[newtonick]

Description

bdk_electrum has populate_tx_cache and I wanted to expose it for my iOS multisig wallet (Birch) that uses Electrum. This PR exposes it on ElectrumClient, so the wallet's transactions can be loaded into the cache before the scan and the scan only downloads new ones. This improves the speed of syncing dramatically. In Swift it looks like this:

let client = try ElectrumClient(url: url)
client.populateTxCache(txs: wallet.transactions().map(\.transaction))
let update = try client.fullScan(request: wallet.startFullScan().build(), stopGap: 20, batchSize: 50, fetchPrevTxouts: false)

I benchmarked a full scan on a new client against a Fulcrum server on testnet4 (stop gap 20, batch size 50, median of 5 runs, in Rust with bdk_wallet 3.2.0 and bdk_electrum 0.24.1):

Wallet	No cache	populate_tx_cache
622 transactions	43.0 s	2.1 s
10 transactions	1.35 s	0.66 s

I made the same change on top of 3.1.1, built bdk-swift with it (release) and tested it with my iOS wallet without any issues.

Notes to the reviewers

I kept the same name as bdk_electrum and had it take a list of transactions instead of a Wallet, since the ADRs in docs/adr say to keep the Rust names and only wrap when needed. The transactions can come from Wallet.transactions.

Documentation

• bdk_wallet
• bitcoin
• uniffi
• Other: BdkElectrumClient::populate_tx_cache

Changelog

I don't have permission to add labels. I think this should be changelog: added.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

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

[thunderbiscuit]

Thanks for opening this! We've been hearing from multiple users now that sync speed and efficiency is paramount, and so I'm happy this API can help you out this way. Will review.

Note the other possible improvements coming in #1012 if you'd like to review/comment on this.

[thunderbiscuit]

ACK 9677671.

This is great; thanks for sending me down that rabbit hole, I was not aware of this method on the client! I would have loved an Android test but I think you'd need to create a wallet, populate it, save to persistence, re-build it, etc. Maybe too involved here.