Add json import/export option to persistence layer for WASM

[riverKanies]

One of the limitations of WASM is persistence, in bdk-wasm changeset we opted to add to_json and from_json methods to the wallet changeset so that the rust code can interface with the js code using a simple json string and the js code can handle persistence.

@0xBEEFCAF3 and I have been discussing whether it makes sense to add these methods in the rust-payjoin crate (using serde_json) or if it makes more sense to do that at the bindings level (like in bdk-wasm, can do just for wasm specific builds if that's the only use case).

Something to consider is that using these methods would put you in a non-persisted assumption for the rust code, but persistence would in fact be happening (if implemented properly) so that may be a third logic path potentially.

My understanding based off what @0xBEEFCAF3 has told me so far is that currently the sender and receiver objects take a persister object. for this to work in wasm we would need to build a compatible persister in js that could be fed into rust context. That's possible but would probably involve pre-packaging the payjoin-wasm node module with a persister object. Since all we need to do on the js end is save/load a complete state object it's easier and simpler to just pass json string (as we opted for in bdk-wasm) so I expect that's a better way to go.

[arminsabouri]

Quick and easy implementation: cd6f9ed

[arminsabouri]

Something to consider is that using these methods would put you in a non-persisted assumption for the rust code, but persistence would in fact be happening (if implemented properly) so that may be a third logic path potentially.

One way to solve this is to combine to/from_json format with approach taken here. i.e we wrap the sender/reciever in a seperate struct and only instantiate the state after the application has explicitly persisted. We should still have the closure but now the js code or other languages have direct access to a json-able format.

let sender = SenderBuilder::new(psbt, uri.clone())
                    .build_recommended(fee_rate)
                    .persist(|key, sender| {
                        let mut sender = sender.to_json()
                        // or if im inclined I could use another format
                        // let mut sender = bincode::serialize(&sender).expect("to serialize");
                        self.db.insert_send_session(&mut sender, key)?;
                        Ok(())
                    })?;

[riverKanies]

hmm, it's interesting because if you're going to bother doing that it feels like it would just make sense to make a prepackaged js persister object, so I'm not totally clear on benefit of one approach vs the other. but I do think the json interface is just nice to have so devs can handle persistence however they like

[DanGould]

I can see having a json-specific serializer implementation in payoin-ffi, or specifically the wasm feature but I'm not sure it makes sense in payjoin itself. It's implementation / environment specific where the trait covers the generic behavior to test for correctness.

[riverKanies]

Yeah, that's what we did for bdk-wasm, looks like we can just implement this on the sender and receiver objects at the bindings layer in payjoin-ffi. we can make it wasm specific for now too as we don't know of another use case.

I will close this and make a note of that on the wasm issue