Make renovate work with cargo-hakari

[sunshowers]

The workspace-hack package results in two issues with dependabot:

1. For some updates, the workspace-hack would need to be regenerated. A workaround is to fetch the branch locally, then run cargo hakari generate and push it to the branch.
2. Dependabot will try and do updates to crates that are only referred to in workspace-hack.

We can solve both these issues with Renovate's support for excluding particular paths, and running post-upgrade commands.

Useful links:

• https://github.com/renovatebot/github-action for self-hosted Renovate (required for post-upgrade commands)
• https://docs.renovatebot.com/configuration-options/#postupgradetasks where we can run hakari after upgrades and before commits
• https://docs.renovatebot.com/configuration-options/#ignorepaths where we can exclude workspace-hack

[ahl]

It looks like dependabot is just broken full stop:

https://github.com/oxidecomputer/omicron/network/updates/9290049/jobs

It would be good to get dependency updates working again soon.

[sunshowers]

Actively working on this.

[ahl]

Previously we had another problem, but now dependabot is just timing out. Perhaps the number of dependencies is beyond what it can handle. https://github.com/oxidecomputer/omicron/network/updates/729281002

[sunshowers]

Weird, it looks like dependabot is stuck in some sort of maybe-quadratic resolution issue (in this log num-rational and futures-task are fetched 78 times each -- why?)

I think part of what might be happening is that Dependabot doesn't know how to handle lockfile-only updates (which is pretty bad!!!), and with the workspace-hack, a number of dependencies made their way from being transitive-only to being direct. So dependabot's issuing a lot of updates that it really should have been all this time, which may be causing resolution slowdowns.

For people with access to Oxide RFDs, https://rfd.shared.oxide.computer/rfd/0434 talks about this and other issues.

[sunshowers]

#4236 should address this.

[sunshowers]

#4241 is an example of a PR which updates the workspace-hack.

We do need #4244 as a followup fix.