Reset Orchard wallet state when rescanning from below NU5 activation.

src/rust/include/rust/orchard/wallet.h

@@ -30,6 +30,15 @@ OrchardWalletPtr* orchard_wallet_new();
  */
 void orchard_wallet_free(OrchardWalletPtr* wallet);
 
+/**
+ * Reset the state of the wallet to be suitable for rescan from the NU5 activation
+ * height.  This removes all witness and spentness information from the wallet. The
+ * keystore is unmodified and decrypted note, nullifier, and conflict data are left
+ * in place with the expectation that they will be overwritten and/or updated in
+ * the rescan process.
+ */
+bool orchard_wallet_reset(OrchardWalletPtr* wallet);
+
 /**
  * Adds a checkpoint to the wallet's note commitment tree to enable
  * a future rewind.

src/rust/src/wallet.rs

@@ -487,6 +487,12 @@ pub extern "C" fn orchard_wallet_free(wallet: *mut Wallet) {
     }
 }
 
+#[no_mangle]
+pub extern "C" fn orchard_wallet_reset(wallet: *mut Wallet) {
+    let wallet = unsafe { wallet.as_mut() }.expect("Wallet pointer may not be null");
+    wallet.reset();
+}
+
 #[no_mangle]
 pub extern "C" fn orchard_wallet_checkpoint(wallet: *mut Wallet, block_height: u32) -> bool {
     let wallet = unsafe { wallet.as_mut() }.expect("Wallet pointer may not be null");

src/wallet/orchard.h

@@ -66,19 +66,27 @@ public:
     OrchardWallet& operator=(const OrchardWallet&) = delete;
 
     /**
-     * Checkpoint the note commitment tree. This returns `false` and leaves the
-     * note commitment tree unmodified if the block height does not match the
-     * last block height scanned for transactions. This must be called exactly
-     * once per block.
+     * Reset the state of the wallet to be suitable for rescan from the NU5 activation
+     * height.  This removes all witness and spentness information from the wallet. The
+     * keystore is unmodified and decrypted note, nullifier, and conflict data are left
+     * in place with the expectation that they will be overwritten and/or updated in the
+     * rescan process.
+     */
+    bool Reset() {
+        return orchard_wallet_reset(inner.get());
+    }
+
+    /**
+     * Checkpoint the note commitment tree. This returns `false` and leaves the note
+     * commitment tree unmodified if the block height does not match the last block
+     * height scanned for transactions. This must be called exactly once per block.
      */
     bool CheckpointNoteCommitmentTree(int nBlockHeight) {
         return orchard_wallet_checkpoint(inner.get(), (uint32_t) nBlockHeight);
     }
 
     /**
-     * Rewind to the most recent checkpoint, and mark as unspent any notes
-     * previously identified as having been spent by transactions in the
-     * latest block.
+     * Return whether the orchard note commitment tree contains any checkpoints.
      */
     bool IsCheckpointed() const {
         return orchard_wallet_is_checkpointed(inner.get());

src/wallet/wallet.cpp

@@ -3937,11 +3937,11 @@ int CWallet::ScanForWalletTransactions(
         // checkpoints. Note data will be restored by the calls to AddToWalletIfInvolvingMe,
         // and then the call to `ChainTipAdded` that later occurs for each block will restore
         // the witness data that is being removed in the rewind here.
+        auto nu5_height = chainParams.GetConsensus().GetActivationHeight(Consensus::UPGRADE_NU5);
         bool performOrchardWalletUpdates{false};
         if (orchardWallet.IsCheckpointed()) {
             // We have a checkpoint, so attempt to rewind the Orchard wallet at most as
             // far as the NU5 activation block.
-            auto nu5_height = chainParams.GetConsensus().GetActivationHeight(Consensus::UPGRADE_NU5);
             // If there's no activation height, we shouldn't have a checkpoint already,
             // and this is a programming error.
             assert(nu5_height.has_value());
@@ -3962,6 +3962,11 @@ int CWallet::ScanForWalletTransactions(
                     throw std::runtime_error("CWallet::ScanForWalletTransactions(): Orchard wallet is out of sync. Please restart your node with -rescan.");
                 }
             }
+        } else if (isInitScan && pindex->nHeight < nu5_height) {
+            // If it's the initial scan and we're starting below the nu5 activation
+            // height, we're effectively rescanning from genesis and so it's safe
+            // to update the note commitment tree as we progress.
+            performOrchardWalletUpdates = true;
         }
 
         ShowProgress(_("Rescanning..."), 0); // show rescan progress in GUI as dialog or on splashscreen, if -rescan on startup
@@ -6015,8 +6020,17 @@ bool CWallet::InitLoadWallet(const CChainParams& params, bool clearWitnessCaches
                 return UIError(_("Prune: last wallet synchronisation goes beyond pruned data. You need to -reindex (download the whole blockchain again in case of pruned node)"));
         }
 
+        // If a rescan would begin at a point before NU5 activation height, reset
+        // the Orchard wallet state to empty.
+        if (pindexRescan->nHeight <= Params().GetConsensus().GetActivationHeight(Consensus::UPGRADE_NU5)) {
+            walletInstance->orchardWallet.Reset();
+        }
+
         uiInterface.InitMessage(_("Rescanning..."));
-        LogPrintf("Rescanning last %i blocks (from block %i)...\n", chainActive.Height() - pindexRescan->nHeight, pindexRescan->nHeight);
+        LogPrintf(
+                "Rescanning last %i blocks (from block %i)...\n",
+                chainActive.Height() - pindexRescan->nHeight,
+                pindexRescan->nHeight);
         nStart = GetTimeMillis();
         walletInstance->ScanForWalletTransactions(pindexRescan, true, true);
         LogPrintf(" rescan      %15dms\n", GetTimeMillis() - nStart);