refactor: Make CWalletTx sync state type-safe

Current CWalletTx state representation makes it possible to set
inconsistent states that won't be handled correctly by wallet sync code
or serialized & deserialized back into the same form.

For example, it is possible to call setConflicted without setting a
conflicting block hash, or setConfirmed with no transaction index. And
it's possible update individual m_confirm and fInMempool data fields
without setting an overall consistent state that can be serialized and
handled correctly.

Fix this without changing behavior by using std::variant, instead of an
enum and collection of fields, to represent sync state, so state
tracking code is safer and more legible.

This is a first step to fixing state tracking bugs
https://github.com/bitcoin-core/bitcoin-devwiki/wiki/Wallet-Transaction-Conflict-Tracking,
by adding an extra margin of safety that can prevent new bugs from being
introduced as existing bugs are fixed.

4 files changed, 36 insertions, 16 deletions

diff --git a/src/wallet/test/coinselector_tests.cpp b/src/wallet/test/coinselector_tests.cpp
index a8a7e12a32..8f985f31ee 100644
--- a/src/wallet/test/coinselector_tests.cpp
+++ b/src/wallet/test/coinselector_tests.cpp
@@ -74,7 +74,7 @@ static void add_coin(std::vector<COutput>& coins, CWallet& wallet, const CAmount
     uint256 txid = tx.GetHash();
 
     LOCK(wallet.cs_wallet);
-    auto ret = wallet.mapWallet.emplace(std::piecewise_construct, std::forward_as_tuple(txid), std::forward_as_tuple(MakeTransactionRef(std::move(tx))));
+    auto ret = wallet.mapWallet.emplace(std::piecewise_construct, std::forward_as_tuple(txid), std::forward_as_tuple(MakeTransactionRef(std::move(tx)), TxStateInactive{}));
     assert(ret.second);
     CWalletTx& wtx = (*ret.first).second;
     if (fIsFromMe)
diff --git a/src/wallet/test/psbt_wallet_tests.cpp b/src/wallet/test/psbt_wallet_tests.cpp
index dd24fa2c19..7bc2bb5583 100644
--- a/src/wallet/test/psbt_wallet_tests.cpp
+++ b/src/wallet/test/psbt_wallet_tests.cpp
@@ -34,12 +34,12 @@ BOOST_AUTO_TEST_CASE(psbt_updater_test)
     CDataStream s_prev_tx1(ParseHex("0200000000010158e87a21b56daf0c23be8e7070456c336f7cbaa5c8757924f545887bb2abdd7501000000171600145f275f436b09a8cc9a2eb2a2f528485c68a56323feffffff02d8231f1b0100000017a914aed962d6654f9a2b36608eb9d64d2b260db4f1118700c2eb0b0000000017a914b7f5faf40e3d40a5a459b1db3535f2b72fa921e88702483045022100a22edcc6e5bc511af4cc4ae0de0fcd75c7e04d8c1c3a8aa9d820ed4b967384ec02200642963597b9b1bc22c75e9f3e117284a962188bf5e8a74c895089046a20ad770121035509a48eb623e10aace8bfd0212fdb8a8e5af3c94b0b133b95e114cab89e4f7965000000"), SER_NETWORK, PROTOCOL_VERSION);
     CTransactionRef prev_tx1;
     s_prev_tx1 >> prev_tx1;
-    m_wallet.mapWallet.emplace(std::piecewise_construct, std::forward_as_tuple(prev_tx1->GetHash()), std::forward_as_tuple(prev_tx1));
+    m_wallet.mapWallet.emplace(std::piecewise_construct, std::forward_as_tuple(prev_tx1->GetHash()), std::forward_as_tuple(prev_tx1, TxStateInactive{}));
 
     CDataStream s_prev_tx2(ParseHex("0200000001aad73931018bd25f84ae400b68848be09db706eac2ac18298babee71ab656f8b0000000048473044022058f6fc7c6a33e1b31548d481c826c015bd30135aad42cd67790dab66d2ad243b02204a1ced2604c6735b6393e5b41691dd78b00f0c5942fb9f751856faa938157dba01feffffff0280f0fa020000000017a9140fb9463421696b82c833af241c78c17ddbde493487d0f20a270100000017a91429ca74f8a08f81999428185c97b5d852e4063f618765000000"), SER_NETWORK, PROTOCOL_VERSION);
     CTransactionRef prev_tx2;
     s_prev_tx2 >> prev_tx2;
-    m_wallet.mapWallet.emplace(std::piecewise_construct, std::forward_as_tuple(prev_tx2->GetHash()), std::forward_as_tuple(prev_tx2));
+    m_wallet.mapWallet.emplace(std::piecewise_construct, std::forward_as_tuple(prev_tx2->GetHash()), std::forward_as_tuple(prev_tx2, TxStateInactive{}));
 
     // Import descriptors for keys and scripts
     import_descriptor(m_wallet, "sh(multi(2,xprv9s21ZrQH143K2LE7W4Xf3jATf9jECxSb7wj91ZnmY4qEJrS66Qru9RFqq8xbkgT32ya6HqYJweFdJUEDf5Q6JFV7jMiUws7kQfe6Tv4RbfN/0h/0h/0h,xprv9s21ZrQH143K2LE7W4Xf3jATf9jECxSb7wj91ZnmY4qEJrS66Qru9RFqq8xbkgT32ya6HqYJweFdJUEDf5Q6JFV7jMiUws7kQfe6Tv4RbfN/0h/0h/1h))");
diff --git a/src/wallet/test/wallet_tests.cpp b/src/wallet/test/wallet_tests.cpp
index 4499eb5903..9a1398e970 100644
--- a/src/wallet/test/wallet_tests.cpp
+++ b/src/wallet/test/wallet_tests.cpp
@@ -330,7 +330,7 @@ BOOST_FIXTURE_TEST_CASE(importwallet_rescan, TestChain100Setup)
 BOOST_FIXTURE_TEST_CASE(coin_mark_dirty_immature_credit, TestChain100Setup)
 {
     CWallet wallet(m_node.chain.get(), "", m_args, CreateDummyWalletDatabase());
-    CWalletTx wtx(m_coinbase_txns.back());
+    CWalletTx wtx{m_coinbase_txns.back(), TxStateConfirmed{m_node.chainman->ActiveChain().Tip()->GetBlockHash(), m_node.chainman->ActiveChain().Height(), /*position_in_block=*/0}};
 
     LOCK(wallet.cs_wallet);
     wallet.SetWalletFlag(WALLET_FLAG_DESCRIPTORS);
@@ -338,9 +338,6 @@ BOOST_FIXTURE_TEST_CASE(coin_mark_dirty_immature_credit, TestChain100Setup)
 
     wallet.SetLastBlockProcessed(m_node.chainman->ActiveChain().Height(), m_node.chainman->ActiveChain().Tip()->GetBlockHash());
 
-    CWalletTx::Confirmation confirm(CWalletTx::Status::CONFIRMED, m_node.chainman->ActiveChain().Height(), m_node.chainman->ActiveChain().Tip()->GetBlockHash(), 0);
-    wtx.m_confirm = confirm;
-
     // Call GetImmatureCredit() once before adding the key to the wallet to
     // cache the current immature credit amount, which is 0.
     BOOST_CHECK_EQUAL(CachedTxGetImmatureCredit(wallet, wtx), 0);
@@ -355,7 +352,7 @@ BOOST_FIXTURE_TEST_CASE(coin_mark_dirty_immature_credit, TestChain100Setup)
 static int64_t AddTx(ChainstateManager& chainman, CWallet& wallet, uint32_t lockTime, int64_t mockTime, int64_t blockTime)
 {
     CMutableTransaction tx;
-    CWalletTx::Confirmation confirm;
+    TxState state = TxStateInactive{};
     tx.nLockTime = lockTime;
     SetMockTime(mockTime);
     CBlockIndex* block = nullptr;
@@ -367,13 +364,13 @@ static int64_t AddTx(ChainstateManager& chainman, CWallet& wallet, uint32_t lock
         block = inserted.first->second;
         block->nTime = blockTime;
         block->phashBlock = &hash;
-        confirm = {CWalletTx::Status::CONFIRMED, block->nHeight, hash, 0};
+        state = TxStateConfirmed{hash, block->nHeight, /*position_in_block=*/0};
     }
-
-    // If transaction is already in map, to avoid inconsistencies, unconfirmation
-    // is needed before confirm again with different block.
-    return wallet.AddToWallet(MakeTransactionRef(tx), confirm, [&](CWalletTx& wtx, bool /* new_tx */) {
-        wtx.setUnconfirmed();
+    return wallet.AddToWallet(MakeTransactionRef(tx), state, [&](CWalletTx& wtx, bool /* new_tx */) {
+        // Assign wtx.m_state to simplify test and avoid the need to simulate
+        // reorg events. Without this, AddToWallet asserts false when the same
+        // transaction is confirmed in different blocks.
+        wtx.m_state = state;
         return true;
     })->nTimeSmart;
 }
@@ -534,8 +531,7 @@ public:
         wallet->SetLastBlockProcessed(wallet->GetLastBlockHeight() + 1, m_node.chainman->ActiveChain().Tip()->GetBlockHash());
         auto it = wallet->mapWallet.find(tx->GetHash());
         BOOST_CHECK(it != wallet->mapWallet.end());
-        CWalletTx::Confirmation confirm(CWalletTx::Status::CONFIRMED, m_node.chainman->ActiveChain().Height(), m_node.chainman->ActiveChain().Tip()->GetBlockHash(), 1);
-        it->second.m_confirm = confirm;
+        it->second.m_state = TxStateConfirmed{m_node.chainman->ActiveChain().Tip()->GetBlockHash(), m_node.chainman->ActiveChain().Height(), /*position_in_block=*/1};
         return it->second;
     }
 
diff --git a/src/wallet/test/wallet_transaction_tests.cpp b/src/wallet/test/wallet_transaction_tests.cpp
new file mode 100644
index 0000000000..5ef2904f66
--- /dev/null
+++ b/src/wallet/test/wallet_transaction_tests.cpp
@@ -0,0 +1,24 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <wallet/transaction.h>
+
+#include <wallet/test/wallet_test_fixture.h>
+
+#include <boost/test/unit_test.hpp>
+
+BOOST_FIXTURE_TEST_SUITE(wallet_transaction_tests, WalletTestingSetup)
+
+BOOST_AUTO_TEST_CASE(roundtrip)
+{
+    for (uint8_t hash = 0; hash < 5; ++hash) {
+        for (int index = -2; index < 3; ++index) {
+            TxState state = TxStateInterpretSerialized(TxStateUnrecognized{uint256{hash}, index});
+            BOOST_CHECK_EQUAL(TxStateSerializedBlockHash(state), uint256{hash});
+            BOOST_CHECK_EQUAL(TxStateSerializedIndex(state), index);
+        }
+    }
+}
+
+BOOST_AUTO_TEST_SUITE_END()