zcash_client_backend: Precompute IVKs in `scan_cached_blocks`

zcash_client_backend/CHANGELOG.md

@@ -26,6 +26,7 @@ and this library adheres to Rust's notion of
 - `zcash_client_backend::scanning`:
   - `ScanError`
   - `impl<K: ScanningKey> ScanningKey for &K`
+  - `impl ScanningKey for (zip32::Scope, sapling::SaplingIvk, sapling::NullifierDerivingKey)`
 
 ### Changed
 - MSRV is now 1.65.0.

zcash_client_backend/src/data_api/chain.rs

@@ -155,7 +155,7 @@ use crate::{
     data_api::{BlockMetadata, NullifierQuery, WalletWrite},
     proto::compact_formats::CompactBlock,
     scan::BatchRunner,
-    scanning::{add_block_to_runner, check_continuity, scan_block_with_runner},
+    scanning::{add_block_to_runner, check_continuity, scan_block_with_runner, ScanningKey},
 };
 
 pub mod error;
@@ -254,6 +254,15 @@ where
         .iter()
         .filter_map(|(account, ufvk)| ufvk.sapling().map(move |k| (account, k)))
         .collect();
+    // Precompute the IVKs instead of doing so per block.
+    let ivks = dfvks
+        .iter()
+        .flat_map(|(account, dfvk)| {
+            dfvk.to_sapling_keys()
+                .into_iter()
+                .map(|key| (*account, key))
+        })
+        .collect::<Vec<_>>();
 
     // Get the nullifiers for the unspent notes we are tracking
     let mut sapling_nullifiers = data_db
@@ -337,7 +346,7 @@ where
             let scanned_block = scan_block_with_runner(
                 params,
                 block,
-                &dfvks,
+                &ivks,
                 &sapling_nullifiers,
                 prior_block_metadata.as_ref(),
                 Some(&mut batch_runner),

zcash_client_backend/src/scanning.rs

@@ -107,6 +107,21 @@ impl ScanningKey for DiversifiableFullViewingKey {
     }
 }
 
+impl ScanningKey for (Scope, SaplingIvk, sapling::NullifierDerivingKey) {
+    type Scope = Scope;
+    type SaplingNk = sapling::NullifierDerivingKey;
+    type SaplingKeys = [(Self::Scope, SaplingIvk, Self::SaplingNk); 1];
+    type Nf = sapling::Nullifier;
+
+    fn to_sapling_keys(&self) -> Self::SaplingKeys {
+        [self.clone()]
+    }
+
+    fn sapling_nf(key: &Self::SaplingNk, note: &sapling::Note, position: Position) -> Self::Nf {
+        note.nf(key, position.into())
+    }
+}
+
 /// The [`ScanningKey`] implementation for [`SaplingIvk`]s.
 /// Nullifiers cannot be derived when scanning with these keys.
 ///