Implement `PrunedBlockchain` backend: RPC on pruned node

[RCasatta]

Current RpcBlockchain grants full access to transaction history, however, it requires a non-pruned node (a node storing the full blockchain, which at the time of writing is over 400GB).

However, by renouncing to a wallet full transaction history, it is still possible to compute the wallet balance and be able to spend without needing a non-pruned node.

The key to achieve that is using RPC method scantxoutset

Note: if someone want to turn this into a "summer-of-bitcoin" project is welcome

Summer of Bitcoin Project Proposal

Description

Current RpcBlockchain grants full access to transaction history, however, it requires a non-pruned node (a node storing the full blockchain, which at the time of writing is over 400GB).

However, even without a wallet's full transaction history it is still possible to compute the wallet balance and be able to spend with only a pruned core full node.

The key to achieve that is using RPC method scantxoutset

Expected Outcomes

• Implement blockchain::rpc::PrunedRpcBlockchain and blockchain::rpc::PrunedRpcConfig.
• PrunedRpcBlockchain will use core scantxoutset RPC to compute wallet balances.
• PrunedRpcBlockchain will create spending transactions with a pruned core full node.

Resources

• bitcoin core node setup.
• bitcoin rpc commands.

Skills Required

• Experience with git. Guide
• Familiarity with rust. First seven chapters of the book
• Able to setup a local core node in regtest and testnet mode.

Mentor(s)
@afilini @RCasatta

Difficulty
Hard

Competency Test

• Install rust, compile and run all bdk examples and tests.
• Read through the BDK docs.
• Setup a local Bitcoin Core node daemon in Regtest mode.
• Make a dummy wallet with and test with example_bitcoind_rpc_polling client.
• Familiarity with basic rust, should be able to write basic trait implementations on foreign structures.

[afilini]

We have discussed making the summer of bitcoin tasks a bit easier because most students don't have a lot of experience with Bitcoin or even git sometimes. Though, compared to the other tasks we have this seems relatively easy, so I'm adding the tag anyway..

[notmandatory]

@afilini and @RCasatta would one or both of your be willing to mentor this issue for Summer of Bitcoin? I put you both on it for now. 🙂

[evanlinjin]

Imo we should take into consideration that "pruning" does not prune away transactions already found in the Core wallet, and that it is very simple to change a core node from non-pruned to pruned, and that it is also possible that a user will create a Core wallet very early on in bootstrap (even though pruning is enabled).

It would make sense in some cases to "reuse" the original RpcBlockchain for a pruned node. Imo an elegant implementation should take into consideration all possibilities.

[notmandatory]

Closing this in favor of a new issue based on changing new bdk_bitcoind_rpc crate.