Add psbt_signer.rs example

[notmandatory]

Description

Adding a simple example of how to create a PSBT with a watch only wallet and then sign it with a signing wallet.

Notes to the reviewers

This example was inspired by a question from a user.

Changelog notice

none.

Checklists

All Submissions:

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

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

@vladimirfomene please take a look and see if it works for you.

[vladimirfomene]

Tested ACK

[vladimirfomene]

Tested ACK

[danielabrozzoni]

Concept ACK

It looks good to me, I think it should also be added to the Cargo.toml in the examples section :)

[notmandatory]

I incorporated the suggested changes and also added a list of any unconfirmed tx if the wallet balance is less than the desired 10k sats. Ready to re-review! 🙏

[danielabrozzoni]

Looking great!

ACK fa998de

[afilini]

I would use .drain_wallet().drain_to(return_address.script_pubkey()) to make sure a change is not created. It's also a chance to show that BDK can do more than "just send x to this address".

[notmandatory]

That's a good idea but I do like having some coin selection and change going on, which I think will be how most new projects will be doing rather than sweeping all funds.

[danielabrozzoni]

Now that I think about it, the problem of receiving 10_000 and sending 9_800 is that you create a small change (~150 sats?), which will be uneconomical to spend (the fees for spending it are greater than the utxo value) - which BDK's coin selection won't pick, unless it's explicitly instructed to do so.

What this means is: after running this example many times, we might have the wallet full of these rather small utxos which we can't spend, and users will start to see some weird errors: for example, they might see that the balance is 10_000 (~70 utxos, ~150 each), but the coin selection will fail saying that there are not enough funds.

We can prevent this either by sweeping all funds every time, or by asking users to fill the wallet with 10_000 sats, but then spending only a smaller number, like 5_000

[danielabrozzoni]

Oh, nevermind, the CS is smart enough that it picks 200 sats as fee, leaving no residue in the wallet :)

[notmandatory]

Yes I guess also in my testing it used the change for fee and left an empty wallet balance. Yea for BDK coin selection! So we think it's ok to do it this way and not use drain_to to ensure there's no change?

[danielabrozzoni]

I think it's fine :)

[afilini]

Yeah the end result should be the same because the leftover is lower than the dust threshold. I can go ahead and merge it like this, my suggestion was based on what I would have done personally, but it also looks good like this.

[afilini]

I was also about to suggest using the descriptor!() macro but decided not to because I think it could be a bit overwhelming at first. Most users will probably have a string for their descriptor and this would look more familiar to them.

[afilini]

ACK fa998de