coinscache_sim.cpp

// Copyright (c) 2023 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 <coins.h>
#include <crypto/sha256.h>
#include <primitives/transaction.h>
#include <test/fuzz/fuzz.h>
#include <test/fuzz/FuzzedDataProvider.h>
#include <test/fuzz/util.h>

#include <assert.h>
#include <optional>
#include <memory>
#include <stdint.h>
#include <vector>

namespace {

/** Number of distinct COutPoint values used in this test. */
constexpr uint32_t NUM_OUTPOINTS = 256;
/** Number of distinct Coin values used in this test (ignoring nHeight). */
constexpr uint32_t NUM_COINS = 256;
/** Maximum number CCoinsViewCache objects used in this test. */
constexpr uint32_t MAX_CACHES = 4;
/** Data type large enough to hold NUM_COINS-1. */
using coinidx_type = uint8_t;

struct PrecomputedData
{
    //! Randomly generated COutPoint values.
    COutPoint outpoints[NUM_OUTPOINTS];

    //! Randomly generated Coin values.
    Coin coins[NUM_COINS];

    PrecomputedData()
    {
        static const uint8_t PREFIX_O[1] = {'o'}; /** Hash prefix for outpoint hashes. */
        static const uint8_t PREFIX_S[1] = {'s'}; /** Hash prefix for coins scriptPubKeys. */
        static const uint8_t PREFIX_M[1] = {'m'}; /** Hash prefix for coins nValue/fCoinBase. */

        for (uint32_t i = 0; i < NUM_OUTPOINTS; ++i) {
            uint32_t idx = (i * 1200U) >> 12; /* Map 3 or 4 entries to same txid. */
            const uint8_t ser[4] = {uint8_t(idx), uint8_t(idx >> 8), uint8_t(idx >> 16), uint8_t(idx >> 24)};
            uint256 txid;
            CSHA256().Write(PREFIX_O, 1).Write(ser, sizeof(ser)).Finalize(txid.begin());
            outpoints[i].hash = Txid::FromUint256(txid);
            outpoints[i].n = i;
        }

        for (uint32_t i = 0; i < NUM_COINS; ++i) {
            const uint8_t ser[4] = {uint8_t(i), uint8_t(i >> 8), uint8_t(i >> 16), uint8_t(i >> 24)};
            uint256 hash;
            CSHA256().Write(PREFIX_S, 1).Write(ser, sizeof(ser)).Finalize(hash.begin());
            /* Convert hash to scriptPubkeys (of different lengths, so SanityCheck's cached memory
             * usage check has a chance to detect mismatches). */
            switch (i % 5U) {
            case 0: /* P2PKH */
                coins[i].out.scriptPubKey.resize(25);
                coins[i].out.scriptPubKey[0] = OP_DUP;
                coins[i].out.scriptPubKey[1] = OP_HASH160;
                coins[i].out.scriptPubKey[2] = 20;
                std::copy(hash.begin(), hash.begin() + 20, coins[i].out.scriptPubKey.begin() + 3);
                coins[i].out.scriptPubKey[23] = OP_EQUALVERIFY;
                coins[i].out.scriptPubKey[24] = OP_CHECKSIG;
                break;
            case 1: /* P2SH */
                coins[i].out.scriptPubKey.resize(23);
                coins[i].out.scriptPubKey[0] = OP_HASH160;
                coins[i].out.scriptPubKey[1] = 20;
                std::copy(hash.begin(), hash.begin() + 20, coins[i].out.scriptPubKey.begin() + 2);
                coins[i].out.scriptPubKey[12] = OP_EQUAL;
                break;
            case 2: /* P2WPKH */
                coins[i].out.scriptPubKey.resize(22);
                coins[i].out.scriptPubKey[0] = OP_0;
                coins[i].out.scriptPubKey[1] = 20;
                std::copy(hash.begin(), hash.begin() + 20, coins[i].out.scriptPubKey.begin() + 2);
                break;
            case 3: /* P2WSH */
                coins[i].out.scriptPubKey.resize(34);
                coins[i].out.scriptPubKey[0] = OP_0;
                coins[i].out.scriptPubKey[1] = 32;
                std::copy(hash.begin(), hash.begin() + 32, coins[i].out.scriptPubKey.begin() + 2);
                break;
            case 4: /* P2TR */
                coins[i].out.scriptPubKey.resize(34);
                coins[i].out.scriptPubKey[0] = OP_1;
                coins[i].out.scriptPubKey[1] = 32;
                std::copy(hash.begin(), hash.begin() + 32, coins[i].out.scriptPubKey.begin() + 2);
                break;
            }
            /* Hash again to construct nValue and fCoinBase. */
            CSHA256().Write(PREFIX_M, 1).Write(ser, sizeof(ser)).Finalize(hash.begin());
            coins[i].out.nValue = CAmount(hash.GetUint64(0) % MAX_MONEY);
            coins[i].fCoinBase = (hash.GetUint64(1) & 7) == 0;
            coins[i].nHeight = 0; /* Real nHeight used in simulation is set dynamically. */
        }
    }
};

enum class EntryType : uint8_t
{
    /* This entry in the cache does not exist (so we'd have to look in the parent cache). */
    NONE,

    /* This entry in the cache corresponds to an unspent coin. */
    UNSPENT,

    /* This entry in the cache corresponds to a spent coin. */
    SPENT,
};

struct CacheEntry
{
    /* Type of entry. */
    EntryType entrytype;

    /* Index in the coins array this entry corresponds to (only if entrytype == UNSPENT). */
    coinidx_type coinidx;

    /* nHeight value for this entry (so the coins[coinidx].nHeight value is ignored; only if entrytype == UNSPENT). */
    uint32_t height;
};

struct CacheLevel
{
    CacheEntry entry[NUM_OUTPOINTS];

    void Wipe() {
        for (uint32_t i = 0; i < NUM_OUTPOINTS; ++i) {
            entry[i].entrytype = EntryType::NONE;
        }
    }
};

/** Class for the base of the hierarchy (roughly simulating a memory-backed CCoinsViewDB).
 *
 * The initial state consists of the empty UTXO set.
 * Coins whose output index is 4 (mod 5) have GetCoin() always succeed after being spent.
 * This exercises code paths with spent, non-DIRTY cache entries.
 */
class CoinsViewBottom final : public CCoinsView
{
    std::map<COutPoint, Coin> m_data;

public:
    std::optional<Coin> GetCoin(const COutPoint& outpoint) const final
    {
        // TODO GetCoin shouldn't return spent coins
        if (auto it = m_data.find(outpoint); it != m_data.end()) return it->second;
        return std::nullopt;
    }

    bool HaveCoin(const COutPoint& outpoint) const final
    {
        return m_data.count(outpoint);
    }

    uint256 GetBestBlock() const final { return {}; }
    std::vector<uint256> GetHeadBlocks() const final { return {}; }
    std::unique_ptr<CCoinsViewCursor> Cursor() const final { return {}; }
    size_t EstimateSize() const final { return m_data.size(); }

    bool BatchWrite(CoinsViewCacheCursor& cursor, const uint256&) final
    {
        for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) {
            if (it->second.IsDirty()) {
                if (it->second.coin.IsSpent() && (it->first.n % 5) != 4) {
                    m_data.erase(it->first);
                } else if (cursor.WillErase(*it)) {
                    m_data[it->first] = std::move(it->second.coin);
                } else {
                    m_data[it->first] = it->second.coin;
                }
            } else {
                /* For non-dirty entries being written, compare them with what we have. */
                auto it2 = m_data.find(it->first);
                if (it->second.coin.IsSpent()) {
                    assert(it2 == m_data.end() || it2->second.IsSpent());
                } else {
                    assert(it2 != m_data.end());
                    assert(it->second.coin.out == it2->second.out);
                    assert(it->second.coin.fCoinBase == it2->second.fCoinBase);
                    assert(it->second.coin.nHeight == it2->second.nHeight);
                }
            }
        }
        return true;
    }
};

} // namespace

FUZZ_TARGET(coinscache_sim)
{
    /** Precomputed COutPoint and CCoins values. */
    static const PrecomputedData data;

    /** Dummy coinsview instance (base of the hierarchy). */
    CoinsViewBottom bottom;
    /** Real CCoinsViewCache objects. */
    std::vector<std::unique_ptr<CCoinsViewCache>> caches;
    /** Simulated cache data (sim_caches[0] matches bottom, sim_caches[i+1] matches caches[i]). */
    CacheLevel sim_caches[MAX_CACHES + 1];
    /** Current height in the simulation. */
    uint32_t current_height = 1U;

    // Initialize bottom simulated cache.
    sim_caches[0].Wipe();

    /** Helper lookup function in the simulated cache stack. */
    auto lookup = [&](uint32_t outpointidx, int sim_idx = -1) -> std::optional<std::pair<coinidx_type, uint32_t>> {
        uint32_t cache_idx = sim_idx == -1 ? caches.size() : sim_idx;
        while (true) {
            const auto& entry = sim_caches[cache_idx].entry[outpointidx];
            if (entry.entrytype == EntryType::UNSPENT) {
                return {{entry.coinidx, entry.height}};
            } else if (entry.entrytype == EntryType::SPENT) {
                return std::nullopt;
            };
            if (cache_idx == 0) break;
            --cache_idx;
        }
        return std::nullopt;
    };

    /** Flush changes in top cache to the one below. */
    auto flush = [&]() {
        assert(caches.size() >= 1);
        auto& cache = sim_caches[caches.size()];
        auto& prev_cache = sim_caches[caches.size() - 1];
        for (uint32_t outpointidx = 0; outpointidx < NUM_OUTPOINTS; ++outpointidx) {
            if (cache.entry[outpointidx].entrytype != EntryType::NONE) {
                prev_cache.entry[outpointidx] = cache.entry[outpointidx];
                cache.entry[outpointidx].entrytype = EntryType::NONE;
            }
        }
    };

    // Main simulation loop: read commands from the fuzzer input, and apply them
    // to both the real cache stack and the simulation.
    FuzzedDataProvider provider(buffer.data(), buffer.size());
    LIMITED_WHILE(provider.remaining_bytes(), 10000) {
        // Every operation (except "Change height") moves current height forward,
        // so it functions as a kind of epoch, making ~all UTXOs unique.
        ++current_height;
        // Make sure there is always at least one CCoinsViewCache.
        if (caches.empty()) {
            caches.emplace_back(new CCoinsViewCache(&bottom, /*deterministic=*/true));
            sim_caches[caches.size()].Wipe();
        }

        // Execute command.
        CallOneOf(
            provider,

            [&]() { // GetCoin
                uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                // Look up in simulation data.
                auto sim = lookup(outpointidx);
                // Look up in real caches.
                auto realcoin = caches.back()->GetCoin(data.outpoints[outpointidx]);
                // Compare results.
                if (!sim.has_value()) {
                    assert(!realcoin || realcoin->IsSpent());
                } else {
                    assert(realcoin && !realcoin->IsSpent());
                    const auto& simcoin = data.coins[sim->first];
                    assert(realcoin->out == simcoin.out);
                    assert(realcoin->fCoinBase == simcoin.fCoinBase);
                    assert(realcoin->nHeight == sim->second);
                }
            },

            [&]() { // HaveCoin
                uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                // Look up in simulation data.
                auto sim = lookup(outpointidx);
                // Look up in real caches.
                auto real = caches.back()->HaveCoin(data.outpoints[outpointidx]);
                // Compare results.
                assert(sim.has_value() == real);
            },

            [&]() { // HaveCoinInCache
                uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                // Invoke on real cache (there is no equivalent in simulation, so nothing to compare result with).
                (void)caches.back()->HaveCoinInCache(data.outpoints[outpointidx]);
            },

            [&]() { // AccessCoin
                uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                // Look up in simulation data.
                auto sim = lookup(outpointidx);
                // Look up in real caches.
                const auto& realcoin = caches.back()->AccessCoin(data.outpoints[outpointidx]);
                // Compare results.
                if (!sim.has_value()) {
                    assert(realcoin.IsSpent());
                } else {
                    assert(!realcoin.IsSpent());
                    const auto& simcoin = data.coins[sim->first];
                    assert(simcoin.out == realcoin.out);
                    assert(simcoin.fCoinBase == realcoin.fCoinBase);
                    assert(realcoin.nHeight == sim->second);
                }
            },

            [&]() { // AddCoin (only possible_overwrite if necessary)
                uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                uint32_t coinidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_COINS - 1);
                // Look up in simulation data (to know whether we must set possible_overwrite or not).
                auto sim = lookup(outpointidx);
                // Invoke on real caches.
                Coin coin = data.coins[coinidx];
                coin.nHeight = current_height;
                caches.back()->AddCoin(data.outpoints[outpointidx], std::move(coin), sim.has_value());
                // Apply to simulation data.
                auto& entry = sim_caches[caches.size()].entry[outpointidx];
                entry.entrytype = EntryType::UNSPENT;
                entry.coinidx = coinidx;
                entry.height = current_height;
            },

            [&]() { // AddCoin (always possible_overwrite)
                uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                uint32_t coinidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_COINS - 1);
                // Invoke on real caches.
                Coin coin = data.coins[coinidx];
                coin.nHeight = current_height;
                caches.back()->AddCoin(data.outpoints[outpointidx], std::move(coin), true);
                // Apply to simulation data.
                auto& entry = sim_caches[caches.size()].entry[outpointidx];
                entry.entrytype = EntryType::UNSPENT;
                entry.coinidx = coinidx;
                entry.height = current_height;
            },

            [&]() { // SpendCoin (moveto = nullptr)
                uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                // Invoke on real caches.
                caches.back()->SpendCoin(data.outpoints[outpointidx], nullptr);
                // Apply to simulation data.
                sim_caches[caches.size()].entry[outpointidx].entrytype = EntryType::SPENT;
            },

            [&]() { // SpendCoin (with moveto)
                uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                // Look up in simulation data (to compare the returned *moveto with).
                auto sim = lookup(outpointidx);
                // Invoke on real caches.
                Coin realcoin;
                caches.back()->SpendCoin(data.outpoints[outpointidx], &realcoin);
                // Apply to simulation data.
                sim_caches[caches.size()].entry[outpointidx].entrytype = EntryType::SPENT;
                // Compare *moveto with the value expected based on simulation data.
                if (!sim.has_value()) {
                    assert(realcoin.IsSpent());
                } else {
                    assert(!realcoin.IsSpent());
                    const auto& simcoin = data.coins[sim->first];
                    assert(simcoin.out == realcoin.out);
                    assert(simcoin.fCoinBase == realcoin.fCoinBase);
                    assert(realcoin.nHeight == sim->second);
                }
            },

            [&]() { // Uncache
                uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                // Apply to real caches (there is no equivalent in our simulation).
                caches.back()->Uncache(data.outpoints[outpointidx]);
            },

            [&]() { // Add a cache level (if not already at the max).
                if (caches.size() != MAX_CACHES) {
                    // Apply to real caches.
                    caches.emplace_back(new CCoinsViewCache(&*caches.back(), /*deterministic=*/true));
                    // Apply to simulation data.
                    sim_caches[caches.size()].Wipe();
                }
            },

            [&]() { // Remove a cache level.
                // Apply to real caches (this reduces caches.size(), implicitly doing the same on the simulation data).
                caches.back()->SanityCheck();
                caches.pop_back();
            },

            [&]() { // Flush.
                // Apply to simulation data.
                flush();
                // Apply to real caches.
                caches.back()->Flush();
            },

            [&]() { // Sync.
                // Apply to simulation data (note that in our simulation, syncing and flushing is the same thing).
                flush();
                // Apply to real caches.
                caches.back()->Sync();
            },

            [&]() { // Flush + ReallocateCache.
                // Apply to simulation data.
                flush();
                // Apply to real caches.
                caches.back()->Flush();
                caches.back()->ReallocateCache();
            },

            [&]() { // GetCacheSize
                (void)caches.back()->GetCacheSize();
            },

            [&]() { // DynamicMemoryUsage
                (void)caches.back()->DynamicMemoryUsage();
            },

            [&]() { // Change height
                current_height = provider.ConsumeIntegralInRange<uint32_t>(1, current_height - 1);
            }
        );
    }

    // Sanity check all the remaining caches
    for (const auto& cache : caches) {
        cache->SanityCheck();
    }

    // Full comparison between caches and simulation data, from bottom to top,
    // as AccessCoin on a higher cache may affect caches below it.
    for (unsigned sim_idx = 1; sim_idx <= caches.size(); ++sim_idx) {
        auto& cache = *caches[sim_idx - 1];
        size_t cache_size = 0;

        for (uint32_t outpointidx = 0; outpointidx < NUM_OUTPOINTS; ++outpointidx) {
            cache_size += cache.HaveCoinInCache(data.outpoints[outpointidx]);
            const auto& real = cache.AccessCoin(data.outpoints[outpointidx]);
            auto sim = lookup(outpointidx, sim_idx);
            if (!sim.has_value()) {
                assert(real.IsSpent());
            } else {
                assert(!real.IsSpent());
                assert(real.out == data.coins[sim->first].out);
                assert(real.fCoinBase == data.coins[sim->first].fCoinBase);
                assert(real.nHeight == sim->second);
            }
        }

        // HaveCoinInCache ignores spent coins, so GetCacheSize() may exceed it. */
        assert(cache.GetCacheSize() >= cache_size);
    }

    // Compare the bottom coinsview (not a CCoinsViewCache) with sim_cache[0].
    for (uint32_t outpointidx = 0; outpointidx < NUM_OUTPOINTS; ++outpointidx) {
        auto realcoin = bottom.GetCoin(data.outpoints[outpointidx]);
        auto sim = lookup(outpointidx, 0);
        if (!sim.has_value()) {
            assert(!realcoin || realcoin->IsSpent());
        } else {
            assert(realcoin && !realcoin->IsSpent());
            assert(realcoin->out == data.coins[sim->first].out);
            assert(realcoin->fCoinBase == data.coins[sim->first].fCoinBase);
            assert(realcoin->nHeight == sim->second);
        }
    }
}