[blockchain] Add traits to reuse Blockchains across multiple wallets

Add two new traits:
- `StatelessBlockchain` is used to tag `Blockchain`s that don't have any
  wallet-specic state, i.e. they can be used as-is to sync multiple wallets.
- `BlockchainFactory` is a trait for objects that can build multiple
  blockchains for different descriptors. It's implemented automatically
  for every `Arc<T>` where `T` is a `StatelessBlockchain`. This allows a
  piece of code that deals with multiple sub-wallets to just get a
  `&B: BlockchainFactory` to sync all of them.

These new traits have been implemented for Electrum, Esplora and RPC
(the first two being stateless and the latter having a dedicated
`RpcBlockchainFactory` struct). It hasn't been implemented on the CBF
blockchain, because I don't think it would work in its current form
(it throws away old block filters, so it's hard to go back and rescan).

This is the first step for bitcoindevkit#549, as BIP47 needs to sync many different
descriptors internally.

It's also very useful for bitcoindevkit#486.

Author: afilini

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add `get_internal_address` to allow you to get internal addresses just as you get external addresses.
 - added `ensure_addresses_cached` to `Wallet` to let offline wallets load and cache addresses in their database
 - Add `is_spent` field to `LocalUtxo`; when we notice that a utxo has been spent we set `is_spent` field to true instead of deleting it from the db.
+- Add traits to reuse `Blockchain`s across multiple wallets (`BlockchainFactory` and `StatelessBlockchain`).
 
 ### Sync API change
 
@@ -437,4 +438,4 @@ final transaction is created by calling `finish` on the builder.
 [v0.16.0]: https://github.com/bitcoindevkit/bdk/compare/v0.15.0...v0.16.0
 [v0.16.1]: https://github.com/bitcoindevkit/bdk/compare/v0.16.0...v0.16.1
 [v0.17.0]: https://github.com/bitcoindevkit/bdk/compare/v0.16.1...v0.17.0
-[unreleased]: https://github.com/bitcoindevkit/bdk/compare/v0.17.0...HEAD
+[unreleased]: https://github.com/bitcoindevkit/bdk/compare/v0.17.0...HEAD

src/blockchain/electrum.rs

@@ -79,6 +79,8 @@ impl Blockchain for ElectrumBlockchain {
     }
 }
 
+impl StatelessBlockchain for ElectrumBlockchain {}
+
 impl GetHeight for ElectrumBlockchain {
     fn get_height(&self) -> Result<u32, Error> {
         // TODO: unsubscribe when added to the client, or is there a better call to use here?

src/blockchain/esplora/reqwest.rs

@@ -101,6 +101,8 @@ impl Blockchain for EsploraBlockchain {
     }
 }
 
+impl StatelessBlockchain for EsploraBlockchain {}
+
 #[maybe_async]
 impl GetHeight for EsploraBlockchain {
     fn get_height(&self) -> Result<u32, Error> {

src/blockchain/esplora/ureq.rs

@@ -98,6 +98,8 @@ impl Blockchain for EsploraBlockchain {
     }
 }
 
+impl StatelessBlockchain for EsploraBlockchain {}
+
 impl GetHeight for EsploraBlockchain {
     fn get_height(&self) -> Result<u32, Error> {
         Ok(self.url_client._get_height()?)

src/blockchain/mod.rs

@@ -164,6 +164,65 @@ pub trait ConfigurableBlockchain: Blockchain + Sized {
     fn from_config(config: &Self::Config) -> Result<Self, Error>;
 }
 
+/// Trait for blockchains that don't contain any state
+///
+/// Statless blockchains can be used to sync multiple wallets with different descriptors.
+///
+/// [`BlockchainFactory`] is automatically implemented for `Arc<T>` where `T` is a stateless
+/// blockchain.
+pub trait StatelessBlockchain: Blockchain {}
+
+/// Trait for a factory of blockchains that share the underlying connection or configuration
+#[cfg_attr(
+    not(feature = "async-interface"),
+    doc = r##"
+## Example
+
+This example shows how to sync multiple walles and return the sum of their balances
+
+```no_run
+# use bdk::Error;
+# use bdk::blockchain::*;
+# use bdk::database::*;
+# use bdk::wallet::*;
+fn sum_of_balances<B: BlockchainFactory>(blockchain_factory: B, wallets: &[Wallet<MemoryDatabase>]) -> Result<u64, Error> {
+    Ok(wallets
+        .iter()
+        .map(|w| -> Result<_, Error> {
+            w.sync(&blockchain_factory.build("wallet_1", None)?, SyncOptions::default())?;
+            w.get_balance()
+        })
+        .collect::<Result<Vec<_>, _>>()?
+        .into_iter()
+        .sum())
+}
+```
+"##
+)]
+pub trait BlockchainFactory {
+    /// The type returned when building a blockchain from this factory
+    type Inner: Blockchain;
+
+    /// Build a new blockchain for the given descriptor checksum
+    ///
+    /// If `override_skip_blocks` is `None`, the returned blockchain will inherit the number of blocks
+    /// from the factory. Since it's not possible to override the value to `None`, set it to
+    /// `Some(0)` to rescan from the genesis.
+    fn build(
+        &self,
+        checksum: &str,
+        override_skip_blocks: Option<u32>,
+    ) -> Result<Self::Inner, Error>;
+}
+
+impl<T: StatelessBlockchain> BlockchainFactory for Arc<T> {
+    type Inner = Self;
+
+    fn build(&self, _checksum: &str, _override_skip_blocks: Option<u32>) -> Result<Self, Error> {
+        Ok(Arc::clone(self))
+    }
+}
+
 /// Data sent with a progress update over a [`channel`]
 pub type ProgressData = (f32, Option<String>);
 

src/blockchain/rpc.rs

@@ -441,6 +441,59 @@ fn list_wallet_dir(client: &Client) -> Result<Vec<String>, Error> {
     Ok(result.wallets.into_iter().map(|n| n.name).collect())
 }
 
+/// Factory of [`RpcBlockchain`] instances, implements [`BlockchainFactory`]
+///
+/// Internally caches the node url and authentication params and allows getting many different [`RpcBlockchain`]
+/// objects for different wallet names and with different rescan heights.
+///
+/// ## Example
+///
+/// ```no_run
+/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
+/// let factory = RpcBlockchainFactory {
+///     url: "http://127.0.0.1:18332",
+///     auth: Auth::Cookie {
+///         file: "/home/user/.bitcoin/.cookie".to_string(),
+///     },
+///     network: Network::Testnet,
+///     wallet_name_prefix: "prefix-".to_string(),
+///     default_skip_blocks: 100_000,
+/// };
+/// let main_wallet_blockchain = factory.build("main_wallet", Some(200_000))?;
+/// # Ok(())
+/// ```
+#[derive(Debug, Clone)]
+pub struct RpcBlockchainFactory {
+    /// The bitcoin node url
+    pub url: String,
+    /// The bitcoin node authentication mechanism
+    pub auth: Auth,
+    /// The network we are using (it will be checked the bitcoin node network matches this)
+    pub network: Network,
+    /// The prefix used to build the full wallet name for blockchains
+    pub wallet_name_prefix: String,
+    /// Default number of blocks to skip which will be inherited by blockchain unless overridden
+    pub default_skip_blocks: u32,
+}
+
+impl BlockchainFactory for RpcBlockchainFactory {
+    type Inner = RpcBlockchain;
+
+    fn build(
+        &self,
+        checksum: &str,
+        override_skip_blocks: Option<u32>,
+    ) -> Result<Self::Inner, Error> {
+        Ok(RpcBlockchain::from_config(&RpcConfig {
+            url: self.url.clone(),
+            auth: self.auth.clone(),
+            network: self.network,
+            wallet_name: format!("{}{}", self.wallet_name_prefix, checksum),
+            skip_blocks: Some(override_skip_blocks.unwrap_or(self.default_skip_blocks)),
+        })?)
+    }
+}
+
 #[cfg(test)]
 #[cfg(feature = "test-rpc")]
 crate::bdk_blockchain_tests! {
@@ -456,3 +509,49 @@ crate::bdk_blockchain_tests! {
         RpcBlockchain::from_config(&config).unwrap()
     }
 }
+
+#[cfg(test)]
+#[cfg(feature = "test-rpc")]
+mod test {
+    use super::*;
+    use crate::blockchain::*;
+    use crate::testutils::blockchain_tests::TestClient;
+
+    use bitcoin::Network;
+    use bitcoincore_rpc::RpcApi;
+
+    #[test]
+    fn test_rpc_blockchain_factory() {
+        let test_client = TestClient::default();
+
+        let factory = RpcBlockchainFactory {
+            url: test_client.bitcoind.rpc_url(),
+            auth: Auth::Cookie {
+                file: test_client.bitcoind.params.cookie_file.clone(),
+            },
+            network: Network::Regtest,
+            wallet_name_prefix: "prefix-".into(),
+            default_skip_blocks: 0,
+        };
+
+        let a = factory.build("aaaaaa", None).unwrap();
+        assert_eq!(a.skip_blocks, Some(0));
+        assert_eq!(
+            a.client
+                .get_wallet_info()
+                .expect("Node connection is working")
+                .wallet_name,
+            "prefix-aaaaaa"
+        );
+
+        let b = factory.build("bbbbbb", Some(100)).unwrap();
+        assert_eq!(b.skip_blocks, Some(100));
+        assert_eq!(
+            a.client
+                .get_wallet_info()
+                .expect("Node connection is working")
+                .wallet_name,
+            "prefix-bbbbbb"
+        );
+    }
+}