wallet: batch and simplify ZapSelectTx process

The goal of the function is to erase the wallet transactions that
match the inputted hashes. There is no need to traverse the database,
reading record by record, to then perform single entry removals for
each of them.

To ensure consistency and improve performance, this change-set removes
all tx records within a single atomic db batch operation, as well as
it cleans up code, improves error handling and simplifies the
transactions removal process entirely.

This optimizes the removal of watch-only transactions during the wallet
migration process and the 'removeprunedfunds' RPC command.

7 files changed, 44 insertions, 113 deletions

diff --git a/src/wallet/rpc/backup.cpp b/src/wallet/rpc/backup.cpp
index 3e1ec667e0..401c0b2d23 100644
--- a/src/wallet/rpc/backup.cpp
+++ b/src/wallet/rpc/backup.cpp
@@ -394,14 +394,8 @@ RPCHelpMan removeprunedfunds()
     uint256 hash(ParseHashV(request.params[0], "txid"));
     std::vector<uint256> vHash;
     vHash.push_back(hash);
-    std::vector<uint256> vHashOut;
-
-    if (pwallet->ZapSelectTx(vHash, vHashOut) != DBErrors::LOAD_OK) {
-        throw JSONRPCError(RPC_WALLET_ERROR, "Could not properly delete the transaction.");
-    }
-
-    if(vHashOut.empty()) {
-        throw JSONRPCError(RPC_INVALID_PARAMETER, "Transaction does not exist in wallet.");
+    if (auto res = pwallet->ZapSelectTx(vHash); !res) {
+        throw JSONRPCError(RPC_WALLET_ERROR, util::ErrorString(res).original);
     }
 
     return UniValue::VNULL;
diff --git a/src/wallet/test/wallet_tests.cpp b/src/wallet/test/wallet_tests.cpp
index dd43705a84..cbab4f2ee9 100644
--- a/src/wallet/test/wallet_tests.cpp
+++ b/src/wallet/test/wallet_tests.cpp
@@ -918,8 +918,8 @@ BOOST_FIXTURE_TEST_CASE(ZapSelectTx, TestChain100Setup)
         BOOST_CHECK(wallet->HasWalletSpend(prev_tx));
         BOOST_CHECK_EQUAL(wallet->mapWallet.count(block_hash), 1u);
 
-        std::vector<uint256> vHashIn{ block_hash }, vHashOut;
-        BOOST_CHECK_EQUAL(wallet->ZapSelectTx(vHashIn, vHashOut), DBErrors::LOAD_OK);
+        std::vector<uint256> vHashIn{ block_hash };
+        BOOST_CHECK(wallet->ZapSelectTx(vHashIn));
 
         BOOST_CHECK(!wallet->HasWalletSpend(prev_tx));
         BOOST_CHECK_EQUAL(wallet->mapWallet.count(block_hash), 0u);
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 1c879fb976..3f2b26ef99 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2311,12 +2311,41 @@ DBErrors CWallet::LoadWallet()
     return nLoadWalletRet;
 }
 
-DBErrors CWallet::ZapSelectTx(std::vector<uint256>& vHashIn, std::vector<uint256>& vHashOut)
+util::Result<void> CWallet::ZapSelectTx(std::vector<uint256>& txs_to_remove)
 {
     AssertLockHeld(cs_wallet);
-    DBErrors nZapSelectTxRet = WalletBatch(GetDatabase()).ZapSelectTx(vHashIn, vHashOut);
-    for (const uint256& hash : vHashOut) {
-        const auto& it = mapWallet.find(hash);
+    WalletBatch batch(GetDatabase());
+    if (!batch.TxnBegin()) return util::Error{_("Error starting db txn for wallet transactions removal")};
+
+    // Check for transaction existence and remove entries from disk
+    using TxIterator = std::unordered_map<uint256, CWalletTx, SaltedTxidHasher>::const_iterator;
+    std::vector<TxIterator> erased_txs;
+    bilingual_str str_err;
+    for (const uint256& hash : txs_to_remove) {
+        auto it_wtx = mapWallet.find(hash);
+        if (it_wtx == mapWallet.end()) {
+            str_err = strprintf(_("Transaction %s does not belong to this wallet"), hash.GetHex());
+            break;
+        }
+        if (!batch.EraseTx(hash)) {
+            str_err = strprintf(_("Failure removing transaction: %s"), hash.GetHex());
+            break;
+        }
+        erased_txs.emplace_back(it_wtx);
+    }
+
+    // Roll back removals in case of an error
+    if (!str_err.empty()) {
+        batch.TxnAbort();
+        return util::Error{str_err};
+    }
+
+    // Dump changes to disk
+    if (!batch.TxnCommit()) return util::Error{_("Error committing db txn for wallet transactions removal")};
+
+    // Update the in-memory state and notify upper layers about the removals
+    for (const auto& it : erased_txs) {
+        const uint256 hash{it->first};
         wtxOrdered.erase(it->second.m_it_wtxOrdered);
         for (const auto& txin : it->second.tx->vin)
             mapTxSpends.erase(txin.prevout);
@@ -2324,12 +2353,9 @@ DBErrors CWallet::ZapSelectTx(std::vector<uint256>& vHashIn, std::vector<uint256
         NotifyTransactionChanged(hash, CT_DELETED);
     }
 
-    if (nZapSelectTxRet != DBErrors::LOAD_OK)
-        return nZapSelectTxRet;
-
     MarkDirty();
 
-    return DBErrors::LOAD_OK;
+    return {}; // all good
 }
 
 bool CWallet::SetAddressBookWithDB(WalletBatch& batch, const CTxDestination& address, const std::string& strName, const std::optional<AddressPurpose>& new_purpose)
@@ -3925,13 +3951,8 @@ bool CWallet::ApplyMigrationData(MigrationData& data, bilingual_str& error)
     watchonly_batch.reset(); // Flush
     // Do the removes
     if (txids_to_delete.size() > 0) {
-        std::vector<uint256> deleted_txids;
-        if (ZapSelectTx(txids_to_delete, deleted_txids) != DBErrors::LOAD_OK) {
-            error = _("Error: Could not delete watchonly transactions");
-            return false;
-        }
-        if (deleted_txids != txids_to_delete) {
-            error = _("Error: Not all watchonly txs could be deleted");
+        if (auto res = ZapSelectTx(txids_to_delete); !res) {
+            error = _("Error: Could not delete watchonly transactions. ") + util::ErrorString(res);
             return false;
         }
     }
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index bc45010200..5eddaac6d8 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -787,7 +787,9 @@ public:
     void chainStateFlushed(ChainstateRole role, const CBlockLocator& loc) override;
 
     DBErrors LoadWallet();
-    DBErrors ZapSelectTx(std::vector<uint256>& vHashIn, std::vector<uint256>& vHashOut) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
+
+    /** Erases the provided transactions from the wallet. */
+    util::Result<void> ZapSelectTx(std::vector<uint256>& txs_to_remove) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
 
     bool SetAddressBook(const CTxDestination& address, const std::string& strName, const std::optional<AddressPurpose>& purpose);
 
diff --git a/src/wallet/walletdb.cpp b/src/wallet/walletdb.cpp
index 96999de97b..6f99da7aea 100644
--- a/src/wallet/walletdb.cpp
+++ b/src/wallet/walletdb.cpp
@@ -1230,90 +1230,6 @@ DBErrors WalletBatch::LoadWallet(CWallet* pwallet)
     return result;
 }
 
-DBErrors WalletBatch::FindWalletTxHashes(std::vector<uint256>& tx_hashes)
-{
-    DBErrors result = DBErrors::LOAD_OK;
-
-    try {
-        int nMinVersion = 0;
-        if (m_batch->Read(DBKeys::MINVERSION, nMinVersion)) {
-            if (nMinVersion > FEATURE_LATEST)
-                return DBErrors::TOO_NEW;
-        }
-
-        // Get cursor
-        std::unique_ptr<DatabaseCursor> cursor = m_batch->GetNewCursor();
-        if (!cursor)
-        {
-            LogPrintf("Error getting wallet database cursor\n");
-            return DBErrors::CORRUPT;
-        }
-
-        while (true)
-        {
-            // Read next record
-            DataStream ssKey{};
-            DataStream ssValue{};
-            DatabaseCursor::Status status = cursor->Next(ssKey, ssValue);
-            if (status == DatabaseCursor::Status::DONE) {
-                break;
-            } else if (status == DatabaseCursor::Status::FAIL) {
-                LogPrintf("Error reading next record from wallet database\n");
-                return DBErrors::CORRUPT;
-            }
-
-            std::string strType;
-            ssKey >> strType;
-            if (strType == DBKeys::TX) {
-                uint256 hash;
-                ssKey >> hash;
-                tx_hashes.push_back(hash);
-            }
-        }
-    } catch (...) {
-        result = DBErrors::CORRUPT;
-    }
-
-    return result;
-}
-
-DBErrors WalletBatch::ZapSelectTx(std::vector<uint256>& vTxHashIn, std::vector<uint256>& vTxHashOut)
-{
-    // build list of wallet TX hashes
-    std::vector<uint256> vTxHash;
-    DBErrors err = FindWalletTxHashes(vTxHash);
-    if (err != DBErrors::LOAD_OK) {
-        return err;
-    }
-
-    std::sort(vTxHash.begin(), vTxHash.end());
-    std::sort(vTxHashIn.begin(), vTxHashIn.end());
-
-    // erase each matching wallet TX
-    bool delerror = false;
-    std::vector<uint256>::iterator it = vTxHashIn.begin();
-    for (const uint256& hash : vTxHash) {
-        while (it < vTxHashIn.end() && (*it) < hash) {
-            it++;
-        }
-        if (it == vTxHashIn.end()) {
-            break;
-        }
-        else if ((*it) == hash) {
-            if(!EraseTx(hash)) {
-                LogPrint(BCLog::WALLETDB, "Transaction was found for deletion but returned database error: %s\n", hash.GetHex());
-                delerror = true;
-            }
-            vTxHashOut.push_back(hash);
-        }
-    }
-
-    if (delerror) {
-        return DBErrors::CORRUPT;
-    }
-    return DBErrors::LOAD_OK;
-}
-
 void MaybeCompactWalletDB(WalletContext& context)
 {
     static std::atomic<bool> fOneThread(false);
diff --git a/src/wallet/walletdb.h b/src/wallet/walletdb.h
index dad0b18a78..62449eb64e 100644
--- a/src/wallet/walletdb.h
+++ b/src/wallet/walletdb.h
@@ -275,8 +275,6 @@ public:
     bool EraseActiveScriptPubKeyMan(uint8_t type, bool internal);
 
     DBErrors LoadWallet(CWallet* pwallet);
-    DBErrors FindWalletTxHashes(std::vector<uint256>& tx_hashes);
-    DBErrors ZapSelectTx(std::vector<uint256>& vHashIn, std::vector<uint256>& vHashOut);
 
     //! write the hdchain model (external chain child index counter)
     bool WriteHDChain(const CHDChain& chain);
diff --git a/test/functional/wallet_importprunedfunds.py b/test/functional/wallet_importprunedfunds.py
index 5fe7c4b591..b3ae22cc44 100755
--- a/test/functional/wallet_importprunedfunds.py
+++ b/test/functional/wallet_importprunedfunds.py
@@ -120,7 +120,7 @@ class ImportPrunedFundsTest(BitcoinTestFramework):
         assert_equal(address_info['ismine'], True)
 
         # Remove transactions
-        assert_raises_rpc_error(-8, "Transaction does not exist in wallet.", w1.removeprunedfunds, txnid1)
+        assert_raises_rpc_error(-4, f'Transaction {txnid1} does not belong to this wallet', w1.removeprunedfunds, txnid1)
         assert not [tx for tx in w1.listtransactions(include_watchonly=True) if tx['txid'] == txnid1]
 
         wwatch.removeprunedfunds(txnid2)