Your db migrations may support updates from #1 to #99 but your OS does not directly support updates of MySQL 5 to MySQL 8 with issues. For example there are plenty of examples of deprecated my.cnf configuration values. Similarly APT on Ubuntu will prompt how to handle a my.cnf that differs from the Distribution release when upgrading Versions. Often times this is more painful than minor version updates.
I think the version milestones in Gitlab are akin to dependency changes for self-hosted Gitlab. An example is the Gitlab v9 (?) Postgres upgrade to Postgres v11 I think, it was opt-in for a prior version of Gitlab than required at that version milestone. It's difficult to make db migration scripts for Gitlab,as in your example, that may depend on newer Postgres idioms not available in the legacy db version. So you can't simply support gitlab updates from X to Y version due to underlyng dependency constraints...
> Your db migrations may support updates from #1 to #99 but your OS does not directly support updates of MySQL 5 to MySQL 8 with issues.
That's just the thing - more software out there should have a clear separation between the files needed to run it (binaries, other libraries), its configuration (either files, environment variables or a mix of both) and the data that's generated by it.
The binaries and libraries can easily have breaking changes and be incompatible with one another (essentially treat them as a blob that fits together, though dynamic linking muddies this). The configuration can also change, though it should be documented and the binaries should output warnings in the logs in such cases (like GitLab actually already does!). The data should have extra care taken to make it compatible between most versions, with at least forwards only migrations available in all other cases (since backwards compatible migrations are just too hard to do in practice).
Alas, i don't install most software on my servers anymore, merely Docker (or Podman, basically any OCI compatible technology) containers with specific volumes or bind mounts for the persistent data. GitLab is pretty good in this regard with its Omnibus install, though there are certainly a few problems with it if you try to do too many updates or have a non-standard configuration.
Of course, i'll still use GitLab in my company because there it's someone else's job to keep it running with a hopefully appropriate amount of resources to keep it that way with minimal downtime and all the relevant updates. But at the same time, for certain circumstances (like my memory constrained homelab setup), it makes sense to look into multiple lightweight integrated solutions.
You can actually find more information about what broke for me while doing updates in particular, seemingly something cgroups related with gitaly related stuff not having the write permissions needed inside of the container, which later lead to the embedded PostgreSQL failing catastrophically. In comparison, right now i just have Gitea for similar goals which is a single binary that uses an SQLite database, as well as the other aforementioned tools for CI and storage of artefacts, which are similarly decoupled.
It's probably all about constraints, drawbacks and finding what works for you best!
I think the version milestones in Gitlab are akin to dependency changes for self-hosted Gitlab. An example is the Gitlab v9 (?) Postgres upgrade to Postgres v11 I think, it was opt-in for a prior version of Gitlab than required at that version milestone. It's difficult to make db migration scripts for Gitlab,as in your example, that may depend on newer Postgres idioms not available in the legacy db version. So you can't simply support gitlab updates from X to Y version due to underlyng dependency constraints...
Thanks for the insightful discourse.