Use a builder pattern in WantsOutputs and WantsInputs

- Output substitution functions return a modified WantsOutputs instead
  of a WantsInputs. `freeze_outputs` is used to proceed to the
  WantsInputs step.
- Similarly, contribute_witness_inputs returns a modified WantsInputs
  and `freeze_inputs` returns the ProvisionalProposal
- One change output (aka drain script) must be identified if outputs are
  replaced. The change output receives all excess input amount from the
  receiver, and will be responsible for paying receiver fees if
  applicable.

payjoin-cli/src/app/v1.rs

@@ -329,7 +329,7 @@ impl App {
         })?;
         log::trace!("check4");
 
-        let provisional_payjoin = payjoin.identify_receiver_outputs(|output_script| {
+        let payjoin = payjoin.identify_receiver_outputs(|output_script| {
             if let Ok(address) = bitcoin::Address::from_script(output_script, network) {
                 bitcoind
                     .get_address_info(&address)
@@ -340,17 +340,22 @@ impl App {
             }
         })?;
 
-        let provisional_payjoin = provisional_payjoin.try_substitute_receiver_output(|| {
-            Ok(bitcoind
-                .get_new_address(None, None)
-                .map_err(|e| Error::Server(e.into()))?
-                .require_network(network)
-                .map_err(|e| Error::Server(e.into()))?
-                .script_pubkey())
-        })?;
-
-        let provisional_payjoin = try_contributing_inputs(provisional_payjoin, &bitcoind)
-            .map_err(|e| Error::Server(e.into()))?;
+        let payjoin = payjoin
+            .substitute_receiver_script(
+                &bitcoind
+                    .get_new_address(None, None)
+                    .map_err(|e| Error::Server(e.into()))?
+                    .require_network(network)
+                    .map_err(|e| Error::Server(e.into()))?
+                    .script_pubkey(),
+            )?
+            .commit_outputs();
+
+        let provisional_payjoin = try_contributing_inputs(payjoin.clone(), &bitcoind)
+            .unwrap_or_else(|e| {
+                log::warn!("Failed to contribute inputs: {}", e);
+                payjoin.commit_inputs()
+            });
 
         let payjoin_proposal = provisional_payjoin.finalize_proposal(
             |psbt: &Psbt| {
@@ -390,7 +395,9 @@ fn try_contributing_inputs(
         script_pubkey: selected_utxo.script_pub_key.clone(),
     };
 
-    Ok(payjoin.contribute_witness_inputs(vec![(selected_outpoint, txo_to_contribute)]))
+    Ok(payjoin
+        .contribute_witness_inputs(vec![(selected_outpoint, txo_to_contribute)])
+        .commit_inputs())
 }
 
 fn full<T: Into<Bytes>>(chunk: T) -> BoxBody<Bytes, hyper::Error> {

payjoin-cli/src/app/v2.rs

@@ -316,21 +316,24 @@ impl App {
         })?;
         log::trace!("check4");
 
-        let provisional_payjoin = payjoin.identify_receiver_outputs(|output_script| {
-            if let Ok(address) = bitcoin::Address::from_script(output_script, network) {
-                bitcoind
-                    .get_address_info(&address)
-                    .map(|info| info.is_mine.unwrap_or(false))
-                    .map_err(|e| Error::Server(e.into()))
-            } else {
-                Ok(false)
-            }
-        })?;
-
-        let provisional_payjoin = provisional_payjoin.try_substitute_receiver_outputs(None)?;
+        let payjoin = payjoin
+            .identify_receiver_outputs(|output_script| {
+                if let Ok(address) = bitcoin::Address::from_script(output_script, network) {
+                    bitcoind
+                        .get_address_info(&address)
+                        .map(|info| info.is_mine.unwrap_or(false))
+                        .map_err(|e| Error::Server(e.into()))
+                } else {
+                    Ok(false)
+                }
+            })?
+            .commit_outputs();
 
-        let provisional_payjoin = try_contributing_inputs(provisional_payjoin, &bitcoind)
-            .map_err(|e| Error::Server(e.into()))?;
+        let provisional_payjoin = try_contributing_inputs(payjoin.clone(), &bitcoind)
+            .unwrap_or_else(|e| {
+                log::warn!("Failed to contribute inputs: {}", e);
+                payjoin.commit_inputs()
+            });
 
         let payjoin_proposal = provisional_payjoin.finalize_proposal(
             |psbt: &Psbt| {
@@ -372,7 +375,9 @@ fn try_contributing_inputs(
         script_pubkey: selected_utxo.script_pub_key.clone(),
     };
 
-    Ok(payjoin.contribute_witness_inputs(vec![(selected_outpoint, txo_to_contribute)]))
+    Ok(payjoin
+        .contribute_witness_inputs(vec![(selected_outpoint, txo_to_contribute)])
+        .commit_inputs())
 }
 
 async fn unwrap_ohttp_keys_or_else_fetch(config: &AppConfig) -> Result<payjoin::OhttpKeys> {