chore(bdk-electrum): use new batch_transaction_get_merkle API

- FIXME: bump the electrum-client version used.
- removes the now unused `serde_json` dependency from `bdk_electrum`.
- update `batch_fetch_anchors` to use `batch_transaction_get_merkle`
  method instead of manually creating the batch call.

Author: oleonardolima

Cargo.toml

@@ -20,3 +20,7 @@ authors = ["Bitcoin Dev Kit Developers"]
 [workspace.lints.clippy]
 print_stdout = "deny"
 print_stderr = "deny"
+
+# FIXME: should be removed once the new API lands in electrum-client and a new release is done.
+[patch.crates-io]
+electrum-client = { git = "https://github.com/oleonardolima/rust-electrum-client.git", branch = "feat/add-batch-transaction-get-merkle" }

crates/electrum/Cargo.toml

@@ -15,7 +15,6 @@ workspace = true
 [dependencies]
 bdk_core = { path = "../core", version = "0.6.0" }
 electrum-client = { version = "0.23.1", features = [ "proxy" ], default-features = false }
-serde_json = "1.0"
 
 [dev-dependencies]
 bdk_testenv = { path = "../testenv" }
@@ -35,3 +34,7 @@ required-features = ["use-rustls"]
 [[bench]]
 name = "test_sync"
 harness = false
+
+# FIXME: should be removed once the new API lands in electrum-client and a new release is done.
+[patch.crates-io]
+electrum-client = { git = "https://github.com/oleonardolima/rust-electrum-client.git", rev = "2dfc299b7ab175e4a62f0797b8f69070415d2afc" }

crates/electrum/src/bdk_electrum_client.rs

@@ -506,23 +506,17 @@ impl<E: ElectrumApi> BdkElectrumClient<E> {
             }
         }
 
-        // Batch all get_merkle calls.
-        let mut batch = electrum_client::Batch::default();
-        for &(txid, height, _) in &to_fetch {
-            batch.raw(
-                "blockchain.transaction.get_merkle".into(),
-                vec![
-                    electrum_client::Param::String(format!("{txid:x}")),
-                    electrum_client::Param::Usize(height),
-                ],
-            );
-        }
-        let resps = self.inner.batch_call(&batch)?;
+        // Fetch merkle proofs.
+        let txids_and_heights: Vec<(Txid, usize)> = to_fetch
+            .iter()
+            .map(|element| (element.0, element.1))
+            .collect();
+        let proofs = self
+            .inner
+            .batch_transaction_get_merkle(&txids_and_heights)?;
 
         // Validate each proof, retrying once for each stale header.
-        for ((txid, height, hash), resp) in to_fetch.into_iter().zip(resps.into_iter()) {
-            let proof: electrum_client::GetMerkleRes = serde_json::from_value(resp)?;
-
+        for ((txid, height, hash), proof) in to_fetch.into_iter().zip(proofs.into_iter()) {
             let mut header = {
                 let cache = self.block_header_cache.lock().unwrap();
                 cache