From ba3011a9c94713fc8ae410491b04a59bed2b4703 Mon Sep 17 00:00:00 2001
From: Lucas Soriano del Pino <l.soriano.del.pino@gmail.com>
Date: Thu, 15 Oct 2020 18:34:55 +1100
Subject: [PATCH] Trigger refund if the publication of Monero TxLock takes too
 long

---
 xmr-btc/Cargo.toml                      |  1 +
 xmr-btc/src/lib.rs                      | 52 +++++++++++++++++++------
 xmr-btc/tests/harness/wallet/bitcoin.rs | 21 +++++++++-
 3 files changed, 61 insertions(+), 13 deletions(-)

diff --git a/xmr-btc/Cargo.toml b/xmr-btc/Cargo.toml
index d82790f5..0b6cf7e7 100644
--- a/xmr-btc/Cargo.toml
+++ b/xmr-btc/Cargo.toml
@@ -12,6 +12,7 @@ cross-curve-dleq = { git = "https://github.com/comit-network/cross-curve-dleq",
 curve25519-dalek = "2"
 ecdsa_fun = { version = "0.3.1", features = ["libsecp_compat"] }
 ed25519-dalek = "1.0.0-pre.4" # Cannot be 1 because they depend on curve25519-dalek version 3
+futures = "0.3"
 genawaiter = "0.99.1"
 miniscript = "1"
 monero = "0.9"
diff --git a/xmr-btc/src/lib.rs b/xmr-btc/src/lib.rs
index f69f1a7a..a028f85b 100644
--- a/xmr-btc/src/lib.rs
+++ b/xmr-btc/src/lib.rs
@@ -51,7 +51,9 @@ pub mod bob;
 pub mod monero;
 pub mod transport;
 
+use async_trait::async_trait;
 use ecdsa_fun::{adaptor::Adaptor, nonce::Deterministic};
+use futures::future::Either;
 use genawaiter::sync::{Gen, GenBoxed};
 use sha2::Sha256;
 
@@ -75,6 +77,11 @@ pub trait ReceiveTransferProof {
     fn receive_transfer_proof(&self) -> monero::TransferProof;
 }
 
+#[async_trait]
+pub trait MedianTime {
+    async fn median_time(&self) -> u32;
+}
+
 /// Perform the on-chain protocol to swap monero and bitcoin as Bob.
 ///
 /// This is called post handshake, after all the keys, addresses and most of the
@@ -104,16 +111,31 @@ pub fn action_generator_bob<N, M, B>(
 where
     N: ReceiveTransferProof + Send + Sync,
     M: monero::WatchForTransfer + Send + Sync,
-    B: bitcoin::WatchForRawTransaction + Send + Sync,
+    B: MedianTime + bitcoin::WatchForRawTransaction + Send + Sync,
 {
-    enum SwapFailedRefund {
+    enum SwapFailed {
+        TimelockReached,
         InsufficientXMR(monero::InsufficientFunds),
     }
 
+    async fn poll_until_bitcoin_time<B>(bitcoin_client: &B, timestamp: u32)
+    where
+        B: MedianTime,
+    {
+        loop {
+            if bitcoin_client.median_time().await >= timestamp {
+                return;
+            }
+        }
+    }
+
     Gen::new_boxed(|co| async move {
-        let swap_result: Result<(), SwapFailedRefund> = async {
+        let swap_result: Result<(), SwapFailed> = async {
             co.yield_(Action::LockBitcoin(tx_lock.clone())).await;
 
+            let poll_until_expiry = poll_until_bitcoin_time(bitcoin_ledger, refund_timelock);
+            futures::pin_mut!(poll_until_expiry);
+
             // the source of this could be the database, this layer doesn't care
             let transfer_proof = network.receive_transfer_proof();
 
@@ -122,12 +144,22 @@ where
             ));
             let S = S_a_monero + S_b_monero;
 
-            // TODO: We should require a specific number of confirmations on the lock
-            // transaction
-            monero_ledger
-                .watch_for_transfer(S, v.public(), transfer_proof, xmr, 10)
-                .await
-                .map_err(|e| SwapFailedRefund::InsufficientXMR(e))?;
+            match futures::future::select(
+                monero_ledger.watch_for_transfer(
+                    S,
+                    v.public(),
+                    transfer_proof,
+                    xmr,
+                    monero::MIN_CONFIRMATIONS,
+                ),
+                poll_until_expiry,
+            )
+            .await
+            {
+                Either::Left((Err(e), _)) => return Err(SwapFailed::InsufficientXMR(e)),
+                Either::Right(_) => return Err(SwapFailed::TimelockReached),
+                _ => {}
+            }
 
             let tx_redeem = bitcoin::TxRedeem::new(&tx_lock, &redeem_address);
             let tx_redeem_encsig = b.encsign(S_a_bitcoin.clone(), tx_redeem.digest());
@@ -166,8 +198,6 @@ where
         }
         .await;
 
-        // NOTE: swap result should only be `Err` if we have reached the
-        // `refund_timelock`. Therefore, we should always yield the refund action
         if swap_result.is_err() {
             let tx_cancel =
                 bitcoin::TxCancel::new(&tx_lock, refund_timelock, A.clone(), b.public());
diff --git a/xmr-btc/tests/harness/wallet/bitcoin.rs b/xmr-btc/tests/harness/wallet/bitcoin.rs
index 524baaa4..723cd8fb 100644
--- a/xmr-btc/tests/harness/wallet/bitcoin.rs
+++ b/xmr-btc/tests/harness/wallet/bitcoin.rs
@@ -1,12 +1,16 @@
 use anyhow::Result;
 use async_trait::async_trait;
+use backoff::{future::FutureOperation as _, ExponentialBackoff};
 use bitcoin::{util::psbt::PartiallySignedTransaction, Address, Amount, Transaction, Txid};
 use bitcoin_harness::{bitcoind_rpc::PsbtBase64, Bitcoind};
 use reqwest::Url;
 use std::time::Duration;
 use tokio::time;
-use xmr_btc::bitcoin::{
-    BroadcastSignedTransaction, BuildTxLockPsbt, SignTxLock, TxLock, WatchForRawTransaction,
+use xmr_btc::{
+    bitcoin::{
+        BroadcastSignedTransaction, BuildTxLockPsbt, SignTxLock, TxLock, WatchForRawTransaction,
+    },
+    MedianTime,
 };
 
 #[derive(Debug)]
@@ -117,3 +121,16 @@ impl WatchForRawTransaction for Wallet {
         }
     }
 }
+
+#[async_trait]
+impl MedianTime for Wallet {
+    async fn median_time(&self) -> u32 {
+        (|| async { Ok(self.0.median_time().await?) })
+            .retry(ExponentialBackoff {
+                max_elapsed_time: None,
+                ..Default::default()
+            })
+            .await
+            .expect("transient errors to be retried")
+    }
+}