Allow receiver to set minimum fee_rate

  Payjoin receiver can set minimum `fee_rate`
  in order to reject payjoin requests that contain
  a psbt which doesnt meet the minimum `fee_rate`
  threshold.

  We add this functionality to `check_broadcast_suitability`
  function which previously was called `check_can_broadcast`.

payjoin-cli/src/app.rs

@@ -250,7 +250,7 @@ impl App {
             )?;
 
         // Receive Check 1: Can Broadcast
-        let proposal = proposal.check_can_broadcast(|tx| {
+        let proposal = proposal.check_broadcast_suitability(None, |tx| {
             let raw_tx = bitcoin::consensus::encode::serialize_hex(&tx);
             let mempool_results = self
                 .bitcoind

payjoin/src/receive/error.rs

@@ -65,6 +65,10 @@ pub(crate) enum InternalRequestError {
     /// Original PSBT input has been seen before. Only automatic receivers, aka "interactive" in the spec
     /// look out for these to prevent probing attacks.
     InputSeen(bitcoin::OutPoint),
+    /// Fee rate provided by sender is too low
+    /// First argument is the fee rate provided by the sender
+    /// Second argument is the minimum fee rate required by the receiver
+    FeeTooLow(bitcoin::Amount, bitcoin::FeeRate),
 }
 
 impl From<InternalRequestError> for RequestError {
@@ -125,6 +129,11 @@ impl fmt::Display for RequestError {
                 write_error(f, "original-psbt-rejected", &format!("Input Type Error: {}.", e)),
             InternalRequestError::InputSeen(_) =>
                 write_error(f, "original-psbt-rejected", "The receiver rejected the original PSBT."),
+            InternalRequestError::FeeTooLow(sender_fee_rate, receiver_fee_rate) => write_error(
+                f,
+                "dont meet minimum required fee rate defined by receiver",
+                &format!("Fee too low: {} < {}.", sender_fee_rate, receiver_fee_rate),
+            ),
         }
     }
 }

payjoin/src/receive/mod.rs

@@ -123,7 +123,7 @@
 //! We need to know this transaction is consensus-valid.
 //!
 //! ```
-//! let checked_1 = proposal.check_can_broadcast(|tx| {
+//! let checked_1 = proposal.check_broadcast_suitability(None, |tx| {
 //!         let raw_tx = bitcoin::consensus::encode::serialize(&tx).to_hex();
 //!         let mempool_results = self
 //!             .bitcoind
@@ -295,7 +295,7 @@ pub trait Headers {
 ///
 /// If you are implementing an interactive payment processor, you should get extract the original
 /// transaction with extract_tx_to_schedule_broadcast() and schedule, followed by checking
-/// that the transaction can be broadcast with check_can_broadcast. Otherwise it is safe to
+/// that the transaction can be broadcast with check_broadcast_suitability. Otherwise it is safe to
 /// call assume_interactive_receive to proceed with validation.
 #[derive(Debug, Clone)]
 pub struct UncheckedProposal {
@@ -360,10 +360,18 @@ impl UncheckedProposal {
     /// Broadcasting the Original PSBT after some time in the failure case makes incurs sender cost and prevents probing.
     ///
     /// Call this after checking downstream.
-    pub fn check_can_broadcast(
+    pub fn check_broadcast_suitability(
         self,
+        min_feerate: Option<FeeRate>,
         can_broadcast: impl Fn(&bitcoin::Transaction) -> Result<bool, Error>,
     ) -> Result<MaybeInputsOwned, Error> {
+        if let (Some(receiver_min_feerate), Ok(psbt_feerate)) = (min_feerate, self.psbt.fee()) {
+            if receiver_min_feerate.to_sat_per_kwu() > psbt_feerate.to_sat() {
+                return Err(Error::BadRequest(
+                    InternalRequestError::FeeTooLow(psbt_feerate, receiver_min_feerate).into(),
+                ));
+            }
+        }
         if can_broadcast(&self.psbt.clone().extract_tx())? {
             Ok(MaybeInputsOwned { psbt: self.psbt, params: self.params })
         } else {

payjoin/tests/integration.rs

@@ -143,8 +143,24 @@ mod integration {
         let _to_broadcast_in_failure_case = proposal.extract_tx_to_schedule_broadcast();
 
         // Receive Check 1: Can Broadcast
+        // Here we set receiver min feerate to be higher than the proposal's psbt feerate
+        // This should fail the check
+        let receiver_min_feerate = bitcoin::FeeRate::from_sat_per_kwu(283);
+        assert!(proposal
+            .clone()
+            .check_broadcast_suitability(Some(receiver_min_feerate), |tx| {
+                Ok(receiver
+                    .test_mempool_accept(&[bitcoin::consensus::encode::serialize_hex(&tx)])
+                    .unwrap()
+                    .first()
+                    .unwrap()
+                    .allowed)
+            })
+            .is_err());
+        // Here we set receiver min feerate to be lower than the proposal's psbt feerate
+        let receiver_min_feerate = bitcoin::FeeRate::from_sat_per_kwu(281);
         let proposal = proposal
-            .check_can_broadcast(|tx| {
+            .check_broadcast_suitability(Some(receiver_min_feerate), |tx| {
                 Ok(receiver
                     .test_mempool_accept(&[bitcoin::consensus::encode::serialize_hex(&tx)])
                     .unwrap()