Create a Dependency that can load checkpoints from bundle

[pacu]

Create a Dependency that can load checkpoints from bundle

Closes #1315

This PR introduces small changes on each commit.
Things done:

• Moved Checkpoint code around so it is all in one place.
• Split "constants" and "helpers" of Checkpoint to their own files
• Remove dead code
• Create the protocol
• Create tests that fail
• Implement the protocol and make sure the tests pass
• Replace use of Checkpoint.birthday(with:network) with CheckpointSource

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[nuttycom]

utACK with minor reservations about the default BundleCheckpointURLProvider.

[pacu]

Github Action keeps failing. I tested if updating to the latest release of Xcode fixes it and it does. I don't have much details about that because the logs don't show any details.

I created another PR for that so that change is not mixed up with this one.

#1319

[daira]

Am I misremembering what we decided for this? I thought that wallets were supposed to scan the block at the birthday height (thus making the - 1 here unnecessary).

[pacu]

This could be me having outdated concepts in my mind. Last time I discussed checkpoints with @str4d he had mentioned that to spend funds at block X, one would have to know tree state of height X - 1 or prior. If it's not the case I should change this documentation.

[daira]

That could be correct; my understanding of this might not be up-to-date. @str4d ?

[str4d]

In zcash/librustzcash#907 the definition of "wallet birthday" was changed from "height at which the wallet was created" (which is by construction before any block containing a wallet transaction) to "height at which the wallet first received a transaction". The old definition meant that the highest valid value for wallet birthday was indeed transaction.height - 1, but with the new definition it is just transaction.height. The X - 1 is now handled in the create_account API: you are required to pass in a Sapling tree state that is for height X - 1 (meaning that if what you have is the wallet's seed phrase and birthday, you need to look up the checkpoint at birthday - 1), and the wallet's birthday is calculated internally by the Rust side from that as tree_state.height + 1.

[nuttycom]

This is also getting more complicated shortly, as wallets may have differing Orchard and Sapling birthday heights in the future.

[pacu]

Based on what @str4d commented, then the note is accurate.

The current checkpoint from bundle logic is to retrieve the given height if available and or return the closes preceding height available.

If the bundle has heights 1000 and 999, if you request 1000 it will give you 1000, not 999. That logic of -1 or +1 is actually is left to the caller.

Do we agree this is the needed behavior? Otherwise we can file a new issue for it and make those changes.

[pacu]

This is also getting more complicated shortly, as wallets may have differing Orchard and Sapling birthday heights in the future.

🫠 probably it is for the best to actually keep themin(saplingBirthday, orchardBirthday) so be "the" birthday for UX purposes, where None is mapped to BlockHeight.max so it picks the available value if one is absent

[daira]

Suggested change

	// Created by Francisco Gindre on 7/28/21.
	// Created by Francisco Gindre on 2021-07-28.

[daira]

utACK with minor comments.

[pacu]

Thank you @daira, I will address this comments and tidy up a bit the commits.