Fix KeychainTxOutIndex::lookahead_to_target

[evanlinjin]

Description

This method was not used (so it was untested) and it was not working. This fixes it.

The old implementation used .next_store_index which returned the keychain's last index stored in .inner (which include lookahead spks). This is WRONG because .replenish_lookahead needs the difference from last revealed.

Changelog notice

Fix KeychainTxOutIndex::lookahead_to_target

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Bugfixes:

• I've added tests to reproduce the issue which are now passing

[danielabrozzoni]

Unrelated, but going through replenish_lookahead I realized it could just use self.next_index instead of let next_reveal_index = self.last_revealed.get(&descriptor_id).map_or(0, |v| *v + 1);. Can you fix? Or I can open an issue if you prefer!

[evanlinjin]

Unrelated, but going through replenish_lookahead I realized it could just use self.next_index instead of let next_reveal_index = self.last_revealed.get(&descriptor_id).map_or(0, |v| *v + 1);. Can you fix?

I don't mind adding to this PR (as it is small). However, I'm not sure what we are trying to fix here?

[LLFourn]

Did we consider removing this method? Do we believe there is still a good reason to do lookaheads outside of the lookahead setting?

[evanlinjin]

Did we consider removing this method? Do we believe there is still a good reason to do lookaheads outside of the lookahead setting?

I'm currently using it for something 😅 (that's how I found the bug). To be fair, what I'm doing is pretty niche but I don't think it hurts to keep it?

[evanlinjin]

This is ready for review!

[notmandatory]

ACK b290b29

The test framework was more complicated to review than the change! but good coverage.