Test promptness of tentative followers

[amesgen]

Description

Closes CAD-4195

See the module description for motivation/approach.

Checklist

• Branch
  • Commit sequence broadly makes sense
  • Commits have useful messages
  • New tests are added if needed and existing tests are updated
  • If this branch changes Consensus and has any consequences for downstream repositories or end users, said changes must be documented in interface-CHANGELOG.md
  • If this branch changes Network and has any consequences for downstream repositories or end users, said changes must be documented in interface-CHANGELOG.md
  • If serialization changes, user-facing consequences (e.g. replay from genesis) are confirmed to be intentional.
• Pull Request
  • Self-reviewed the diff
  • Useful pull request description at least containing the following information:
    • What does this PR change?
    • Why these changes were needed?
    • How does this affect downstream repositories and/or end-users?
    • Which ticket does this PR close (if any)? If it does, is it linked?
  • Reviewer requested

[nfrisby]

I found some typos in the comments, and made a few comment/layout suggestions. Approved! Really nice. As you say, there's a good bit of ceremony in the setup, but it seems minimal and tenable.

[nfrisby]

Are there any properties of genChainUpdates that we're assuming here?

I just noticed it doesn't have a comment on it :(

EG, at a glance, it looks like it can generate SwitchFork n bs where length bs == n, but I didn't see anything ensuring that in that case the last bs is preferred to the youngest rolled back block. Thus these ChainUpdates do not necessarily monotonically improve the chain tip 🤔

That might not matter to this test, but perhaps a one line comment here clarifying the extent to which it does or doesn't would be worthwhile.

[amesgen]

Yes, it is not guaranteed that every chain update will result in a tentative header, but this is not important for/orthogonal to this test, as we still mainly get pipelined blocks. I am adding a comment stressing this 👍

[nfrisby]

Discussed on a call.

• Esgen pointed out that this TestBlock(With) uses the Bft protocol, which has no tie-breaker. And so length bs == n will never cause a switch (with no tiebreaker, the behavior is just to keep the chain you already have).

• We'll open a ticket to enrich other tentative header tests specifically with interesting tiebreaker behaviors. But for this promptness test, it's a bit off-topic: AddBlock will trigger the tentative pathway.

[amesgen]

bors merge

[iohk-bors]

Build succeeded:

• build (8.10.7, ubuntu-latest)
• build (8.10.7, windows-latest)
• ci/hydra:Cardano:ouroboros-network:required