zcash_client_backend and sqlite work items

[AArnott]

As part of an exploration in consuming the zcash_client_backend and zcash_client_sqlite crates, the following feature gaps need to be filled in these crates to address requirements:

To be assigned

The following items are required but haven't been formally assigned to either this repo's contributors or others in the community:

• P0: Record of UTXOs that are consumed by a transaction that was merely downloaded, rather than only for transactions created in that particular wallet.
  Transparent input details should be retrieved with transactions on account recovery zcash/librustzcash#1074

• P0: Default transparent address doesn't match what other wallets generate when default sapling address comes from a non-zero diversifier index.
  Scan for 0-indexed transparent address even when scanning with a UIVK. zcash/librustzcash#599

• P1: Record of the diversifier or receiving address used for incoming funds
  Expose diversifier column from sapling_received_notes in the v_tx_outputs view zcash/librustzcash#1073
  A workaround is in place as of aa72e7f.

• P1: Importing of accounts with direct keys, or additional seed phrases.
  Design for importing FVKs, IVKs, and even Unified spending keys zcash/librustzcash#1075

  • Support multiple seed phrases (zcash_client_sqlite: Rework accounts to support imported viewing keys zcash/librustzcash#1235)
  • Support full viewing keys
  • Support incoming viewing keys

• P2: Enable downloading of transactions from the mempool. Possibly mitigate the DoS attack of flooding the mempool with transactions to slow down all clients through many trial decryptions by only considering transactions with a ZIP-317 compliant fee when the mempool is unusually full.
  zcash_client_backend: Support scanning for transactions in the mempool. zcash/librustzcash#911

• P2: Record fees for downloaded transactions, whether inbound or outbound. Especially inbound when they are still in the mempool so that the receiver can be warned that the sender may be sending a transaction that they intend to not be mined.
  Depends on Transparent input details should be retrieved with transactions on account recovery zcash/librustzcash#1074, and possibly other work.

• P2: Calculate birthday heights, in their 3 varieties
  zcash_client_sqlite should expose whether a note has been spent, and what block created it zcash/librustzcash#1076
  Implemented locally as of fc92642.

Direct contributions

The following items are planned to be contributed by a regular contributor to this repo:

TBD

Others in community

The following items are expected to be addressed by others in the community:

• P0: Orchard support
  Add Orchard support to zcash_client_sqlite zcash/librustzcash#404
  Add Orchard support to zcash_client_backend zcash/librustzcash#403

Many of the above are already filled by Zingolib, but zingolib has its own limitations and bugs. This effort is an exploration to see whether these lower-level crates can be used as a substitute.