Add simulation-based CCoinsViewCache fuzzer

The fuzzer goes through a sequence of operations that get applied to both a
real stack of CCoinsViewCache objects, and to simulation data, comparing
the two at the end.

2 files changed, 462 insertions, 0 deletions

diff --git a/src/Makefile.test.include b/src/Makefile.test.include
index 4d867fdc2f..d6992640ff 100644
--- a/src/Makefile.test.include
+++ b/src/Makefile.test.include
@@ -248,6 +248,7 @@ test_fuzz_fuzz_SOURCES = \
  test/fuzz/chain.cpp \
  test/fuzz/checkqueue.cpp \
  test/fuzz/coins_view.cpp \
+ test/fuzz/coinscache_sim.cpp \
  test/fuzz/connman.cpp \
  test/fuzz/crypto.cpp \
  test/fuzz/crypto_aes256.cpp \
diff --git a/src/test/fuzz/coinscache_sim.cpp b/src/test/fuzz/coinscache_sim.cpp
new file mode 100644
index 0000000000..c3f732f075
--- /dev/null
+++ b/src/test/fuzz/coinscache_sim.cpp
@@ -0,0 +1,461 @@
+// 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)};
+            CSHA256().Write(PREFIX_O, 1).Write(ser, sizeof(ser)).Finalize(outpoints[i].hash.begin());
+            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. */
+            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). */
+class CoinsViewBottom final : public CCoinsView
+{
+    std::map<COutPoint, Coin> m_data;
+
+public:
+    bool GetCoin(const COutPoint& outpoint, Coin& coin) const final
+    {
+        auto it = m_data.find(outpoint);
+        if (it == m_data.end()) {
+            return false;
+        } else {
+            coin = it->second;
+            return true;
+        }
+    }
+
+    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(CCoinsMap& data, const uint256&, bool erase) final
+    {
+        for (auto it = data.begin(); it != data.end(); it = erase ? data.erase(it) : std::next(it)) {
+            if (it->second.flags & CCoinsCacheEntry::DIRTY) {
+                if (it->second.coin.IsSpent()) {
+                    m_data.erase(it->first);
+                } else if (erase) {
+                    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. */
+                if (it->second.coin.IsSpent()) {
+                    assert(m_data.count(it->first) == 0);
+                } else {
+                    auto it2 = m_data.find(it->first);
+                    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));
+            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.
+                Coin realcoin;
+                auto real = caches.back()->GetCoin(data.outpoints[outpointidx], realcoin);
+                // Compare results.
+                if (!sim.has_value()) {
+                    assert(!real);
+                } else {
+                    assert(!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()));
+                    // 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.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);
+            }
+        );
+    }
+
+    // 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) {
+        Coin realcoin;
+        bool real = bottom.GetCoin(data.outpoints[outpointidx], realcoin);
+        auto sim = lookup(outpointidx, 0);
+        if (!sim.has_value()) {
+            assert(!real);
+        } else {
+            assert(!realcoin.IsSpent());
+            assert(realcoin.out == data.coins[sim->first].out);
+            assert(realcoin.fCoinBase == data.coins[sim->first].fCoinBase);
+            assert(realcoin.nHeight == sim->second);
+        }
+    }
+}