chain: add new method LookupInputMempoolSpend

This commit adds a new method to look up mempool spent for a given
outpoint.

chain/bitcoind_client.go

@@ -1400,3 +1400,11 @@ func (c *BitcoindClient) filterTx(txDetails *btcutil.Tx,
 
 	return true, rec, nil
 }
+
+// LookupInputMempoolSpend returns the transaction hash and true if the given
+// input is found being spent in mempool, otherwise it returns nil and false.
+func (c *BitcoindClient) LookupInputMempoolSpend(op wire.OutPoint) (
+	chainhash.Hash, bool) {
+
+	return c.chainConn.events.LookupInputSpend(op)
+}

chain/bitcoind_events_test.go

@@ -67,6 +67,9 @@ func TestBitcoindEvents(t *testing.T) {
 			// mempool.
 			btcClient = setupBitcoind(t, addr, test.rpcPolling)
 			testNotifySpentMempool(t, miner1, btcClient)
+
+			// Test looking up mempool for input spent.
+			testLookupInputMempoolSpend(t, miner1, btcClient)
 		})
 	}
 }
@@ -214,6 +217,46 @@ func testNotifySpentMempool(t *testing.T, miner *rpctest.Harness,
 	}
 }
 
+// testLookupInputMempoolSpend tests that LookupInputMempoolSpend returns the
+// correct tx hash and whether the input has been spent in the mempool.
+func testLookupInputMempoolSpend(t *testing.T, miner *rpctest.Harness,
+	client *BitcoindClient) {
+
+	require := require.New(t)
+
+	script, _, err := randPubKeyHashScript()
+	require.NoError(err)
+
+	// Create a test tx.
+	tx, err := miner.CreateTransaction(
+		[]*wire.TxOut{{Value: 1000, PkScript: script}}, 5, false,
+	)
+	require.NoError(err)
+
+	// Lookup the input in mempool.
+	op := tx.TxIn[0].PreviousOutPoint
+	txid, found := client.LookupInputMempoolSpend(op)
+
+	// Expect that the input has not been spent in the mempool.
+	require.False(found)
+	require.Zero(txid)
+
+	// Send the tx which will put it in the mempool.
+	_, err = client.SendRawTransaction(tx, true)
+	require.NoError(err)
+
+	// Lookup the input again should return the spending tx.
+	//
+	// NOTE: We need to wait for the tx to propagate to the mempool.
+	require.Eventually(func() bool {
+		txid, found = client.LookupInputMempoolSpend(op)
+		return found
+	}, 5*time.Second, 100*time.Millisecond)
+
+	// Check the expected txid is returned.
+	require.Equal(tx.TxHash(), txid)
+}
+
 // testReorg tests that the given BitcoindClient correctly responds to a chain
 // re-org.
 func testReorg(t *testing.T, miner1, miner2 *rpctest.Harness,

chain/btcd.go

@@ -465,3 +465,11 @@ func (c *RPCClient) POSTClient() (*rpcclient.Client, error) {
 	configCopy.HTTPPostMode = true
 	return rpcclient.New(&configCopy, nil)
 }
+
+// LookupInputMempoolSpend returns the transaction hash and true if the given
+// input is found being spent in mempool, otherwise it returns nil and false.
+func (c *RPCClient) LookupInputMempoolSpend(op wire.OutPoint) (
+	chainhash.Hash, bool) {
+
+	return getTxSpendingPrevOut(op, c.Client)
+}