Move more domain knowledge onto the `TxCancel` type

swap/src/bitcoin/cancel.rs

@@ -1,12 +1,12 @@
+use crate::bitcoin;
 use crate::bitcoin::wallet::Watchable;
 use crate::bitcoin::{
     build_shared_output_descriptor, Address, Amount, BlockHeight, PublicKey, Transaction, TxLock,
     TX_FEE,
 };
 use ::bitcoin::util::bip143::SigHashCache;
-use ::bitcoin::{OutPoint, SigHash, SigHashType, TxIn, TxOut, Txid};
+use ::bitcoin::{OutPoint, Script, SigHash, SigHashType, TxIn, TxOut, Txid};
 use anyhow::Result;
-use bitcoin::Script;
 use ecdsa_fun::Signature;
 use miniscript::{Descriptor, DescriptorTrait};
 use serde::{Deserialize, Serialize};
@@ -149,7 +149,39 @@ impl TxCancel {
         OutPoint::new(self.inner.txid(), 0)
     }
 
-    pub fn add_signatures(
+    pub fn complete_as_alice(
+        self,
+        a: bitcoin::SecretKey,
+        B: bitcoin::PublicKey,
+        tx_cancel_sig_B: bitcoin::Signature,
+    ) -> Result<Transaction> {
+        let sig_a = a.sign(self.digest());
+        let sig_b = tx_cancel_sig_B;
+
+        let tx_cancel = self
+            .add_signatures((a.public(), sig_a), (B, sig_b))
+            .expect("sig_{a,b} to be valid signatures for tx_cancel");
+
+        Ok(tx_cancel)
+    }
+
+    pub fn complete_as_bob(
+        self,
+        A: bitcoin::PublicKey,
+        b: bitcoin::SecretKey,
+        tx_cancel_sig_A: bitcoin::Signature,
+    ) -> Result<Transaction> {
+        let sig_a = tx_cancel_sig_A;
+        let sig_b = b.sign(self.digest());
+
+        let tx_cancel = self
+            .add_signatures((A, sig_a), (b.public(), sig_b))
+            .expect("sig_{a,b} to be valid signatures for tx_cancel");
+
+        Ok(tx_cancel)
+    }
+
+    fn add_signatures(
         self,
         (A, sig_a): (PublicKey, Signature),
         (B, sig_b): (PublicKey, Signature),

swap/src/protocol/alice/steps.rs

@@ -1,6 +1,6 @@
 use crate::bitcoin;
 use crate::bitcoin::{CancelTimelock, PunishTimelock, TxCancel, TxLock, TxRefund};
-use anyhow::{bail, Result};
+use anyhow::{bail, Context, Result};
 
 pub async fn publish_cancel_transaction(
     tx_lock: TxLock,
@@ -25,15 +25,12 @@ pub async fn publish_cancel_transaction(
         // TODO(Franck): Maybe the cancel transaction is already mined, in this case,
         // the broadcast will error out.
 
-        let sig_a = a.sign(tx_cancel.digest());
-        let sig_b = tx_cancel_sig_bob.clone();
-
-        let tx_cancel = tx_cancel
-            .add_signatures((a.public(), sig_a), (B, sig_b))
-            .expect("sig_{a,b} to be valid signatures for tx_cancel");
+        let transaction = tx_cancel
+            .complete_as_alice(a, B, tx_cancel_sig_bob)
+            .context("Failed to complete Bitcoin cancel transaction")?;
 
         // TODO(Franck): Error handling is delicate, why can't we broadcast?
-        let (..) = bitcoin_wallet.broadcast(tx_cancel, "cancel").await?;
+        let (..) = bitcoin_wallet.broadcast(transaction, "cancel").await?;
 
         // TODO(Franck): Wait until transaction is mined and returned mined
         // block height

swap/src/protocol/bob/state.rs

@@ -8,7 +8,7 @@ use crate::monero_ext::ScalarExt;
 use crate::protocol::alice::{Message1, Message3};
 use crate::protocol::bob::{EncryptedSignature, Message0, Message2, Message4};
 use crate::protocol::CROSS_CURVE_PROOF_SYSTEM;
-use anyhow::{anyhow, bail, Result};
+use anyhow::{anyhow, bail, Context, Result};
 use ecdsa_fun::adaptor::{Adaptor, HashTranscript};
 use ecdsa_fun::nonce::Deterministic;
 use ecdsa_fun::Signature;
@@ -441,20 +441,12 @@ impl State4 {
     }
 
     pub async fn submit_tx_cancel(&self, bitcoin_wallet: &bitcoin::Wallet) -> Result<Txid> {
-        let tx_cancel =
-            bitcoin::TxCancel::new(&self.tx_lock, self.cancel_timelock, self.A, self.b.public());
-
-        let sig_a = self.tx_cancel_sig_a.clone();
-        let sig_b = self.b.sign(tx_cancel.digest());
-
-        let tx_cancel = tx_cancel
-            .add_signatures((self.A, sig_a), (self.b.public(), sig_b))
-            .expect(
-                "sig_{a,b} to be valid signatures for
-                tx_cancel",
-            );
+        let transaction =
+            bitcoin::TxCancel::new(&self.tx_lock, self.cancel_timelock, self.A, self.b.public())
+                .complete_as_bob(self.A, self.b.clone(), self.tx_cancel_sig_a.clone())
+                .context("Failed to complete Bitcoin cancel transaction")?;
 
-        let (tx_id, _) = bitcoin_wallet.broadcast(tx_cancel, "cancel").await?;
+        let (tx_id, _) = bitcoin_wallet.broadcast(transaction, "cancel").await?;
 
         Ok(tx_id)
     }