Restore the previous non-finalized chain if a block is invalid

[teor2345]

Motivation

1. In PR Refactor: Return errors from state update methods to prepare for contextual validation #2417, we started returning errors from the non-finalized state UpdateWith methods.

But when there is an error, we implicitly drop the full non-finalized chain, and reset the state to the finalized tip. This is surprising, and might cause bugs in future. It might also make the code hard to test.

(And throwing away 100 blocks is pretty inefficient, too.)

2. The mandatory checkpoint check only applies to existing chains, which is inconsistent. It could be a source of bugs, and it makes testing harder.

Solution

• Restore the parent chain when a block update fails
• Change the Chain.push method to make it easier to use correctly
• Move the mandatory checkpoint check to validate_and_commit, so it applies to new chains and existing chains

Review

@conradoplg and I worked on the original PR together. This fix conflicts with a bunch of upcoming state PRs, so we should try to get it in soon.

These upcoming PRs will also add tests for this fix.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

[conradoplg]

Good catch!