Merge #2005: Add populate_anchor_cache method to bdk

191bdb1 feat(electrum): Add `populate_anchor_cache` method (+Sharon)

Pull request description:

  This PR addresses #1982
  It introduces functionality to populate the anchor cache, improving how BDK handles stateful or performance-critical anchor data.

  ### Changes
  - Add `populate_anchor_cache` method to `BdkElectrumClient `in `bdk-core`
  - Improves sync speed by caching anchor transactions in advance
  - Enables pre-caching of anchor transactions to reduce redundant network calls

  Follow-up work will extend this functionality to `bdk-wallet`.

ACKs for top commit:
  evanlinjin:
    ACK 191bdb1

Tree-SHA512: b14e9ee5a5deb439951175195419c2e07e513c23a6c242254e610b1f6d38a37cb71f2ae8d82f9c34628acf665126256eab7f37bf9ba6634547e7218433136cbe

Author: evanlinjin

crates/electrum/src/bdk_electrum_client.rs

@@ -7,6 +7,7 @@ use bdk_core::{
     BlockId, CheckPoint, ConfirmationBlockTime, TxUpdate,
 };
 use electrum_client::{ElectrumApi, Error, HeaderNotification};
+use std::convert::TryInto;
 use std::sync::{Arc, Mutex};
 
 /// We include a chain suffix of a certain length for the purpose of robustness.
@@ -37,8 +38,24 @@ impl<E: ElectrumApi> BdkElectrumClient<E> {
         }
     }
 
-    /// Inserts transactions into the transaction cache so that the client will not fetch these
-    /// transactions.
+    /// Insert anchors into the anchor cache so that the client will not re-fetch them.
+    ///
+    /// Typically used to pre-populate the cache from an existing `TxGraph`.
+    pub fn populate_anchor_cache(
+        &self,
+        tx_anchors: impl IntoIterator<Item = (Txid, impl IntoIterator<Item = ConfirmationBlockTime>)>,
+    ) {
+        let mut cache = self.anchor_cache.lock().unwrap();
+        for (txid, anchors) in tx_anchors {
+            for anchor in anchors {
+                cache.insert((txid, anchor.block_id.hash), anchor);
+            }
+        }
+    }
+
+    /// Insert transactions into the transaction cache so that the client will not re-fetch them.
+    ///
+    /// Typically used to pre-populate the cache from an existing `TxGraph`.
     pub fn populate_tx_cache(&self, txs: impl IntoIterator<Item = impl Into<Arc<Transaction>>>) {
         let mut tx_cache = self.tx_cache.lock().unwrap();
         for tx in txs {

examples/example_electrum/src/main.rs

@@ -126,16 +126,13 @@ fn main() -> anyhow::Result<()> {
 
     let client = BdkElectrumClient::new(electrum_cmd.electrum_args().client(network)?);
 
-    // Tell the electrum client about the txs we've already got locally so it doesn't re-download
-    // them
-    client.populate_tx_cache(
-        graph
-            .lock()
-            .unwrap()
-            .graph()
-            .full_txs()
-            .map(|tx_node| tx_node.tx),
-    );
+    // Tell the electrum client about the txs and anchors we've already got locally so it doesn't
+    // re-download .them
+    {
+        let graph = graph.lock().unwrap();
+        client.populate_tx_cache(graph.graph().full_txs().map(|tx_node| tx_node.tx));
+        client.populate_anchor_cache(graph.graph().all_anchors().clone());
+    }
 
     let (chain_update, tx_update, keychain_update) = match electrum_cmd.clone() {
         ElectrumCommands::Scan {