wallet: Persist Orchard note positions with the witness tree

We rewrite the entire witness tree each time we update the wallet's best
chain state, and we need the note positions to always be consistent with
the tree state (otherwise mined notes become unspendable after the node
restarts).

src/rust/src/wallet.rs

@@ -8,9 +8,10 @@ use std::ptr;
 use std::slice;
 use tracing::error;
 
-use zcash_encoding::Optional;
+use zcash_encoding::{Optional, Vector};
 use zcash_primitives::{
     consensus::BlockHeight,
+    merkle_tree::incremental::{read_position, write_position},
     transaction::{components::Amount, TxId},
 };
 
@@ -1189,7 +1190,26 @@ pub extern "C" fn orchard_wallet_write_note_commitment_tree(
         Optional::write(&mut writer, wallet.last_checkpoint, |w, h| {
             w.write_u32::<LittleEndian>(h.into())
         })?;
-        write_tree(&mut writer, &wallet.witness_tree)
+        write_tree(&mut writer, &wallet.witness_tree)?;
+
+        // Write note positions.
+        Vector::write_sized(
+            &mut writer,
+            wallet.wallet_note_positions.iter(),
+            |mut w, (txid, tx_notes)| {
+                txid.write(&mut w)?;
+                Vector::write_sized(
+                    w,
+                    tx_notes.note_positions.iter(),
+                    |w, (action_idx, position)| {
+                        w.write_u32::<LittleEndian>(*action_idx as u32)?;
+                        write_position(w, *position)
+                    },
+                )
+            },
+        )?;
+
+        Ok(())
     };
 
     match write_all() {
@@ -1216,6 +1236,22 @@ pub extern "C" fn orchard_wallet_load_note_commitment_tree(
         })?;
         let witness_tree = read_tree(&mut reader)?;
 
+        // Read note positions.
+        wallet.wallet_note_positions = Vector::read_collected(&mut reader, |mut r| {
+            Ok((
+                TxId::read(&mut r)?,
+                NotePositions {
+                    tx_height: None,
+                    note_positions: Vector::read_collected(r, |r| {
+                        Ok((
+                            r.read_u32::<LittleEndian>().map(|idx| idx as usize)?,
+                            read_position(r)?,
+                        ))
+                    })?,
+                },
+            ))
+        })?;
+
         wallet.last_checkpoint = last_checkpoint;
         wallet.witness_tree = witness_tree;
         Ok(())