test(electrum): fetch_prev_txout does not process coinbase transact…

…ions

crates/electrum/src/bdk_electrum_client.rs

@@ -531,3 +531,52 @@ fn chain_update(
     }
     Ok(tip)
 }
+
+#[cfg(test)]
+mod test {
+    use crate::{bdk_electrum_client::TxUpdate, BdkElectrumClient};
+    use bdk_chain::bitcoin::{hashes::Hash, OutPoint, Transaction, TxIn, Txid};
+    use bdk_core::collections::BTreeMap;
+    use bdk_testenv::{utils::new_tx, TestEnv};
+    use std::sync::Arc;
+
+    #[cfg(feature = "default")]
+    #[test]
+    fn test_fetch_prev_txout_with_coinbase() {
+        let env = TestEnv::new().unwrap();
+        let electrum_client =
+            electrum_client::Client::new(env.electrsd.electrum_url.as_str()).unwrap();
+        let client = BdkElectrumClient::new(electrum_client);
+
+        // Calling `fetch_prev_txout` on a coinbase transaction triggers a `fetch_tx` on a
+        // transaction with a txid of all zeros. However, Electrum's `transaction_get` may not
+        // return an error for an all-zero txid.  Therefore, we insert a transaction with a txid of
+        // all zeros into the `tx_cache` to guarantee a crash if `fetch_prev_txout` tries to fetch
+        // an all-zero txid.
+        let mut tx_cache = client.tx_cache.lock().unwrap();
+        tx_cache.insert(Txid::all_zeros(), new_tx(0).into());
+        drop(tx_cache);
+
+        // Create a coinbase transaction.
+        let coinbase_tx = Transaction {
+            input: vec![TxIn {
+                previous_output: OutPoint::null(),
+                ..Default::default()
+            }],
+            ..new_tx(0)
+        };
+
+        assert!(coinbase_tx.is_coinbase());
+
+        // Test that `fetch_prev_txout` does not process our coinbase transaction. If it attempts to
+        // fetch our previously inserted transaction with a txid of all zeros, this test will crash.
+        let mut tx_update = TxUpdate {
+            txs: vec![Arc::new(coinbase_tx)],
+            ..Default::default()
+        };
+        let _ = client.fetch_prev_txout(&mut tx_update);
+
+        // Ensure that the txouts are empty.
+        assert_eq!(tx_update.txouts, BTreeMap::default());
+    }
+}