refactor: Drop util::Result operator=

`util::Result` objects are aggregates that can hold multiple fields with
different information. Currently Result objects can only hold a success value
of an arbitrary type or a single bilingual_str error message. In followup PR
https://github.com/bitcoin/bitcoin/pull/25722, Result objects may be able to
hold both success and failure values of different types, plus error and warning
messages.

Having a Result::operator= assignment operator that completely erases all
existing Result information before assigning new information is potentially
dangerous in this case. For example, code that looks like it is assigning a
warning value could erase previously-assigned success or failure values.
Conversely, code that looks like it is just assigning a success or failure
value could erase previously assigned error and warning messages.

To prevent potential bugs like this, disable Result::operator= assignment
operator.

It is possible in the future we may want to re-enable operator= in limited
cases (such as when implicit conversions are not used) or add a Replace() or
Reset() method that mimicks default operator= behavior. Followup PR
https://github.com/bitcoin/bitcoin/pull/25722 also adds a Result::Update()
method providing another way to update an existing Result object.

Co-authored-by: stickies-v <stickies-v@protonmail.com>

src/init.cpp

@@ -992,10 +992,8 @@ bool AppInitParameterInteraction(const ArgsManager& args)
         InitWarning(strprintf(_("Reducing -maxconnections from %d to %d, because of system limitations."), nUserMaxConnections, nMaxConnections));
 
     // ********************************************************* Step 3: parameter-to-internal-flags
-    auto result = init::SetLoggingCategories(args);
-    if (!result) return InitError(util::ErrorString(result));
-    result = init::SetLoggingLevel(args);
-    if (!result) return InitError(util::ErrorString(result));
+    if (auto result{init::SetLoggingCategories(args)}; !result) return InitError(util::ErrorString(result));
+    if (auto result{init::SetLoggingLevel(args)}; !result) return InitError(util::ErrorString(result));
 
     nConnectTimeout = args.GetIntArg("-timeout", DEFAULT_CONNECT_TIMEOUT);
     if (nConnectTimeout <= 0) {

src/qt/addresstablemodel.cpp

@@ -369,21 +369,22 @@ QString AddressTableModel::addRow(const QString &type, const QString &label, con
     else if(type == Receive)
     {
         // Generate a new address to associate with given label
-        auto op_dest = walletModel->wallet().getNewDestination(address_type, strLabel);
-        if (!op_dest) {
+        if (auto dest{walletModel->wallet().getNewDestination(address_type, strLabel)}) {
+            strAddress = EncodeDestination(*dest);
+        } else {
             WalletModel::UnlockContext ctx(walletModel->requestUnlock());
             if (!ctx.isValid()) {
                 // Unlock wallet failed or was cancelled
                 editStatus = WALLET_UNLOCK_FAILURE;
                 return QString();
             }
-            op_dest = walletModel->wallet().getNewDestination(address_type, strLabel);
-            if (!op_dest) {
+            if (auto dest_retry{walletModel->wallet().getNewDestination(address_type, strLabel)}) {
+                strAddress = EncodeDestination(*dest_retry);
+            } else {
                 editStatus = KEY_GENERATION_FAILURE;
                 return QString();
             }
         }
-        strAddress = EncodeDestination(*op_dest);
     }
     else
     {

src/test/mempool_tests.cpp

@@ -202,9 +202,11 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
     tx7.vout[1].scriptPubKey = CScript() << OP_11 << OP_EQUAL;
     tx7.vout[1].nValue = 1 * COIN;
 
-    auto ancestors_calculated{pool.CalculateMemPoolAncestors(entry.Fee(2000000LL).FromTx(tx7), CTxMemPool::Limits::NoLimits())};
-    BOOST_REQUIRE(ancestors_calculated.has_value());
-    BOOST_CHECK(*ancestors_calculated == setAncestors);
+    {
+        auto ancestors_calculated{pool.CalculateMemPoolAncestors(entry.Fee(2000000LL).FromTx(tx7), CTxMemPool::Limits::NoLimits())};
+        BOOST_REQUIRE(ancestors_calculated.has_value());
+        BOOST_CHECK(*ancestors_calculated == setAncestors);
+    }
 
     pool.addUnchecked(entry.FromTx(tx7), setAncestors);
     BOOST_CHECK_EQUAL(pool.size(), 7U);
@@ -260,9 +262,11 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
     tx10.vout[0].scriptPubKey = CScript() << OP_11 << OP_EQUAL;
     tx10.vout[0].nValue = 10 * COIN;
 
-    ancestors_calculated = pool.CalculateMemPoolAncestors(entry.Fee(200000LL).Time(NodeSeconds{4s}).FromTx(tx10), CTxMemPool::Limits::NoLimits());
-    BOOST_REQUIRE(ancestors_calculated);
-    BOOST_CHECK(*ancestors_calculated == setAncestors);
+    {
+        auto ancestors_calculated{pool.CalculateMemPoolAncestors(entry.Fee(200000LL).Time(NodeSeconds{4s}).FromTx(tx10), CTxMemPool::Limits::NoLimits())};
+        BOOST_REQUIRE(ancestors_calculated);
+        BOOST_CHECK(*ancestors_calculated == setAncestors);
+    }
 
     pool.addUnchecked(entry.FromTx(tx10), setAncestors);
 

src/util/result.h

@@ -39,6 +39,13 @@ private:
 
     std::variant<bilingual_str, T> m_variant;
 
+    //! Disallow operator= to avoid confusion in the future when the Result
+    //! class gains support for richer error reporting, and callers should have
+    //! ability to set a new result value without clearing existing error
+    //! messages.
+    Result& operator=(const Result&) = delete;
+    Result& operator=(Result&&) = delete;
+
     template <typename FT>
     friend bilingual_str ErrorString(const Result<FT>& result);
 
@@ -46,6 +53,9 @@ public:
     Result() : m_variant{std::in_place_index_t<1>{}, std::monostate{}} {}  // constructor for void
     Result(T obj) : m_variant{std::in_place_index_t<1>{}, std::move(obj)} {}
     Result(Error error) : m_variant{std::in_place_index_t<0>{}, std::move(error.message)} {}
+    Result(const Result&) = default;
+    Result(Result&&) = default;
+    ~Result() = default;
 
     //! std::optional methods, so functions returning optional<T> can change to
     //! return Result<T> with minimal changes to existing code, and vice versa.

src/validation.cpp

@@ -946,8 +946,9 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         maybe_rbf_limits.descendant_size_vbytes += conflict->GetSizeWithDescendants();
     }
 
-    auto ancestors{m_pool.CalculateMemPoolAncestors(*entry, maybe_rbf_limits)};
-    if (!ancestors) {
+    if (auto ancestors{m_pool.CalculateMemPoolAncestors(*entry, maybe_rbf_limits)}) {
+        ws.m_ancestors = std::move(*ancestors);
+    } else {
         // If CalculateMemPoolAncestors fails second time, we want the original error string.
         // Contracting/payment channels CPFP carve-out:
         // If the new transaction is relatively small (up to 40k weight)
@@ -970,11 +971,13 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         if (ws.m_vsize > EXTRA_DESCENDANT_TX_SIZE_LIMIT) {
             return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "too-long-mempool-chain", error_message);
         }
-        ancestors = m_pool.CalculateMemPoolAncestors(*entry, cpfp_carve_out_limits);
-        if (!ancestors) return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "too-long-mempool-chain", error_message);
+        if (auto ancestors_retry{m_pool.CalculateMemPoolAncestors(*entry, cpfp_carve_out_limits)}) {
+            ws.m_ancestors = std::move(*ancestors_retry);
+        } else {
+            return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "too-long-mempool-chain", error_message);
+        }
     }
 
-    ws.m_ancestors = *ancestors;
     // Even though just checking direct mempool parents for inheritance would be sufficient, we
     // check using the full ancestor set here because it's more convenient to use what we have
     // already calculated.

src/wallet/test/fuzz/notifications.cpp

@@ -106,13 +106,11 @@ struct FuzzedWallet {
     CTxDestination GetDestination(FuzzedDataProvider& fuzzed_data_provider)
     {
         auto type{fuzzed_data_provider.PickValueInArray(OUTPUT_TYPES)};
-        util::Result<CTxDestination> op_dest{util::Error{}};
         if (fuzzed_data_provider.ConsumeBool()) {
-            op_dest = wallet->GetNewDestination(type, "");
+            return *Assert(wallet->GetNewDestination(type, ""));
         } else {
-            op_dest = wallet->GetNewChangeDestination(type);
+            return *Assert(wallet->GetNewChangeDestination(type));
         }
-        return *Assert(op_dest);
     }
     CScript GetScriptPubKey(FuzzedDataProvider& fuzzed_data_provider) { return GetScriptForDestination(GetDestination(fuzzed_data_provider)); }
     void FundTx(FuzzedDataProvider& fuzzed_data_provider, CMutableTransaction tx)

src/wallet/test/spend_tests.cpp

@@ -97,13 +97,11 @@ BOOST_FIXTURE_TEST_CASE(wallet_duplicated_preset_inputs_test, TestChain100Setup)
     // so that the recipient's amount is no longer equal to the user's selected target of 299 BTC.
 
     // First case, use 'subtract_fee_from_outputs=true'
-    util::Result<CreatedTransactionResult> res_tx = CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control);
-    BOOST_CHECK(!res_tx.has_value());
+    BOOST_CHECK(!CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control));
 
     // Second case, don't use 'subtract_fee_from_outputs'.
     recipients[0].fSubtractFeeFromAmount = false;
-    res_tx = CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control);
-    BOOST_CHECK(!res_tx.has_value());
+    BOOST_CHECK(!CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control));
 }
 
 BOOST_AUTO_TEST_SUITE_END()