pczt: Add the Spend Finalizer role

pczt/Cargo.toml

@@ -56,4 +56,5 @@ zcp-builder = ["dep:zcash_primitives", "dep:zcash_protocol"]
 io-finalizer = ["orchard", "sapling"]
 prover = ["dep:rand_core", "sapling?/temporary-zcashd"]
 signer = ["dep:blake2b_simd", "dep:rand_core", "orchard", "sapling", "transparent"]
+spend-finalizer = ["transparent"]
 tx-extractor = ["dep:rand_core", "orchard", "sapling", "transparent"]

pczt/src/lib.rs

@@ -33,6 +33,8 @@
 //!     - A source of randomness.
 //! - Combiner (anyone can execute)
 //!   - Combines several PCZTs that represent the same transaction into a single PCZT.
+//! - Spend Finalizer (anyone can execute)
+//!   - Combines partial transparent signatures into `script_sig`s.
 //! - Transaction Extractor (anyone can execute)
 //!   - Creates bindingSig and extracts the final transaction.
 

pczt/src/roles.rs

@@ -11,6 +11,9 @@ pub mod signer;
 
 pub mod combiner;
 
+#[cfg(feature = "spend-finalizer")]
+pub mod spend_finalizer;
+
 #[cfg(feature = "tx-extractor")]
 pub mod tx_extractor;
 

pczt/src/roles/spend_finalizer/mod.rs

@@ -0,0 +1,44 @@
+use zcash_primitives::transaction::components::transparent;
+
+use crate::Pczt;
+
+pub struct SpendFinalizer {
+    pczt: Pczt,
+}
+
+impl SpendFinalizer {
+    /// Instantiates the Spend Finalizer role with the given PCZT.
+    pub fn new(pczt: Pczt) -> Self {
+        Self { pczt }
+    }
+
+    /// Finalizes the spends of the PCZT.
+    pub fn finalize_spends(self) -> Result<Pczt, Error> {
+        let Pczt {
+            global,
+            transparent,
+            sapling,
+            orchard,
+        } = self.pczt;
+
+        let mut transparent = transparent.into_parsed().map_err(Error::TransparentParse)?;
+
+        transparent
+            .finalize_spends()
+            .map_err(Error::TransparentFinalize)?;
+
+        Ok(Pczt {
+            global,
+            transparent: crate::transparent::Bundle::serialize_from(transparent),
+            sapling,
+            orchard,
+        })
+    }
+}
+
+/// Errors that can occur while finalizing the spends of a PCZT.
+#[derive(Debug)]
+pub enum Error {
+    TransparentFinalize(transparent::pczt::SpendFinalizerError),
+    TransparentParse(transparent::pczt::ParseError),
+}