coins_tests.cpp

// Copyright (c) 2014-2022 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 <clientversion.h>
#include <coins.h>
#include <script/standard.h>
#include <streams.h>
#include <test/util/random.h>
#include <test/util/setup_common.h>
#include <txdb.h>
#include <uint256.h>
#include <undo.h>
#include <util/strencodings.h>

#include <map>
#include <vector>

#include <boost/test/unit_test.hpp>

int ApplyTxInUndo(Coin&& undo, CCoinsViewCache& view, const COutPoint& out);
void UpdateCoins(const CTransaction& tx, CCoinsViewCache& inputs, CTxUndo &txundo, int nHeight);

namespace
{
//! equality test
bool operator==(const Coin &a, const Coin &b) {
    // Empty Coin objects are always equal.
    if (a.IsSpent() && b.IsSpent()) return true;
    return a.fCoinBase == b.fCoinBase &&
           a.nHeight == b.nHeight &&
           a.out == b.out;
}

class CCoinsViewTest : public CCoinsView
{
    uint256 hashBestBlock_;
    std::map<COutPoint, Coin> map_;

public:
    [[nodiscard]] bool GetCoin(const COutPoint& outpoint, Coin& coin) const override
    {
        std::map<COutPoint, Coin>::const_iterator it = map_.find(outpoint);
        if (it == map_.end()) {
            return false;
        }
        coin = it->second;
        if (coin.IsSpent() && InsecureRandBool() == 0) {
            // Randomly return false in case of an empty entry.
            return false;
        }
        return true;
    }

    uint256 GetBestBlock() const override { return hashBestBlock_; }

    bool BatchWrite(CCoinsMap& mapCoins, const uint256& hashBlock, bool erase = true) override
    {
        for (CCoinsMap::iterator it = mapCoins.begin(); it != mapCoins.end(); it = erase ? mapCoins.erase(it) : std::next(it)) {
            if (it->second.flags & CCoinsCacheEntry::DIRTY) {
                // Same optimization used in CCoinsViewDB is to only write dirty entries.
                map_[it->first] = it->second.coin;
                if (it->second.coin.IsSpent() && InsecureRandRange(3) == 0) {
                    // Randomly delete empty entries on write.
                    map_.erase(it->first);
                }
            }
        }
        if (!hashBlock.IsNull())
            hashBestBlock_ = hashBlock;
        return true;
    }
};

class CCoinsViewCacheTest : public CCoinsViewCache
{
public:
    explicit CCoinsViewCacheTest(CCoinsView* _base) : CCoinsViewCache(_base) {}

    void SelfTest() const
    {
        // Manually recompute the dynamic usage of the whole data, and compare it.
        size_t ret = memusage::DynamicUsage(cacheCoins);
        size_t count = 0;
        for (const auto& entry : cacheCoins) {
            ret += entry.second.coin.DynamicMemoryUsage();
            ++count;
        }
        BOOST_CHECK_EQUAL(GetCacheSize(), count);
        BOOST_CHECK_EQUAL(DynamicMemoryUsage(), ret);
    }

    CCoinsMap& map() const { return cacheCoins; }
    size_t& usage() const { return cachedCoinsUsage; }
};

} // namespace

BOOST_FIXTURE_TEST_SUITE(coins_tests, BasicTestingSetup)

static const unsigned int NUM_SIMULATION_ITERATIONS = 40000;

// This is a large randomized insert/remove simulation test on a variable-size
// stack of caches on top of CCoinsViewTest.
//
// It will randomly create/update/delete Coin entries to a tip of caches, with
// txids picked from a limited list of random 256-bit hashes. Occasionally, a
// new tip is added to the stack of caches, or the tip is flushed and removed.
//
// During the process, booleans are kept to make sure that the randomized
// operation hits all branches.
//
// If fake_best_block is true, assign a random uint256 to mock the recording
// of best block on flush. This is necessary when using CCoinsViewDB as the base,
// otherwise we'll hit an assertion in BatchWrite.
//
void SimulationTest(CCoinsView* base, bool fake_best_block)
{
    // Various coverage trackers.
    bool removed_all_caches = false;
    bool reached_4_caches = false;
    bool added_an_entry = false;
    bool added_an_unspendable_entry = false;
    bool removed_an_entry = false;
    bool updated_an_entry = false;
    bool found_an_entry = false;
    bool missed_an_entry = false;
    bool uncached_an_entry = false;
    bool flushed_without_erase = false;

    // A simple map to track what we expect the cache stack to represent.
    std::map<COutPoint, Coin> result;

    // The cache stack.
    std::vector<std::unique_ptr<CCoinsViewCacheTest>> stack; // A stack of CCoinsViewCaches on top.
    stack.push_back(std::make_unique<CCoinsViewCacheTest>(base)); // Start with one cache.

    // Use a limited set of random transaction ids, so we do test overwriting entries.
    std::vector<uint256> txids;
    txids.resize(NUM_SIMULATION_ITERATIONS / 8);
    for (unsigned int i = 0; i < txids.size(); i++) {
        txids[i] = InsecureRand256();
    }

    for (unsigned int i = 0; i < NUM_SIMULATION_ITERATIONS; i++) {
        // Do a random modification.
        {
            uint256 txid = txids[InsecureRandRange(txids.size())]; // txid we're going to modify in this iteration.
            Coin& coin = result[COutPoint(txid, 0)];

            // Determine whether to test HaveCoin before or after Access* (or both). As these functions
            // can influence each other's behaviour by pulling things into the cache, all combinations
            // are tested.
            bool test_havecoin_before = InsecureRandBits(2) == 0;
            bool test_havecoin_after = InsecureRandBits(2) == 0;

            bool result_havecoin = test_havecoin_before ? stack.back()->HaveCoin(COutPoint(txid, 0)) : false;

            // Infrequently, test usage of AccessByTxid instead of AccessCoin - the
            // former just delegates to the latter and returns the first unspent in a txn.
            const Coin& entry = (InsecureRandRange(500) == 0) ?
                AccessByTxid(*stack.back(), txid) : stack.back()->AccessCoin(COutPoint(txid, 0));
            BOOST_CHECK(coin == entry);

            if (test_havecoin_before) {
                BOOST_CHECK(result_havecoin == !entry.IsSpent());
            }

            if (test_havecoin_after) {
                bool ret = stack.back()->HaveCoin(COutPoint(txid, 0));
                BOOST_CHECK(ret == !entry.IsSpent());
            }

            if (InsecureRandRange(5) == 0 || coin.IsSpent()) {
                Coin newcoin;
                newcoin.out.nValue = InsecureRandMoneyAmount();
                newcoin.nHeight = 1;

                // Infrequently test adding unspendable coins.
                if (InsecureRandRange(16) == 0 && coin.IsSpent()) {
                    newcoin.out.scriptPubKey.assign(1 + InsecureRandBits(6), OP_RETURN);
                    BOOST_CHECK(newcoin.out.scriptPubKey.IsUnspendable());
                    added_an_unspendable_entry = true;
                } else {
                    // Random sizes so we can test memory usage accounting
                    newcoin.out.scriptPubKey.assign(InsecureRandBits(6), 0);
                    (coin.IsSpent() ? added_an_entry : updated_an_entry) = true;
                    coin = newcoin;
                }
                bool is_overwrite = !coin.IsSpent() || InsecureRand32() & 1;
                stack.back()->AddCoin(COutPoint(txid, 0), std::move(newcoin), is_overwrite);
            } else {
                // Spend the coin.
                removed_an_entry = true;
                coin.Clear();
                BOOST_CHECK(stack.back()->SpendCoin(COutPoint(txid, 0)));
            }
        }

        // Once every 10 iterations, remove a random entry from the cache
        if (InsecureRandRange(10) == 0) {
            COutPoint out(txids[InsecureRand32() % txids.size()], 0);
            int cacheid = InsecureRand32() % stack.size();
            stack[cacheid]->Uncache(out);
            uncached_an_entry |= !stack[cacheid]->HaveCoinInCache(out);
        }

        // Once every 1000 iterations and at the end, verify the full cache.
        if (InsecureRandRange(1000) == 1 || i == NUM_SIMULATION_ITERATIONS - 1) {
            for (const auto& entry : result) {
                bool have = stack.back()->HaveCoin(entry.first);
                const Coin& coin = stack.back()->AccessCoin(entry.first);
                BOOST_CHECK(have == !coin.IsSpent());
                BOOST_CHECK(coin == entry.second);
                if (coin.IsSpent()) {
                    missed_an_entry = true;
                } else {
                    BOOST_CHECK(stack.back()->HaveCoinInCache(entry.first));
                    found_an_entry = true;
                }
            }
            for (const auto& test : stack) {
                test->SelfTest();
            }
        }

        if (InsecureRandRange(100) == 0) {
            // Every 100 iterations, flush an intermediate cache
            if (stack.size() > 1 && InsecureRandBool() == 0) {
                unsigned int flushIndex = InsecureRandRange(stack.size() - 1);
                if (fake_best_block) stack[flushIndex]->SetBestBlock(InsecureRand256());
                bool should_erase = InsecureRandRange(4) < 3;
                BOOST_CHECK(should_erase ? stack[flushIndex]->Flush() : stack[flushIndex]->Sync());
                flushed_without_erase |= !should_erase;
            }
        }
        if (InsecureRandRange(100) == 0) {
            // Every 100 iterations, change the cache stack.
            if (stack.size() > 0 && InsecureRandBool() == 0) {
                //Remove the top cache
                if (fake_best_block) stack.back()->SetBestBlock(InsecureRand256());
                bool should_erase = InsecureRandRange(4) < 3;
                BOOST_CHECK(should_erase ? stack.back()->Flush() : stack.back()->Sync());
                flushed_without_erase |= !should_erase;
                stack.pop_back();
            }
            if (stack.size() == 0 || (stack.size() < 4 && InsecureRandBool())) {
                //Add a new cache
                CCoinsView* tip = base;
                if (stack.size() > 0) {
                    tip = stack.back().get();
                } else {
                    removed_all_caches = true;
                }
                stack.push_back(std::make_unique<CCoinsViewCacheTest>(tip));
                if (stack.size() == 4) {
                    reached_4_caches = true;
                }
            }
        }
    }

    // Verify coverage.
    BOOST_CHECK(removed_all_caches);
    BOOST_CHECK(reached_4_caches);
    BOOST_CHECK(added_an_entry);
    BOOST_CHECK(added_an_unspendable_entry);
    BOOST_CHECK(removed_an_entry);
    BOOST_CHECK(updated_an_entry);
    BOOST_CHECK(found_an_entry);
    BOOST_CHECK(missed_an_entry);
    BOOST_CHECK(uncached_an_entry);
    BOOST_CHECK(flushed_without_erase);
}

// Run the above simulation for multiple base types.
BOOST_AUTO_TEST_CASE(coins_cache_simulation_test)
{
    CCoinsViewTest base;
    SimulationTest(&base, false);

    CCoinsViewDB db_base{{.path = "test", .cache_bytes = 1 << 23, .memory_only = true}, {}};
    SimulationTest(&db_base, true);
}

// Store of all necessary tx and undo data for next test
typedef std::map<COutPoint, std::tuple<CTransaction,CTxUndo,Coin>> UtxoData;
UtxoData utxoData;

UtxoData::iterator FindRandomFrom(const std::set<COutPoint> &utxoSet) {
    assert(utxoSet.size());
    auto utxoSetIt = utxoSet.lower_bound(COutPoint(InsecureRand256(), 0));
    if (utxoSetIt == utxoSet.end()) {
        utxoSetIt = utxoSet.begin();
    }
    auto utxoDataIt = utxoData.find(*utxoSetIt);
    assert(utxoDataIt != utxoData.end());
    return utxoDataIt;
}


// This test is similar to the previous test
// except the emphasis is on testing the functionality of UpdateCoins
// random txs are created and UpdateCoins is used to update the cache stack
// In particular it is tested that spending a duplicate coinbase tx
// has the expected effect (the other duplicate is overwritten at all cache levels)
BOOST_AUTO_TEST_CASE(updatecoins_simulation_test)
{
    SeedInsecureRand(SeedRand::ZEROS);
    g_mock_deterministic_tests = true;

    bool spent_a_duplicate_coinbase = false;
    // A simple map to track what we expect the cache stack to represent.
    std::map<COutPoint, Coin> result;

    // The cache stack.
    CCoinsViewTest base; // A CCoinsViewTest at the bottom.
    std::vector<std::unique_ptr<CCoinsViewCacheTest>> stack; // A stack of CCoinsViewCaches on top.
    stack.push_back(std::make_unique<CCoinsViewCacheTest>(&base)); // Start with one cache.

    // Track the txids we've used in various sets
    std::set<COutPoint> coinbase_coins;
    std::set<COutPoint> disconnected_coins;
    std::set<COutPoint> duplicate_coins;
    std::set<COutPoint> utxoset;

    for (unsigned int i = 0; i < NUM_SIMULATION_ITERATIONS; i++) {
        uint32_t randiter = InsecureRand32();

        // 19/20 txs add a new transaction
        if (randiter % 20 < 19) {
            CMutableTransaction tx;
            tx.vin.resize(1);
            tx.vout.resize(1);
            tx.vout[0].nValue = i; //Keep txs unique unless intended to duplicate
            tx.vout[0].scriptPubKey.assign(InsecureRand32() & 0x3F, 0); // Random sizes so we can test memory usage accounting
            const int height{int(InsecureRand32() >> 1)};
            Coin old_coin;

            // 2/20 times create a new coinbase
            if (randiter % 20 < 2 || coinbase_coins.size() < 10) {
                // 1/10 of those times create a duplicate coinbase
                if (InsecureRandRange(10) == 0 && coinbase_coins.size()) {
                    auto utxod = FindRandomFrom(coinbase_coins);
                    // Reuse the exact same coinbase
                    tx = CMutableTransaction{std::get<0>(utxod->second)};
                    // shouldn't be available for reconnection if it's been duplicated
                    disconnected_coins.erase(utxod->first);

                    duplicate_coins.insert(utxod->first);
                }
                else {
                    coinbase_coins.insert(COutPoint(tx.GetHash(), 0));
                }
                assert(CTransaction(tx).IsCoinBase());
            }

            // 17/20 times reconnect previous or add a regular tx
            else {

                COutPoint prevout;
                // 1/20 times reconnect a previously disconnected tx
                if (randiter % 20 == 2 && disconnected_coins.size()) {
                    auto utxod = FindRandomFrom(disconnected_coins);
                    tx = CMutableTransaction{std::get<0>(utxod->second)};
                    prevout = tx.vin[0].prevout;
                    if (!CTransaction(tx).IsCoinBase() && !utxoset.count(prevout)) {
                        disconnected_coins.erase(utxod->first);
                        continue;
                    }

                    // If this tx is already IN the UTXO, then it must be a coinbase, and it must be a duplicate
                    if (utxoset.count(utxod->first)) {
                        assert(CTransaction(tx).IsCoinBase());
                        assert(duplicate_coins.count(utxod->first));
                    }
                    disconnected_coins.erase(utxod->first);
                }

                // 16/20 times create a regular tx
                else {
                    auto utxod = FindRandomFrom(utxoset);
                    prevout = utxod->first;

                    // Construct the tx to spend the coins of prevouthash
                    tx.vin[0].prevout = prevout;
                    assert(!CTransaction(tx).IsCoinBase());
                }
                // In this simple test coins only have two states, spent or unspent, save the unspent state to restore
                old_coin = result[prevout];
                // Update the expected result of prevouthash to know these coins are spent
                result[prevout].Clear();

                utxoset.erase(prevout);

                // The test is designed to ensure spending a duplicate coinbase will work properly
                // if that ever happens and not resurrect the previously overwritten coinbase
                if (duplicate_coins.count(prevout)) {
                    spent_a_duplicate_coinbase = true;
                }

            }
            // Update the expected result to know about the new output coins
            assert(tx.vout.size() == 1);
            const COutPoint outpoint(tx.GetHash(), 0);
            result[outpoint] = Coin{tx.vout[0], height, CTransaction{tx}.IsCoinBase()};

            // Call UpdateCoins on the top cache
            CTxUndo undo;
            UpdateCoins(CTransaction{tx}, *(stack.back()), undo, height);

            // Update the utxo set for future spends
            utxoset.insert(outpoint);

            // Track this tx and undo info to use later
            utxoData.emplace(outpoint, std::make_tuple(tx,undo,old_coin));
        } else if (utxoset.size()) {
            //1/20 times undo a previous transaction
            auto utxod = FindRandomFrom(utxoset);

            CTransaction &tx = std::get<0>(utxod->second);
            CTxUndo &undo = std::get<1>(utxod->second);
            Coin &orig_coin = std::get<2>(utxod->second);

            // Update the expected result
            // Remove new outputs
            result[utxod->first].Clear();
            // If not coinbase restore prevout
            if (!tx.IsCoinBase()) {
                result[tx.vin[0].prevout] = orig_coin;
            }

            // Disconnect the tx from the current UTXO
            // See code in DisconnectBlock
            // remove outputs
            BOOST_CHECK(stack.back()->SpendCoin(utxod->first));
            // restore inputs
            if (!tx.IsCoinBase()) {
                const COutPoint &out = tx.vin[0].prevout;
                Coin coin = undo.vprevout[0];
                ApplyTxInUndo(std::move(coin), *(stack.back()), out);
            }
            // Store as a candidate for reconnection
            disconnected_coins.insert(utxod->first);

            // Update the utxoset
            utxoset.erase(utxod->first);
            if (!tx.IsCoinBase())
                utxoset.insert(tx.vin[0].prevout);
        }

        // Once every 1000 iterations and at the end, verify the full cache.
        if (InsecureRandRange(1000) == 1 || i == NUM_SIMULATION_ITERATIONS - 1) {
            for (const auto& entry : result) {
                bool have = stack.back()->HaveCoin(entry.first);
                const Coin& coin = stack.back()->AccessCoin(entry.first);
                BOOST_CHECK(have == !coin.IsSpent());
                BOOST_CHECK(coin == entry.second);
            }
        }

        // One every 10 iterations, remove a random entry from the cache
        if (utxoset.size() > 1 && InsecureRandRange(30) == 0) {
            stack[InsecureRand32() % stack.size()]->Uncache(FindRandomFrom(utxoset)->first);
        }
        if (disconnected_coins.size() > 1 && InsecureRandRange(30) == 0) {
            stack[InsecureRand32() % stack.size()]->Uncache(FindRandomFrom(disconnected_coins)->first);
        }
        if (duplicate_coins.size() > 1 && InsecureRandRange(30) == 0) {
            stack[InsecureRand32() % stack.size()]->Uncache(FindRandomFrom(duplicate_coins)->first);
        }

        if (InsecureRandRange(100) == 0) {
            // Every 100 iterations, flush an intermediate cache
            if (stack.size() > 1 && InsecureRandBool() == 0) {
                unsigned int flushIndex = InsecureRandRange(stack.size() - 1);
                BOOST_CHECK(stack[flushIndex]->Flush());
            }
        }
        if (InsecureRandRange(100) == 0) {
            // Every 100 iterations, change the cache stack.
            if (stack.size() > 0 && InsecureRandBool() == 0) {
                BOOST_CHECK(stack.back()->Flush());
                stack.pop_back();
            }
            if (stack.size() == 0 || (stack.size() < 4 && InsecureRandBool())) {
                CCoinsView* tip = &base;
                if (stack.size() > 0) {
                    tip = stack.back().get();
                }
                stack.push_back(std::make_unique<CCoinsViewCacheTest>(tip));
            }
        }
    }

    // Verify coverage.
    BOOST_CHECK(spent_a_duplicate_coinbase);

    g_mock_deterministic_tests = false;
}

BOOST_AUTO_TEST_CASE(ccoins_serialization)
{
    // Good example
    DataStream ss1{ParseHex("97f23c835800816115944e077fe7c803cfa57f29b36bf87c1d35")};
    Coin cc1;
    ss1 >> cc1;
    BOOST_CHECK_EQUAL(cc1.fCoinBase, false);
    BOOST_CHECK_EQUAL(cc1.nHeight, 203998U);
    BOOST_CHECK_EQUAL(cc1.out.nValue, CAmount{60000000000});
    BOOST_CHECK_EQUAL(HexStr(cc1.out.scriptPubKey), HexStr(GetScriptForDestination(PKHash(uint160(ParseHex("816115944e077fe7c803cfa57f29b36bf87c1d35"))))));

    // Good example
    DataStream ss2{ParseHex("8ddf77bbd123008c988f1a4a4de2161e0f50aac7f17e7f9555caa4")};
    Coin cc2;
    ss2 >> cc2;
    BOOST_CHECK_EQUAL(cc2.fCoinBase, true);
    BOOST_CHECK_EQUAL(cc2.nHeight, 120891U);
    BOOST_CHECK_EQUAL(cc2.out.nValue, 110397);
    BOOST_CHECK_EQUAL(HexStr(cc2.out.scriptPubKey), HexStr(GetScriptForDestination(PKHash(uint160(ParseHex("8c988f1a4a4de2161e0f50aac7f17e7f9555caa4"))))));

    // Smallest possible example
    DataStream ss3{ParseHex("000006")};
    Coin cc3;
    ss3 >> cc3;
    BOOST_CHECK_EQUAL(cc3.fCoinBase, false);
    BOOST_CHECK_EQUAL(cc3.nHeight, 0U);
    BOOST_CHECK_EQUAL(cc3.out.nValue, 0);
    BOOST_CHECK_EQUAL(cc3.out.scriptPubKey.size(), 0U);

    // scriptPubKey that ends beyond the end of the stream
    DataStream ss4{ParseHex("000007")};
    try {
        Coin cc4;
        ss4 >> cc4;
        BOOST_CHECK_MESSAGE(false, "We should have thrown");
    } catch (const std::ios_base::failure&) {
    }

    // Very large scriptPubKey (3*10^9 bytes) past the end of the stream
    DataStream tmp{};
    uint64_t x = 3000000000ULL;
    tmp << VARINT(x);
    BOOST_CHECK_EQUAL(HexStr(tmp), "8a95c0bb00");
    DataStream ss5{ParseHex("00008a95c0bb00")};
    try {
        Coin cc5;
        ss5 >> cc5;
        BOOST_CHECK_MESSAGE(false, "We should have thrown");
    } catch (const std::ios_base::failure&) {
    }
}

const static COutPoint OUTPOINT;
const static CAmount SPENT = -1;
const static CAmount ABSENT = -2;
const static CAmount FAIL = -3;
const static CAmount VALUE1 = 100;
const static CAmount VALUE2 = 200;
const static CAmount VALUE3 = 300;
const static char DIRTY = CCoinsCacheEntry::DIRTY;
const static char FRESH = CCoinsCacheEntry::FRESH;
const static char NO_ENTRY = -1;

const static auto FLAGS = {char(0), FRESH, DIRTY, char(DIRTY | FRESH)};
const static auto CLEAN_FLAGS = {char(0), FRESH};
const static auto ABSENT_FLAGS = {NO_ENTRY};

static void SetCoinsValue(CAmount value, Coin& coin)
{
    assert(value != ABSENT);
    coin.Clear();
    assert(coin.IsSpent());
    if (value != SPENT) {
        coin.out.nValue = value;
        coin.nHeight = 1;
        assert(!coin.IsSpent());
    }
}

static size_t InsertCoinsMapEntry(CCoinsMap& map, CAmount value, char flags)
{
    if (value == ABSENT) {
        assert(flags == NO_ENTRY);
        return 0;
    }
    assert(flags != NO_ENTRY);
    CCoinsCacheEntry entry;
    entry.flags = flags;
    SetCoinsValue(value, entry.coin);
    auto inserted = map.emplace(OUTPOINT, std::move(entry));
    assert(inserted.second);
    return inserted.first->second.coin.DynamicMemoryUsage();
}

void GetCoinsMapEntry(const CCoinsMap& map, CAmount& value, char& flags, const COutPoint& outp = OUTPOINT)
{
    auto it = map.find(outp);
    if (it == map.end()) {
        value = ABSENT;
        flags = NO_ENTRY;
    } else {
        if (it->second.coin.IsSpent()) {
            value = SPENT;
        } else {
            value = it->second.coin.out.nValue;
        }
        flags = it->second.flags;
        assert(flags != NO_ENTRY);
    }
}

void WriteCoinsViewEntry(CCoinsView& view, CAmount value, char flags)
{
    CCoinsMap map;
    InsertCoinsMapEntry(map, value, flags);
    BOOST_CHECK(view.BatchWrite(map, {}));
}

class SingleEntryCacheTest
{
public:
    SingleEntryCacheTest(CAmount base_value, CAmount cache_value, char cache_flags)
    {
        WriteCoinsViewEntry(base, base_value, base_value == ABSENT ? NO_ENTRY : DIRTY);
        cache.usage() += InsertCoinsMapEntry(cache.map(), cache_value, cache_flags);
    }

    CCoinsView root;
    CCoinsViewCacheTest base{&root};
    CCoinsViewCacheTest cache{&base};
};

static void CheckAccessCoin(CAmount base_value, CAmount cache_value, CAmount expected_value, char cache_flags, char expected_flags)
{
    SingleEntryCacheTest test(base_value, cache_value, cache_flags);
    test.cache.AccessCoin(OUTPOINT);
    test.cache.SelfTest();

    CAmount result_value;
    char result_flags;
    GetCoinsMapEntry(test.cache.map(), result_value, result_flags);
    BOOST_CHECK_EQUAL(result_value, expected_value);
    BOOST_CHECK_EQUAL(result_flags, expected_flags);
}

BOOST_AUTO_TEST_CASE(ccoins_access)
{
    /* Check AccessCoin behavior, requesting a coin from a cache view layered on
     * top of a base view, and checking the resulting entry in the cache after
     * the access.
     *
     *               Base    Cache   Result  Cache        Result
     *               Value   Value   Value   Flags        Flags
     */
    CheckAccessCoin(ABSENT, ABSENT, ABSENT, NO_ENTRY   , NO_ENTRY   );
    CheckAccessCoin(ABSENT, SPENT , SPENT , 0          , 0          );
    CheckAccessCoin(ABSENT, SPENT , SPENT , FRESH      , FRESH      );
    CheckAccessCoin(ABSENT, SPENT , SPENT , DIRTY      , DIRTY      );
    CheckAccessCoin(ABSENT, SPENT , SPENT , DIRTY|FRESH, DIRTY|FRESH);
    CheckAccessCoin(ABSENT, VALUE2, VALUE2, 0          , 0          );
    CheckAccessCoin(ABSENT, VALUE2, VALUE2, FRESH      , FRESH      );
    CheckAccessCoin(ABSENT, VALUE2, VALUE2, DIRTY      , DIRTY      );
    CheckAccessCoin(ABSENT, VALUE2, VALUE2, DIRTY|FRESH, DIRTY|FRESH);
    CheckAccessCoin(SPENT , ABSENT, ABSENT, NO_ENTRY   , NO_ENTRY   );
    CheckAccessCoin(SPENT , SPENT , SPENT , 0          , 0          );
    CheckAccessCoin(SPENT , SPENT , SPENT , FRESH      , FRESH      );
    CheckAccessCoin(SPENT , SPENT , SPENT , DIRTY      , DIRTY      );
    CheckAccessCoin(SPENT , SPENT , SPENT , DIRTY|FRESH, DIRTY|FRESH);
    CheckAccessCoin(SPENT , VALUE2, VALUE2, 0          , 0          );
    CheckAccessCoin(SPENT , VALUE2, VALUE2, FRESH      , FRESH      );
    CheckAccessCoin(SPENT , VALUE2, VALUE2, DIRTY      , DIRTY      );
    CheckAccessCoin(SPENT , VALUE2, VALUE2, DIRTY|FRESH, DIRTY|FRESH);
    CheckAccessCoin(VALUE1, ABSENT, VALUE1, NO_ENTRY   , 0          );
    CheckAccessCoin(VALUE1, SPENT , SPENT , 0          , 0          );
    CheckAccessCoin(VALUE1, SPENT , SPENT , FRESH      , FRESH      );
    CheckAccessCoin(VALUE1, SPENT , SPENT , DIRTY      , DIRTY      );
    CheckAccessCoin(VALUE1, SPENT , SPENT , DIRTY|FRESH, DIRTY|FRESH);
    CheckAccessCoin(VALUE1, VALUE2, VALUE2, 0          , 0          );
    CheckAccessCoin(VALUE1, VALUE2, VALUE2, FRESH      , FRESH      );
    CheckAccessCoin(VALUE1, VALUE2, VALUE2, DIRTY      , DIRTY      );
    CheckAccessCoin(VALUE1, VALUE2, VALUE2, DIRTY|FRESH, DIRTY|FRESH);
}

static void CheckSpendCoins(CAmount base_value, CAmount cache_value, CAmount expected_value, char cache_flags, char expected_flags)
{
    SingleEntryCacheTest test(base_value, cache_value, cache_flags);
    test.cache.SpendCoin(OUTPOINT);
    test.cache.SelfTest();

    CAmount result_value;
    char result_flags;
    GetCoinsMapEntry(test.cache.map(), result_value, result_flags);
    BOOST_CHECK_EQUAL(result_value, expected_value);
    BOOST_CHECK_EQUAL(result_flags, expected_flags);
};

BOOST_AUTO_TEST_CASE(ccoins_spend)
{
    /* Check SpendCoin behavior, requesting a coin from a cache view layered on
     * top of a base view, spending, and then checking
     * the resulting entry in the cache after the modification.
     *
     *              Base    Cache   Result  Cache        Result
     *              Value   Value   Value   Flags        Flags
     */
    CheckSpendCoins(ABSENT, ABSENT, ABSENT, NO_ENTRY   , NO_ENTRY   );
    CheckSpendCoins(ABSENT, SPENT , SPENT , 0          , DIRTY      );
    CheckSpendCoins(ABSENT, SPENT , ABSENT, FRESH      , NO_ENTRY   );
    CheckSpendCoins(ABSENT, SPENT , SPENT , DIRTY      , DIRTY      );
    CheckSpendCoins(ABSENT, SPENT , ABSENT, DIRTY|FRESH, NO_ENTRY   );
    CheckSpendCoins(ABSENT, VALUE2, SPENT , 0          , DIRTY      );
    CheckSpendCoins(ABSENT, VALUE2, ABSENT, FRESH      , NO_ENTRY   );
    CheckSpendCoins(ABSENT, VALUE2, SPENT , DIRTY      , DIRTY      );
    CheckSpendCoins(ABSENT, VALUE2, ABSENT, DIRTY|FRESH, NO_ENTRY   );
    CheckSpendCoins(SPENT , ABSENT, ABSENT, NO_ENTRY   , NO_ENTRY   );
    CheckSpendCoins(SPENT , SPENT , SPENT , 0          , DIRTY      );
    CheckSpendCoins(SPENT , SPENT , ABSENT, FRESH      , NO_ENTRY   );
    CheckSpendCoins(SPENT , SPENT , SPENT , DIRTY      , DIRTY      );
    CheckSpendCoins(SPENT , SPENT , ABSENT, DIRTY|FRESH, NO_ENTRY   );
    CheckSpendCoins(SPENT , VALUE2, SPENT , 0          , DIRTY      );
    CheckSpendCoins(SPENT , VALUE2, ABSENT, FRESH      , NO_ENTRY   );
    CheckSpendCoins(SPENT , VALUE2, SPENT , DIRTY      , DIRTY      );
    CheckSpendCoins(SPENT , VALUE2, ABSENT, DIRTY|FRESH, NO_ENTRY   );
    CheckSpendCoins(VALUE1, ABSENT, SPENT , NO_ENTRY   , DIRTY      );
    CheckSpendCoins(VALUE1, SPENT , SPENT , 0          , DIRTY      );
    CheckSpendCoins(VALUE1, SPENT , ABSENT, FRESH      , NO_ENTRY   );
    CheckSpendCoins(VALUE1, SPENT , SPENT , DIRTY      , DIRTY      );
    CheckSpendCoins(VALUE1, SPENT , ABSENT, DIRTY|FRESH, NO_ENTRY   );
    CheckSpendCoins(VALUE1, VALUE2, SPENT , 0          , DIRTY      );
    CheckSpendCoins(VALUE1, VALUE2, ABSENT, FRESH      , NO_ENTRY   );
    CheckSpendCoins(VALUE1, VALUE2, SPENT , DIRTY      , DIRTY      );
    CheckSpendCoins(VALUE1, VALUE2, ABSENT, DIRTY|FRESH, NO_ENTRY   );
}

static void CheckAddCoinBase(CAmount base_value, CAmount cache_value, CAmount modify_value, CAmount expected_value, char cache_flags, char expected_flags, bool coinbase)
{
    SingleEntryCacheTest test(base_value, cache_value, cache_flags);

    CAmount result_value;
    char result_flags;
    try {
        CTxOut output;
        output.nValue = modify_value;
        test.cache.AddCoin(OUTPOINT, Coin(std::move(output), 1, coinbase), coinbase);
        test.cache.SelfTest();
        GetCoinsMapEntry(test.cache.map(), result_value, result_flags);
    } catch (std::logic_error&) {
        result_value = FAIL;
        result_flags = NO_ENTRY;
    }

    BOOST_CHECK_EQUAL(result_value, expected_value);
    BOOST_CHECK_EQUAL(result_flags, expected_flags);
}

// Simple wrapper for CheckAddCoinBase function above that loops through
// different possible base_values, making sure each one gives the same results.
// This wrapper lets the coins_add test below be shorter and less repetitive,
// while still verifying that the CoinsViewCache::AddCoin implementation
// ignores base values.
template <typename... Args>
static void CheckAddCoin(Args&&... args)
{
    for (const CAmount base_value : {ABSENT, SPENT, VALUE1})
        CheckAddCoinBase(base_value, std::forward<Args>(args)...);
}

BOOST_AUTO_TEST_CASE(ccoins_add)
{
    /* Check AddCoin behavior, requesting a new coin from a cache view,
     * writing a modification to the coin, and then checking the resulting
     * entry in the cache after the modification. Verify behavior with the
     * AddCoin possible_overwrite argument set to false, and to true.
     *
     *           Cache   Write   Result  Cache        Result       possible_overwrite
     *           Value   Value   Value   Flags        Flags
     */
    CheckAddCoin(ABSENT, VALUE3, VALUE3, NO_ENTRY   , DIRTY|FRESH, false);
    CheckAddCoin(ABSENT, VALUE3, VALUE3, NO_ENTRY   , DIRTY      , true );
    CheckAddCoin(SPENT , VALUE3, VALUE3, 0          , DIRTY|FRESH, false);
    CheckAddCoin(SPENT , VALUE3, VALUE3, 0          , DIRTY      , true );
    CheckAddCoin(SPENT , VALUE3, VALUE3, FRESH      , DIRTY|FRESH, false);
    CheckAddCoin(SPENT , VALUE3, VALUE3, FRESH      , DIRTY|FRESH, true );
    CheckAddCoin(SPENT , VALUE3, VALUE3, DIRTY      , DIRTY      , false);
    CheckAddCoin(SPENT , VALUE3, VALUE3, DIRTY      , DIRTY      , true );
    CheckAddCoin(SPENT , VALUE3, VALUE3, DIRTY|FRESH, DIRTY|FRESH, false);
    CheckAddCoin(SPENT , VALUE3, VALUE3, DIRTY|FRESH, DIRTY|FRESH, true );
    CheckAddCoin(VALUE2, VALUE3, FAIL  , 0          , NO_ENTRY   , false);
    CheckAddCoin(VALUE2, VALUE3, VALUE3, 0          , DIRTY      , true );
    CheckAddCoin(VALUE2, VALUE3, FAIL  , FRESH      , NO_ENTRY   , false);
    CheckAddCoin(VALUE2, VALUE3, VALUE3, FRESH      , DIRTY|FRESH, true );
    CheckAddCoin(VALUE2, VALUE3, FAIL  , DIRTY      , NO_ENTRY   , false);
    CheckAddCoin(VALUE2, VALUE3, VALUE3, DIRTY      , DIRTY      , true );
    CheckAddCoin(VALUE2, VALUE3, FAIL  , DIRTY|FRESH, NO_ENTRY   , false);
    CheckAddCoin(VALUE2, VALUE3, VALUE3, DIRTY|FRESH, DIRTY|FRESH, true );
}

void CheckWriteCoins(CAmount parent_value, CAmount child_value, CAmount expected_value, char parent_flags, char child_flags, char expected_flags)
{
    SingleEntryCacheTest test(ABSENT, parent_value, parent_flags);

    CAmount result_value;
    char result_flags;
    try {
        WriteCoinsViewEntry(test.cache, child_value, child_flags);
        test.cache.SelfTest();
        GetCoinsMapEntry(test.cache.map(), result_value, result_flags);
    } catch (std::logic_error&) {
        result_value = FAIL;
        result_flags = NO_ENTRY;
    }

    BOOST_CHECK_EQUAL(result_value, expected_value);
    BOOST_CHECK_EQUAL(result_flags, expected_flags);
}

BOOST_AUTO_TEST_CASE(ccoins_write)
{
    /* Check BatchWrite behavior, flushing one entry from a child cache to a
     * parent cache, and checking the resulting entry in the parent cache
     * after the write.
     *
     *              Parent  Child   Result  Parent       Child        Result
     *              Value   Value   Value   Flags        Flags        Flags
     */
    CheckWriteCoins(ABSENT, ABSENT, ABSENT, NO_ENTRY   , NO_ENTRY   , NO_ENTRY   );
    CheckWriteCoins(ABSENT, SPENT , SPENT , NO_ENTRY   , DIRTY      , DIRTY      );
    CheckWriteCoins(ABSENT, SPENT , ABSENT, NO_ENTRY   , DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(ABSENT, VALUE2, VALUE2, NO_ENTRY   , DIRTY      , DIRTY      );
    CheckWriteCoins(ABSENT, VALUE2, VALUE2, NO_ENTRY   , DIRTY|FRESH, DIRTY|FRESH);
    CheckWriteCoins(SPENT , ABSENT, SPENT , 0          , NO_ENTRY   , 0          );
    CheckWriteCoins(SPENT , ABSENT, SPENT , FRESH      , NO_ENTRY   , FRESH      );
    CheckWriteCoins(SPENT , ABSENT, SPENT , DIRTY      , NO_ENTRY   , DIRTY      );
    CheckWriteCoins(SPENT , ABSENT, SPENT , DIRTY|FRESH, NO_ENTRY   , DIRTY|FRESH);
    CheckWriteCoins(SPENT , SPENT , SPENT , 0          , DIRTY      , DIRTY      );
    CheckWriteCoins(SPENT , SPENT , SPENT , 0          , DIRTY|FRESH, DIRTY      );
    CheckWriteCoins(SPENT , SPENT , ABSENT, FRESH      , DIRTY      , NO_ENTRY   );
    CheckWriteCoins(SPENT , SPENT , ABSENT, FRESH      , DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(SPENT , SPENT , SPENT , DIRTY      , DIRTY      , DIRTY      );
    CheckWriteCoins(SPENT , SPENT , SPENT , DIRTY      , DIRTY|FRESH, DIRTY      );
    CheckWriteCoins(SPENT , SPENT , ABSENT, DIRTY|FRESH, DIRTY      , NO_ENTRY   );
    CheckWriteCoins(SPENT , SPENT , ABSENT, DIRTY|FRESH, DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(SPENT , VALUE2, VALUE2, 0          , DIRTY      , DIRTY      );
    CheckWriteCoins(SPENT , VALUE2, VALUE2, 0          , DIRTY|FRESH, DIRTY      );
    CheckWriteCoins(SPENT , VALUE2, VALUE2, FRESH      , DIRTY      , DIRTY|FRESH);
    CheckWriteCoins(SPENT , VALUE2, VALUE2, FRESH      , DIRTY|FRESH, DIRTY|FRESH);
    CheckWriteCoins(SPENT , VALUE2, VALUE2, DIRTY      , DIRTY      , DIRTY      );
    CheckWriteCoins(SPENT , VALUE2, VALUE2, DIRTY      , DIRTY|FRESH, DIRTY      );
    CheckWriteCoins(SPENT , VALUE2, VALUE2, DIRTY|FRESH, DIRTY      , DIRTY|FRESH);
    CheckWriteCoins(SPENT , VALUE2, VALUE2, DIRTY|FRESH, DIRTY|FRESH, DIRTY|FRESH);
    CheckWriteCoins(VALUE1, ABSENT, VALUE1, 0          , NO_ENTRY   , 0          );
    CheckWriteCoins(VALUE1, ABSENT, VALUE1, FRESH      , NO_ENTRY   , FRESH      );
    CheckWriteCoins(VALUE1, ABSENT, VALUE1, DIRTY      , NO_ENTRY   , DIRTY      );
    CheckWriteCoins(VALUE1, ABSENT, VALUE1, DIRTY|FRESH, NO_ENTRY   , DIRTY|FRESH);
    CheckWriteCoins(VALUE1, SPENT , SPENT , 0          , DIRTY      , DIRTY      );
    CheckWriteCoins(VALUE1, SPENT , FAIL  , 0          , DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(VALUE1, SPENT , ABSENT, FRESH      , DIRTY      , NO_ENTRY   );
    CheckWriteCoins(VALUE1, SPENT , FAIL  , FRESH      , DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(VALUE1, SPENT , SPENT , DIRTY      , DIRTY      , DIRTY      );
    CheckWriteCoins(VALUE1, SPENT , FAIL  , DIRTY      , DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(VALUE1, SPENT , ABSENT, DIRTY|FRESH, DIRTY      , NO_ENTRY   );
    CheckWriteCoins(VALUE1, SPENT , FAIL  , DIRTY|FRESH, DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(VALUE1, VALUE2, VALUE2, 0          , DIRTY      , DIRTY      );
    CheckWriteCoins(VALUE1, VALUE2, FAIL  , 0          , DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(VALUE1, VALUE2, VALUE2, FRESH      , DIRTY      , DIRTY|FRESH);
    CheckWriteCoins(VALUE1, VALUE2, FAIL  , FRESH      , DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(VALUE1, VALUE2, VALUE2, DIRTY      , DIRTY      , DIRTY      );
    CheckWriteCoins(VALUE1, VALUE2, FAIL  , DIRTY      , DIRTY|FRESH, NO_ENTRY   );
    CheckWriteCoins(VALUE1, VALUE2, VALUE2, DIRTY|FRESH, DIRTY      , DIRTY|FRESH);
    CheckWriteCoins(VALUE1, VALUE2, FAIL  , DIRTY|FRESH, DIRTY|FRESH, NO_ENTRY   );

    // The checks above omit cases where the child flags are not DIRTY, since
    // they would be too repetitive (the parent cache is never updated in these
    // cases). The loop below covers these cases and makes sure the parent cache
    // is always left unchanged.
    for (const CAmount parent_value : {ABSENT, SPENT, VALUE1})
        for (const CAmount child_value : {ABSENT, SPENT, VALUE2})
            for (const char parent_flags : parent_value == ABSENT ? ABSENT_FLAGS : FLAGS)
                for (const char child_flags : child_value == ABSENT ? ABSENT_FLAGS : CLEAN_FLAGS)
                    CheckWriteCoins(parent_value, child_value, parent_value, parent_flags, child_flags, parent_flags);
}


Coin MakeCoin()
{
    Coin coin;
    coin.out.nValue = InsecureRand32();
    coin.nHeight = InsecureRandRange(4096);
    coin.fCoinBase = 0;
    return coin;
}


//! For CCoinsViewCache instances backed by either another cache instance or
//! leveldb, test cache behavior and flag state (DIRTY/FRESH) by
//!
//! 1. Adding a random coin to the child-most cache,
//! 2. Flushing all caches (without erasing),
//! 3. Ensure the entry still exists in the cache and has been written to parent,
//! 4. (if `do_erasing_flush`) Flushing the caches again (with erasing),
//! 5. (if `do_erasing_flush`) Ensure the entry has been written to the parent and is no longer in the cache,
//! 6. Spend the coin, ensure it no longer exists in the parent.
//!
void TestFlushBehavior(
    CCoinsViewCacheTest* view,
    CCoinsViewDB& base,
    std::vector<std::unique_ptr<CCoinsViewCacheTest>>& all_caches,
    bool do_erasing_flush)
{
    CAmount value;
    char flags;
    size_t cache_usage;

    auto flush_all = [&all_caches](bool erase) {
        // Flush in reverse order to ensure that flushes happen from children up.
        for (auto i = all_caches.rbegin(); i != all_caches.rend(); ++i) {
            auto& cache = *i;
            // hashBlock must be filled before flushing to disk; value is
            // unimportant here. This is normally done during connect/disconnect block.
            cache->SetBestBlock(InsecureRand256());
            erase ? cache->Flush() : cache->Sync();
        }
    };

    uint256 txid = InsecureRand256();
    COutPoint outp = COutPoint(txid, 0);
    Coin coin = MakeCoin();
    // Ensure the coins views haven't seen this coin before.
    BOOST_CHECK(!base.HaveCoin(outp));
    BOOST_CHECK(!view->HaveCoin(outp));

    // --- 1. Adding a random coin to the child cache
    //
    view->AddCoin(outp, Coin(coin), false);

    cache_usage = view->DynamicMemoryUsage();
    // `base` shouldn't have coin (no flush yet) but `view` should have cached it.
    BOOST_CHECK(!base.HaveCoin(outp));
    BOOST_CHECK(view->HaveCoin(outp));

    GetCoinsMapEntry(view->map(), value, flags, outp);
    BOOST_CHECK_EQUAL(value, coin.out.nValue);
    BOOST_CHECK_EQUAL(flags, DIRTY|FRESH);

    // --- 2. Flushing all caches (without erasing)
    //
    flush_all(/*erase=*/ false);

    // CoinsMap usage should be unchanged since we didn't erase anything.
    BOOST_CHECK_EQUAL(cache_usage, view->DynamicMemoryUsage());

    // --- 3. Ensuring the entry still exists in the cache and has been written to parent
    //
    GetCoinsMapEntry(view->map(), value, flags, outp);
    BOOST_CHECK_EQUAL(value, coin.out.nValue);
    BOOST_CHECK_EQUAL(flags, 0);  // Flags should have been wiped.

    // Both views should now have the coin.
    BOOST_CHECK(base.HaveCoin(outp));
    BOOST_CHECK(view->HaveCoin(outp));

    if (do_erasing_flush) {
        // --- 4. Flushing the caches again (with erasing)
        //
        flush_all(/*erase=*/ true);

        // Memory usage should have gone down.
        BOOST_CHECK(view->DynamicMemoryUsage() < cache_usage);

        // --- 5. Ensuring the entry is no longer in the cache
        //
        GetCoinsMapEntry(view->map(), value, flags, outp);
        BOOST_CHECK_EQUAL(value, ABSENT);
        BOOST_CHECK_EQUAL(flags, NO_ENTRY);

        view->AccessCoin(outp);
        GetCoinsMapEntry(view->map(), value, flags, outp);
        BOOST_CHECK_EQUAL(value, coin.out.nValue);
        BOOST_CHECK_EQUAL(flags, 0);
    }

    // Can't overwrite an entry without specifying that an overwrite is
    // expected.
    BOOST_CHECK_THROW(
        view->AddCoin(outp, Coin(coin), /*possible_overwrite=*/ false),
        std::logic_error);

    // --- 6. Spend the coin.
    //
    BOOST_CHECK(view->SpendCoin(outp));

    // The coin should be in the cache, but spent and marked dirty.
    GetCoinsMapEntry(view->map(), value, flags, outp);
    BOOST_CHECK_EQUAL(value, SPENT);
    BOOST_CHECK_EQUAL(flags, DIRTY);
    BOOST_CHECK(!view->HaveCoin(outp)); // Coin should be considered spent in `view`.
    BOOST_CHECK(base.HaveCoin(outp));  // But coin should still be unspent in `base`.

    flush_all(/*erase=*/ false);

    // Coin should be considered spent in both views.
    BOOST_CHECK(!view->HaveCoin(outp));
    BOOST_CHECK(!base.HaveCoin(outp));

    // Spent coin should not be spendable.
    BOOST_CHECK(!view->SpendCoin(outp));

    // --- Bonus check: ensure that a coin added to the base view via one cache
    //     can be spent by another cache which has never seen it.
    //
    txid = InsecureRand256();
    outp = COutPoint(txid, 0);
    coin = MakeCoin();
    BOOST_CHECK(!base.HaveCoin(outp));
    BOOST_CHECK(!all_caches[0]->HaveCoin(outp));
    BOOST_CHECK(!all_caches[1]->HaveCoin(outp));

    all_caches[0]->AddCoin(outp, std::move(coin), false);
    all_caches[0]->Sync();
    BOOST_CHECK(base.HaveCoin(outp));
    BOOST_CHECK(all_caches[0]->HaveCoin(outp));
    BOOST_CHECK(!all_caches[1]->HaveCoinInCache(outp));

    BOOST_CHECK(all_caches[1]->SpendCoin(outp));
    flush_all(/*erase=*/ false);
    BOOST_CHECK(!base.HaveCoin(outp));
    BOOST_CHECK(!all_caches[0]->HaveCoin(outp));
    BOOST_CHECK(!all_caches[1]->HaveCoin(outp));

    flush_all(/*erase=*/ true); // Erase all cache content.

    // --- Bonus check 2: ensure that a FRESH, spent coin is deleted by Sync()
    //
    txid = InsecureRand256();
    outp = COutPoint(txid, 0);
    coin = MakeCoin();
    CAmount coin_val = coin.out.nValue;
    BOOST_CHECK(!base.HaveCoin(outp));
    BOOST_CHECK(!all_caches[0]->HaveCoin(outp));
    BOOST_CHECK(!all_caches[1]->HaveCoin(outp));

    // Add and spend from same cache without flushing.
    all_caches[0]->AddCoin(outp, std::move(coin), false);

    // Coin should be FRESH in the cache.
    GetCoinsMapEntry(all_caches[0]->map(), value, flags, outp);
    BOOST_CHECK_EQUAL(value, coin_val);
    BOOST_CHECK_EQUAL(flags, DIRTY|FRESH);

    // Base shouldn't have seen coin.
    BOOST_CHECK(!base.HaveCoin(outp));

    BOOST_CHECK(all_caches[0]->SpendCoin(outp));
    all_caches[0]->Sync();

    // Ensure there is no sign of the coin after spend/flush.
    GetCoinsMapEntry(all_caches[0]->map(), value, flags, outp);
    BOOST_CHECK_EQUAL(value, ABSENT);
    BOOST_CHECK_EQUAL(flags, NO_ENTRY);
    BOOST_CHECK(!all_caches[0]->HaveCoinInCache(outp));
    BOOST_CHECK(!base.HaveCoin(outp));
}

BOOST_AUTO_TEST_CASE(ccoins_flush_behavior)
{
    // Create two in-memory caches atop a leveldb view.
    CCoinsViewDB base{{.path = "test", .cache_bytes = 1 << 23, .memory_only = true}, {}};
    std::vector<std::unique_ptr<CCoinsViewCacheTest>> caches;
    caches.push_back(std::make_unique<CCoinsViewCacheTest>(&base));
    caches.push_back(std::make_unique<CCoinsViewCacheTest>(caches.back().get()));

    for (const auto& view : caches) {
        TestFlushBehavior(view.get(), base, caches, /*do_erasing_flush=*/false);
        TestFlushBehavior(view.get(), base, caches, /*do_erasing_flush=*/true);
    }
}

BOOST_AUTO_TEST_SUITE_END()