Add a dummy spend to the bundle if the bundle is required to be present.

CHANGELOG.md

@@ -26,14 +26,15 @@ The entries below are relative to the `zcash_primitives::sapling` module as of
   - `OutputInfo`
   - `ProverProgress`
   - `BundleType`
+  - `SigningMetadata`
   - `bundle` bundle builder function.
 - `sapling_crypto::bundle` module:
   - The following types moved from
     `zcash_primitives::transaction::components::sapling`:
     - `Bundle`
     - `SpendDescription, SpendDescriptionV5`
     - `OutputDescription, OutputDescriptionV5`
-    - `Authorization, Authorized, MapAuth`
+    - `Authorization, Authorized`
     - `GrothProofBytes`
   - `Bundle::<InProgress<Unproven, _>>::create_proofs`
   - `Bundle::<InProgress<_, Unsigned>>::prepare`
@@ -42,9 +43,6 @@ The entries below are relative to the `zcash_primitives::sapling` module as of
   - `Bundle::<InProgress<Proven, Unsigned>>::apply_signatures`
   - `Bundle::try_map_authorization`
   - `TryMapAuth`
-  - `impl {MapAuth, TryMapAuth} for (FnMut, FnMut, FnMut, FnMut)`
-    helpers to enable calling `Bundle::{map_authorization, try_map_authorization}`
-    with a set of closures.
   - `testing` module, containing the following functions moved from
     `zcash_primitives::transaction::components::sapling::testing`:
     - `arb_output_description`
@@ -93,6 +91,7 @@ The entries below are relative to the `zcash_primitives::sapling` module as of
   argument as a `NoteValue` instead of as a bare `u64`.
 - `sapling_crypto::builder`:
   - `SaplingBuilder` has been renamed to `Builder`
+  - `MaybeSigned::SigningMetadata` has been renamed to `MaybeSigned::SigningParts`
   - `Builder` no longer has a `P: zcash_primitives::consensus::Parameters`
     type parameter.
   - `Builder::new` now takes a `Zip212Enforcement` argument instead of a
@@ -118,6 +117,9 @@ The entries below are relative to the `zcash_primitives::sapling` module as of
   - `Bundle` now has a second generic parameter `V`.
   - `Bundle::value_balance` now returns `&V` instead of
     `&zcash_primitives::transaction::components::Amount`.
+  - `Bundle::map_authorization` now takes a context argument and explicit 
+    functions for each mappable field, rather than a `MapAuth` value, in 
+    order to simplify handling of context values.
   - `Authorized::binding_sig` now has type `redjubjub::Signature<Binding>`.
   - `Authorized::AuthSig` now has type `redjubjub::Signature<SpendAuth>`.
   - `SpendDescription::temporary_zcashd_from_parts` now takes `rk` as
@@ -128,7 +130,6 @@ The entries below are relative to the `zcash_primitives::sapling` module as of
     `redjubjub::Signature<SpendAuth>` instead of
     `sapling_crypto::redjubjub::Signature`.
   - `testing::arb_bundle` now takes a `value_balance: V` argument.
-  - `MapAuth` trait methods now take `&mut self` instead of `&self`.
 - `sapling_crypto::circuit::ValueCommitmentOpening::value` is now represented as
   a `NoteValue` instead of as a bare `u64`.
 - `sapling_crypto::keys`:
@@ -172,6 +173,7 @@ The entries below are relative to the `zcash_primitives::sapling` module as of
   - `OutputDescription::read`
   - `OutputDescription::{write_v4, write_v5_without_proof}`
   - `OutputDescriptionV5::read`
+  - `MapAuth` trait
 - `sapling_crypto::builder`:
   - `SpendDescriptionInfo`
 - `sapling_crypto::note_encryption::SaplingDomain::for_height` (use

src/builder.rs

@@ -53,27 +53,51 @@ impl BundleType {
         bundle_required: false,
     };
 
+    /// Returns the number of logical spends that a builder will produce in constructing a bundle
+    /// of this type, given the specified numbers of spends and outputs.
+    ///
+    /// Returns an error if the specified number of spends and outputs is incompatible with
+    /// this bundle type.
+    pub fn num_spends(&self, requested_spends: usize) -> Result<usize, &'static str> {
+        match self {
+            BundleType::Transactional { bundle_required } => {
+                Ok(if *bundle_required || requested_spends > 0 {
+                    core::cmp::max(requested_spends, 1)
+                } else {
+                    0
+                })
+            }
+            BundleType::Coinbase => {
+                if requested_spends == 0 {
+                    Ok(0)
+                } else {
+                    Err("Spends not allowed in coinbase bundles")
+                }
+            }
+        }
+    }
+
     /// Returns the number of logical outputs that a builder will produce in constructing a bundle
     /// of this type, given the specified numbers of spends and outputs.
     ///
     /// Returns an error if the specified number of spends and outputs is incompatible with
     /// this bundle type.
     pub fn num_outputs(
         &self,
-        num_spends: usize,
-        num_outputs: usize,
+        requested_spends: usize,
+        requested_outputs: usize,
     ) -> Result<usize, &'static str> {
         match self {
             BundleType::Transactional { bundle_required } => {
-                Ok(if *bundle_required || num_outputs > 0 {
-                    core::cmp::max(num_outputs, MIN_SHIELDED_OUTPUTS)
+                Ok(if *bundle_required || requested_outputs > 0 {
+                    core::cmp::max(requested_outputs, MIN_SHIELDED_OUTPUTS)
                 } else {
                     0
                 })
             }
             BundleType::Coinbase => {
-                if num_spends == 0 {
-                    Ok(num_outputs)
+                if requested_spends == 0 {
+                    Ok(requested_outputs)
                 } else {
                     Err("Spends not allowed in coinbase bundles")
                 }
@@ -550,6 +574,11 @@ pub fn bundle<SP: SpendProver, OP: OutputProver, R: RngCore, V: TryFrom<i64>>(
         }
     }
 
+    let requested_spend_count = spends.len();
+    let bundle_spend_count = bundle_type
+        .num_spends(requested_spend_count)
+        .map_err(|_| Error::BundleTypeNotSatisfiable)?;
+
     let requested_output_count = outputs.len();
     let bundle_output_count = bundle_type
         .num_outputs(spends.len(), requested_output_count)
@@ -562,8 +591,14 @@ pub fn bundle<SP: SpendProver, OP: OutputProver, R: RngCore, V: TryFrom<i64>>(
     tx_metadata.spend_indices.resize(spends.len(), 0);
     tx_metadata.output_indices.resize(requested_output_count, 0);
 
-    // Record initial spend positions
-    let mut indexed_spends: Vec<_> = spends.into_iter().enumerate().collect();
+    // Create any required dummy spends and record initial spend positions
+    let mut indexed_spends: Vec<_> = spends
+        .into_iter()
+        .chain(iter::repeat_with(|| SpendInfo::dummy(&mut rng)))
+        .enumerate()
+        .take(bundle_spend_count)
+        .collect();
+
     // Create any required dummy outputs and record initial output positions
     let mut indexed_outputs: Vec<_> = outputs
         .into_iter()
@@ -582,7 +617,9 @@ pub fn bundle<SP: SpendProver, OP: OutputProver, R: RngCore, V: TryFrom<i64>>(
         .enumerate()
         .map(|(i, (pos, spend))| {
             // Record the post-randomized spend location
-            tx_metadata.spend_indices[pos] = i;
+            if pos < requested_spend_count {
+                tx_metadata.spend_indices[pos] = i;
+            }
 
             spend.prepare(&mut rng)
         })