Test non-incentivizing sweep payjoin

Such one-output transactions pay the base costs to transact and provide
an incentive for a receiver to contribute with reduced marginal cost of
adding an input rather than an explicit change output. Allowing payjoin
sweeps breaks common input heuristic for them. Sweeps with many inputs
are common, making ~5.2% or ~54M of ~1.037B monetary, non-coinbase txs.

Source monetary non-coinbase transactions count: https://blockchair.com/bitcoin/transactions?q=is_coinbase(false)#f=hash,is_coinbase,time

Source sweep transactions count: https://blockchair.com/bitcoin/transactions?s=time(desc)&q=is_coinbase(false),input_count(2..),output_count(1)#f=hash,time

payjoin/src/send/mod.rs

@@ -62,6 +62,11 @@ pub struct RequestBuilder<'a> {
     uri: PjUri<'a>,
     disable_output_substitution: bool,
     fee_contribution: Option<(bitcoin::Amount, Option<usize>)>,
+    /// Decreases the fee contribution instead of erroring.
+    ///
+    /// If this option is true and a transaction with change amount lower than fee
+    /// contribution is provided then instead of returning error the fee contribution will
+    /// be just lowered in the request to match the change amount.
     clamp_fee_contribution: bool,
     min_fee_rate: FeeRate,
 }

payjoin/tests/integration.rs

@@ -402,14 +402,9 @@ mod integration {
                     .assume_checked()
                     .check_pj_supported()
                     .unwrap();
-                let psbt = build_original_psbt(&sender, &pj_uri)?;
+                let psbt = build_sweep_psbt(&sender, &pj_uri)?;
                 let mut req_ctx = RequestBuilder::from_psbt_and_uri(psbt.clone(), pj_uri.clone())?
-                    .build_with_additional_fee(
-                        Amount::from_sat(10000),
-                        None,
-                        FeeRate::ZERO,
-                        false,
-                    )?;
+                    .build_non_incentivizing()?;
                 let (Request { url, body, .. }, send_ctx) =
                     req_ctx.extract_v2(directory.to_owned())?;
                 let response = agent
@@ -826,6 +821,31 @@ mod integration {
             Ok(Psbt::from_str(&psbt)?)
         }
 
+        fn build_sweep_psbt(
+            sender: &bitcoincore_rpc::Client,
+            pj_uri: &PjUri,
+        ) -> Result<Psbt, BoxError> {
+            let mut outputs = HashMap::with_capacity(1);
+            outputs.insert(pj_uri.address.to_string(), Amount::from_btc(50.0)?);
+            let options = bitcoincore_rpc::json::WalletCreateFundedPsbtOptions {
+                lock_unspent: Some(true),
+                fee_rate: Some(Amount::from_sat(2000)),
+                subtract_fee_from_outputs: vec![0],
+                ..Default::default()
+            };
+            let psbt = sender
+                .wallet_create_funded_psbt(
+                    &[], // inputs
+                    &outputs,
+                    None, // locktime
+                    Some(options),
+                    Some(true), // check that the sender properly clears keypaths
+                )?
+                .psbt;
+            let psbt = sender.wallet_process_psbt(&psbt, None, None, None)?.psbt;
+            Ok(Psbt::from_str(&psbt)?)
+        }
+
         fn extract_pj_tx(
             sender: &bitcoincore_rpc::Client,
             psbt: Psbt,