo=appstream list | grep -v '\[d\]'
   CentOS Linux 8 - AppStream
   Name                 Stream    Profiles    Summary                                                                
   389-ds               1.4                   389 Directory Server (base)                                                                                                                                                                                               
   libselinux-python    2.8       common      Python 2 bindings for libselinux                                                                                                                                                                                          
   mod_auth_openidc     2.3                   Apache module suporting OpenID Connect authentication                                                                                                                                                                     
   parfait              0.5       common      Parfait Module                                                                                                                                                                                                            
   pki-core             10.6                  PKI Core module for PKI 10.6 or later                                                                                                                                                                                     
   pki-deps             10.6                  PKI Dependencies module for PKI 10.6 or later                          

3. NOT IN 6.6.0 UNSOLVED This leaves open a potential edge-case: if in the future, we require an RPM from a Base OS modules without a default stream but ships with multiple streams (again, current population: 0), it will fail and there is no way to hint

   • NOT IN 6.6.0 We should probably have a way of formally declaring N:S for *pkglist.txt Base OS RPMs in the future. Some possibilities:

     • A separate *pkglist.modularity.txt file

     • N:S-declaring directives in the comments of *pkglist.txt

     • Could this be combined with packages.yaml? (not without a major rewrite)

Unsolved problems

What are the “Fetch RPM” flow differences between Base OS (prune_packages) & External (yumdownloader) packages?

SIMP-9643 - Getting issue details... STATUS

How can we know the URL/path to an RPM’s source repo’s repomd.xml file?

SIMP-9644 - Getting issue details... STATUS

This is simple enough to do by hand for an individual package, but I’m not sure how to automate it yet. Here are some ideas:

1. Option 1: see if yumdownloader can be convinced to display the repo root’s URL, like --urls does with the RPM

   1. I haven’t found an option that does this

2. Option 2: walk up the dir tree until we find metadata (hacky, expensive)

3. Option 3: (somehow) find/define the DNF cache that was used to download the RPM and (somehow) fish out the modulemd data that was used for that specific package

4. Option 4: Do it the other way around:

   1. Before getting packages, get a repo’s repomd.xml file first, then use it to find the xxx-packages.yaml.gz

   2. read the modulemd data from the packages.yaml file

   3. filter the modulemd data down to just the streams and packages you need

   4. then run yumdownloader to acquire those exact packages

Separate yumdownloader runs may result in RPMs for the same N:S having different N:S:V:C:A

Different RPMs could be sourced from different versions (V:) of the same module stream if yumdownloader pulls them from different repo mirrors that are out of sync with each other. Using the heuristic of a “slim” module stream per unique N:S:V:C:A , this would result in multiple module streams instead of one.

This is a rare edge case that V: is specifically intended to catch, and it seems correct to fail instead of building a “mirrored” stream subset using RPMs from a different (stream) versions. However, I can’t demonstrate that the potential impact of this scenario is worth prioritizing its implementation.

The strongest impacts I came up with so far rely on the fact that there’s a good chance that, between two stream versions, the combined set of downloaded RPMs won’t be a precise subset of either stream. But unless the mirrors were really out of sync, this probably wouldn’t matter. The stream version is a snapshot in time of all the modulemd (modular) metadata for the stream—it doesn’t actually affect its RPMs' resolutions.

(I honestly don’t know many details of how/when V: is used other than “highest wins”, but it might lead to weird edge cases:)

1. There’s a (staggeringly) remote change that the newer stream version dropped package(s) or one of its packages has a new dependency

2. The slim repo will use one N:S:V or the other, but neither upstream precisely matches its RPMs. After re-integrating with the full upstream repo or mirror, DNF might miss an update by deciding it already know the stream version resolve using the wrong stream version for some of the packages, to the wrong versions, miss updates

3. There may be other reasons to do with inter-modular dependencies.

4. TL;DR: Not sure if failing is the best way forward—input welcome.

For the time being, I am treating this as a don’t-have-to-solve problem

Are there conditions where streams don’t provide C:A information when packages are noarch?

No. By the time they are built, they will have a context and arch.

Undecided

[Should/how to] persist cached modulemd metadata for already-downloaded RPMs between builds?

The current yumdownloader process

True or false: “Any mirrored “slim” module MUST NOT have multiple streams”

1. This sounds reasonable, but is it actually true?

2. It’s impossible to install multiple streams on a single SIMP server, but do we think we’d need to package multiple “slim” module streams for agents?

   1. My current inclination is to assume “no.” Given our approach toward modularity in general, that seems like a really edgy edge case.