Add zero-amount Receive Chain Swap (#538)

Add support for Zero-Amount Receive Chain Swaps

cli/src/commands.rs

@@ -271,13 +271,21 @@ pub(crate) async fn handle_command(
                 })
                 .await?;
 
-            wait_confirmation!(
-                format!(
-                    "Fees: {} sat. Are the fees acceptable? (y/N) ",
-                    prepare_response.fees_sat
-                ),
-                "Payment receive halted"
-            );
+            let fees = prepare_response.fees_sat;
+            let confirmation_msg = match payer_amount_sat {
+                Some(_) => format!("Fees: {fees} sat. Are the fees acceptable? (y/N)"),
+                None => {
+                    let min = prepare_response.min_payer_amount_sat;
+                    let max = prepare_response.max_payer_amount_sat;
+                    let service_feerate = prepare_response.service_feerate;
+                    format!(
+                        "Fees: {fees} sat + {service_feerate:?}% of the sent amount. \
+                        Sender should send between {min:?} sat and {max:?} sat. \
+                        Are the fees acceptable? (y/N)"
+                    )
+                }
+            };
+            wait_confirmation!(confirmation_msg, "Payment receive halted");
 
             let response = sdk
                 .receive_payment(&ReceivePaymentRequest {

lib/bindings/langs/flutter/breez_sdk_liquid/include/breez_sdk_liquid.h

@@ -367,6 +367,9 @@ typedef struct wire_cst_prepare_receive_response {
   int32_t payment_method;
   uint64_t *payer_amount_sat;
   uint64_t fees_sat;
+  uint64_t *min_payer_amount_sat;
+  uint64_t *max_payer_amount_sat;
+  double *swapper_feerate;
 } wire_cst_prepare_receive_response;
 
 typedef struct wire_cst_receive_payment_request {
@@ -1194,6 +1197,8 @@ struct wire_cst_check_message_request *frbgen_breez_liquid_cst_new_box_autoadd_c
 
 struct wire_cst_connect_request *frbgen_breez_liquid_cst_new_box_autoadd_connect_request(void);
 
+double *frbgen_breez_liquid_cst_new_box_autoadd_f_64(double value);
+
 struct wire_cst_get_payment_request *frbgen_breez_liquid_cst_new_box_autoadd_get_payment_request(void);
 
 int64_t *frbgen_breez_liquid_cst_new_box_autoadd_i_64(int64_t value);
@@ -1306,6 +1311,7 @@ static int64_t dummy_method_to_enforce_bundling(void) {
     dummy_var ^= ((int64_t) (void*) frbgen_breez_liquid_cst_new_box_autoadd_buy_bitcoin_request);
     dummy_var ^= ((int64_t) (void*) frbgen_breez_liquid_cst_new_box_autoadd_check_message_request);
     dummy_var ^= ((int64_t) (void*) frbgen_breez_liquid_cst_new_box_autoadd_connect_request);
+    dummy_var ^= ((int64_t) (void*) frbgen_breez_liquid_cst_new_box_autoadd_f_64);
     dummy_var ^= ((int64_t) (void*) frbgen_breez_liquid_cst_new_box_autoadd_get_payment_request);
     dummy_var ^= ((int64_t) (void*) frbgen_breez_liquid_cst_new_box_autoadd_i_64);
     dummy_var ^= ((int64_t) (void*) frbgen_breez_liquid_cst_new_box_autoadd_liquid_address_data);

lib/bindings/src/breez_sdk_liquid.udl

@@ -431,9 +431,12 @@ dictionary PrepareReceiveRequest {
 };
 
 dictionary PrepareReceiveResponse {
-    u64? payer_amount_sat;
     PaymentMethod payment_method;
     u64 fees_sat;
+    u64? payer_amount_sat;
+    u64? min_payer_amount_sat;
+    u64? max_payer_amount_sat;
+    f64? swapper_feerate;
 };
 
 dictionary ReceivePaymentRequest {

lib/core/src/chain_swap.rs

@@ -436,6 +436,20 @@ impl ChainSwapHandler {
             | ChainSwapStates::TransactionLockupFailed
             | ChainSwapStates::TransactionRefunded
             | ChainSwapStates::SwapExpired => {
+                // Zero-amount Receive Chain Swaps also get to TransactionLockupFailed when user locks up funds
+                let is_zero_amount = swap.payer_amount_sat == 0;
+                if matches!(swap_state, ChainSwapStates::TransactionLockupFailed) && is_zero_amount
+                {
+                    match self.handle_amountless_update(swap).await {
+                        Ok(_) => {
+                            // We successfully accepted the quote, the swap should continue as normal
+                            return Ok(()); // Break from TxLockupFailed branch
+                        }
+                        // In case of error, we continue and mark it as refundable
+                        Err(e) => error!("Failed to accept the quote for swap {}: {e:?}", &swap.id),
+                    }
+                }
+
                 match swap.refund_tx_id.clone() {
                     None => {
                         warn!("Chain Swap {id} is in an unrecoverable state: {swap_state:?}");
@@ -453,7 +467,7 @@ impl ChainSwapHandler {
                         }
                     }
                     Some(refund_tx_id) => warn!(
-                        "Refund tx for Chain Swap {id} was already broadcast: txid {refund_tx_id}"
+                        "Refund for Chain Swap {id} was already broadcast: txid {refund_tx_id}"
                     ),
                 };
                 Ok(())
@@ -466,6 +480,82 @@ impl ChainSwapHandler {
         }
     }
 
+    async fn handle_amountless_update(&self, swap: &ChainSwap) -> Result<(), PaymentError> {
+        let quote = self
+            .swapper
+            .get_zero_amount_chain_swap_quote(&swap.id)
+            .map(|quote| quote.to_sat())?;
+        info!("Got quote of {quote} sat for swap {}", &swap.id);
+
+        self.validate_and_update_amountless_swap(swap, quote)
+            .await?;
+        self.swapper
+            .accept_zero_amount_chain_swap_quote(&swap.id, quote)
+    }
+
+    async fn validate_and_update_amountless_swap(
+        &self,
+        swap: &ChainSwap,
+        quote_server_lockup_amount_sat: u64,
+    ) -> Result<(), PaymentError> {
+        debug!("Validating {swap:?}");
+
+        ensure_sdk!(
+            matches!(swap.direction, Direction::Incoming),
+            PaymentError::generic(&format!(
+                "Only an incoming chain swap can be a zero-amount swap. Swap ID: {}",
+                &swap.id
+            ))
+        );
+
+        let script_pubkey = swap.get_receive_lockup_swap_script_pubkey(self.config.network)?;
+        let script_balance = self
+            .bitcoin_chain_service
+            .lock()
+            .await
+            .script_get_balance(script_pubkey.as_script())?;
+        debug!("Found lockup balance {script_balance:?}");
+        let user_lockup_amount_sat = match script_balance.confirmed > 0 {
+            true => script_balance.confirmed,
+            false => match script_balance.unconfirmed > 0 {
+                true => script_balance.unconfirmed.unsigned_abs(),
+                false => 0,
+            },
+        };
+        ensure_sdk!(
+            user_lockup_amount_sat > 0,
+            PaymentError::generic("Lockup address has no confirmed or unconfirmed balance")
+        );
+
+        let pair = swap.get_boltz_pair()?;
+        let swapper_service_feerate = pair.fees.percentage;
+        let swapper_server_fees_sat = pair.fees.server();
+        let service_fees_sat =
+            ((swapper_service_feerate / 100.0) * user_lockup_amount_sat as f64).ceil() as u64;
+        let fees_sat = swapper_server_fees_sat + service_fees_sat;
+        ensure_sdk!(
+            user_lockup_amount_sat > fees_sat,
+            PaymentError::generic(&format!("Invalid quote: fees ({fees_sat} sat) are higher than user lockup ({user_lockup_amount_sat} sat)"))
+        );
+
+        let expected_server_lockup_amount_sat = user_lockup_amount_sat - fees_sat;
+        debug!("user_lockup_amount_sat = {}, service_fees_sat = {}, server_fees_sat = {}, expected_server_lockup_amount_sat = {}, quote_server_lockup_amount_sat = {}",
+            user_lockup_amount_sat, service_fees_sat, swapper_server_fees_sat, expected_server_lockup_amount_sat, quote_server_lockup_amount_sat);
+        ensure_sdk!(
+            expected_server_lockup_amount_sat <= quote_server_lockup_amount_sat,
+            PaymentError::generic(&format!("Invalid quote: expected at least {expected_server_lockup_amount_sat} sat, got {quote_server_lockup_amount_sat} sat"))
+        );
+
+        let receiver_amount_sat = quote_server_lockup_amount_sat - swap.claim_fees_sat;
+        self.persister.update_zero_amount_swap_values(
+            &swap.id,
+            user_lockup_amount_sat,
+            receiver_amount_sat,
+        )?;
+
+        Ok(())
+    }
+
     async fn on_new_outgoing_status(&self, swap: &ChainSwap, update: &boltz::Update) -> Result<()> {
         let id = &update.id;
         let status = &update.status;

lib/core/src/frb_generated.rs

@@ -3291,6 +3291,17 @@ impl SseDecode for Option<bool> {
     }
 }
 
+impl SseDecode for Option<f64> {
+    // Codec=Sse (Serialization based), see doc to use other codecs
+    fn sse_decode(deserializer: &mut flutter_rust_bridge::for_generated::SseDeserializer) -> Self {
+        if (<bool>::sse_decode(deserializer)) {
+            return Some(<f64>::sse_decode(deserializer));
+        } else {
+            return None;
+        }
+    }
+}
+
 impl SseDecode for Option<i64> {
     // Codec=Sse (Serialization based), see doc to use other codecs
     fn sse_decode(deserializer: &mut flutter_rust_bridge::for_generated::SseDeserializer) -> Self {
@@ -3747,10 +3758,16 @@ impl SseDecode for crate::model::PrepareReceiveResponse {
         let mut var_paymentMethod = <crate::model::PaymentMethod>::sse_decode(deserializer);
         let mut var_payerAmountSat = <Option<u64>>::sse_decode(deserializer);
         let mut var_feesSat = <u64>::sse_decode(deserializer);
+        let mut var_minPayerAmountSat = <Option<u64>>::sse_decode(deserializer);
+        let mut var_maxPayerAmountSat = <Option<u64>>::sse_decode(deserializer);
+        let mut var_swapperFeerate = <Option<f64>>::sse_decode(deserializer);
         return crate::model::PrepareReceiveResponse {
             payment_method: var_paymentMethod,
             payer_amount_sat: var_payerAmountSat,
             fees_sat: var_feesSat,
+            min_payer_amount_sat: var_minPayerAmountSat,
+            max_payer_amount_sat: var_maxPayerAmountSat,
+            swapper_feerate: var_swapperFeerate,
         };
     }
 }
@@ -5807,6 +5824,9 @@ impl flutter_rust_bridge::IntoDart for crate::model::PrepareReceiveResponse {
             self.payment_method.into_into_dart().into_dart(),
             self.payer_amount_sat.into_into_dart().into_dart(),
             self.fees_sat.into_into_dart().into_dart(),
+            self.min_payer_amount_sat.into_into_dart().into_dart(),
+            self.max_payer_amount_sat.into_into_dart().into_dart(),
+            self.swapper_feerate.into_into_dart().into_dart(),
         ]
         .into_dart()
     }
@@ -7282,6 +7302,16 @@ impl SseEncode for Option<bool> {
     }
 }
 
+impl SseEncode for Option<f64> {
+    // Codec=Sse (Serialization based), see doc to use other codecs
+    fn sse_encode(self, serializer: &mut flutter_rust_bridge::for_generated::SseSerializer) {
+        <bool>::sse_encode(self.is_some(), serializer);
+        if let Some(value) = self {
+            <f64>::sse_encode(value, serializer);
+        }
+    }
+}
+
 impl SseEncode for Option<i64> {
     // Codec=Sse (Serialization based), see doc to use other codecs
     fn sse_encode(self, serializer: &mut flutter_rust_bridge::for_generated::SseSerializer) {
@@ -7680,6 +7710,9 @@ impl SseEncode for crate::model::PrepareReceiveResponse {
         <crate::model::PaymentMethod>::sse_encode(self.payment_method, serializer);
         <Option<u64>>::sse_encode(self.payer_amount_sat, serializer);
         <u64>::sse_encode(self.fees_sat, serializer);
+        <Option<u64>>::sse_encode(self.min_payer_amount_sat, serializer);
+        <Option<u64>>::sse_encode(self.max_payer_amount_sat, serializer);
+        <Option<f64>>::sse_encode(self.swapper_feerate, serializer);
     }
 }
 
@@ -8290,6 +8323,12 @@ mod io {
             CstDecode::<crate::model::ConnectRequest>::cst_decode(*wrap).into()
         }
     }
+    impl CstDecode<f64> for *mut f64 {
+        // Codec=Cst (C-struct based), see doc to use other codecs
+        fn cst_decode(self) -> f64 {
+            unsafe { *flutter_rust_bridge::for_generated::box_from_leak_ptr(self) }
+        }
+    }
     impl CstDecode<crate::model::GetPaymentRequest> for *mut wire_cst_get_payment_request {
         // Codec=Cst (C-struct based), see doc to use other codecs
         fn cst_decode(self) -> crate::model::GetPaymentRequest {
@@ -9554,6 +9593,9 @@ mod io {
                 payment_method: self.payment_method.cst_decode(),
                 payer_amount_sat: self.payer_amount_sat.cst_decode(),
                 fees_sat: self.fees_sat.cst_decode(),
+                min_payer_amount_sat: self.min_payer_amount_sat.cst_decode(),
+                max_payer_amount_sat: self.max_payer_amount_sat.cst_decode(),
+                swapper_feerate: self.swapper_feerate.cst_decode(),
             }
         }
     }
@@ -10702,6 +10744,9 @@ mod io {
                 payment_method: Default::default(),
                 payer_amount_sat: core::ptr::null_mut(),
                 fees_sat: Default::default(),
+                min_payer_amount_sat: core::ptr::null_mut(),
+                max_payer_amount_sat: core::ptr::null_mut(),
+                swapper_feerate: core::ptr::null_mut(),
             }
         }
     }
@@ -11481,6 +11526,11 @@ mod io {
         )
     }
 
+    #[no_mangle]
+    pub extern "C" fn frbgen_breez_liquid_cst_new_box_autoadd_f_64(value: f64) -> *mut f64 {
+        flutter_rust_bridge::for_generated::new_leak_box_ptr(value)
+    }
+
     #[no_mangle]
     pub extern "C" fn frbgen_breez_liquid_cst_new_box_autoadd_get_payment_request(
     ) -> *mut wire_cst_get_payment_request {
@@ -12840,6 +12890,9 @@ mod io {
         payment_method: i32,
         payer_amount_sat: *mut u64,
         fees_sat: u64,
+        min_payer_amount_sat: *mut u64,
+        max_payer_amount_sat: *mut u64,
+        swapper_feerate: *mut f64,
     }
     #[repr(C)]
     #[derive(Clone, Copy)]

lib/core/src/model.rs

@@ -2,6 +2,7 @@ use std::path::PathBuf;
 
 use anyhow::{anyhow, Result};
 
+use boltz_client::boltz::ChainPair;
 use boltz_client::{
     bitcoin::ScriptBuf,
     network::Chain,
@@ -302,7 +303,31 @@ pub struct PrepareReceiveRequest {
 pub struct PrepareReceiveResponse {
     pub payment_method: PaymentMethod,
     pub payer_amount_sat: Option<u64>,
+
+    /// Generally represents the total fees that would be paid to send or receive this payment.
+    ///
+    /// In case of Zero-Amount Receive Chain swaps, the swapper service fee (`swapper_feerate` times
+    /// the amount) is paid in addition to `fees_sat`. The swapper service feerate is already known
+    /// in the beginning, but the exact swapper service fee will only be known when the
+    /// `payer_amount_sat` is known.
+    ///
+    /// In all other types of swaps, the swapper service fee is included in `fees_sat`.
     pub fees_sat: u64,
+
+    /// The minimum amount the payer can send for this swap to succeed.
+    ///
+    /// When the method is [PaymentMethod::LiquidAddress], this is empty.
+    pub min_payer_amount_sat: Option<u64>,
+
+    /// The maximum amount the payer can send for this swap to succeed.
+    ///
+    /// When the method is [PaymentMethod::LiquidAddress], this is empty.
+    pub max_payer_amount_sat: Option<u64>,
+
+    /// The percentage of the sent amount that will count towards the service fee.
+    ///
+    /// When the method is [PaymentMethod::LiquidAddress], this is empty.
+    pub swapper_feerate: Option<f64>,
 }
 
 /// An argument when calling [crate::sdk::LiquidSdk::receive_payment].
@@ -671,6 +696,13 @@ impl ChainSwap {
         })
     }
 
+    pub(crate) fn get_boltz_pair(&self) -> Result<ChainPair> {
+        let pair: ChainPair = serde_json::from_str(&self.pair_fees_json)
+            .map_err(|e| anyhow!("Failed to deserialize ChainPair: {e:?}"))?;
+
+        Ok(pair)
+    }
+
     pub(crate) fn get_claim_swap_script(&self) -> SdkResult<SwapScriptV2> {
         let chain_swap_details = self.get_boltz_create_response()?.claim_details;
         let our_pubkey = self.get_claim_keypair()?.public_key();