BDK Descriptor Improvements: Add multi descriptor capability in wallet

[rajarshimaitra]

Description

In order for BDK to extend beyond just a Bitcoin wallet, it needs a more powerful descriptor capability. This project aims at improving the descriptors used in the library. Currently BDK uses only a single descriptor for a single wallet. Giving a new descriptor to BDK will create a brand new wallet.

The goal of this project is to create a wallet that can handle multiple descriptors within itself (a "super wallet"), and can send funds from any or all of them together.

Expected Outcomes

• Through understanding of descriptors.
• Basic understanding of the rust-miniscript library.
• Multi descriptor capability in BDK.

Resources

Skills Required

• Experience with git. Guide
• Basic familiarity with rust. First seven chapters of the book
• Familiarity with BDK’s descriptor usage.

Mentor(s)

Difficulty: Hard

Competency Test (optional)

• Install rust, compile and run all bdk examples and tests.
• Read through the BDK docs.
• Make a dummy wallet with BDK with different blockchain backends.
• Familiarity with basic rust, should be able to write basic custom trait implementations on foreign structures.

[afilini]

Note that the main goal for this task is not really to add more descriptors to the Wallet structure, but mainly to find a way to group multiple wallets into a "super wallet" that can operate on all of them at the same time, for instance by creating a transaction that spends utxos from all of them

[rajarshimaitra]

Noted. Paraphrased the above in the description. Let me know if it can be elaborated more..

[LLFourn]

Note that the main goal for this task is not really to add more descriptors to the Wallet structure, but mainly to find a way to group multiple wallets into a "super wallet" that can operate on all of them at the same time, for instance by creating a transaction that spends utxos from all of them

I think the simplest solution is create a API that allows you to create a PSBT using the databases of multiple wallets. Could even be a TxBuilder method add_utxos_from_wallet(wallet). Internally this could just take list_utxos do a weak version of add_foreign_utxo where the UTXO would be a non-mandatory spend. This would give you all the functionality that isn't available in BDK at the moment.

Of course this leaves you to manually add the balances of the individual wallets to find the joint balance etc. But more sophisticated things can be done later. In my mind wallets already have two descriptors with different semantics. I don't see why they couldn't have three or four or twenty four all in a Vec.

[rajarshimaitra]

I think once we converge on an approach, the description can be elaborated further. I feel this will be a little difficult project for the students and they would need as much context as possible to tackle this issue.

Thanks for all the inputs @LLFourn @afilini , I will keep following this thread and update the project template accordingly.

[afilini]

In my mind wallets already have two descriptors with different semantics. I don't see why they couldn't have three or four or twenty four all in a Vec.

Yes, what worries me is how complex it can get to define all the different semantics for n descriptors inside a wallet. If this "super-wallet" thing turns out well we could also consider changing the wallets to only have one descriptor (which would simplify a ton of stuff) and then handling the semantics outside (for instance the super-wallet create_tx knows that it needs to generate the change address from one specific sub-wallet that is designated as the "internal" one).