From 2777300c68f575565dabdcef4d0498ebc91a401e Mon Sep 17 00:00:00 2001
From: Robin David <dev.robin.david@gmail.com>
Date: Tue, 12 Aug 2025 16:41:40 +0200
Subject: [PATCH] fuzz: Implement connect_block harness

Co-authored-by: marcofleon <marleo23@proton.me>
---
 src/test/fuzz/CMakeLists.txt    |   1 +
 src/test/fuzz/connect_block.cpp | 480 ++++++++++++++++++++++++++++++++
 2 files changed, 481 insertions(+)
 create mode 100644 src/test/fuzz/connect_block.cpp

diff --git a/src/test/fuzz/CMakeLists.txt b/src/test/fuzz/CMakeLists.txt
index 7888f149008..b2e88303fb6 100644
--- a/src/test/fuzz/CMakeLists.txt
+++ b/src/test/fuzz/CMakeLists.txt
@@ -29,6 +29,7 @@ add_executable(fuzz
   cmpctblock.cpp
   coins_view.cpp
   coinscache_sim.cpp
+  connect_block.cpp
   connman.cpp
   crypto.cpp
   crypto_aes256.cpp
diff --git a/src/test/fuzz/connect_block.cpp b/src/test/fuzz/connect_block.cpp
new file mode 100644
index 00000000000..7c70dce173c
--- /dev/null
+++ b/src/test/fuzz/connect_block.cpp
@@ -0,0 +1,480 @@
+// Copyright (c) 2026-present 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 <addresstype.h>
+#include <chain.h>
+#include <consensus/amount.h>
+#include <consensus/merkle.h>
+#include <node/kernel_notifications.h>
+#include <node/mining_types.h>
+#include <primitives/block.h>
+#include <primitives/transaction.h>
+#include <pubkey.h>
+#include <script/interpreter.h>
+#include <script/script.h>
+#include <sync.h>
+#include <test/fuzz/FuzzedDataProvider.h>
+#include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
+#include <test/util/mining.h>
+#include <test/util/script.h>
+#include <test/util/setup_common.h>
+#include <test/util/time.h>
+#include <txmempool.h>
+#include <uint256.h>
+#include <validation.h>
+#include <validationinterface.h>
+
+#include <algorithm>
+#include <cstdint>
+#include <memory>
+#include <utility>
+#include <vector>
+
+
+namespace {
+
+TestingSetup* g_setup;
+
+/** Vector of blocks to keep references to blocks (to enable fuzzing input to pick one to build upon) */
+static std::vector<std::shared_ptr<CBlock>> g_blocks;
+/** CTxIns for spending outputs, which can be unspent, already spent, or an immature coinbase. */
+static std::vector<CTxIn> g_spend_candidate_txins;
+/** Static P2SH_OP_TRUE script */
+static const CScript P2SH_OP_TRUE = CScript() << OP_HASH160 << ToByteVector(ScriptHash(CScript() << OP_TRUE)) << OP_EQUAL;
+/** Static P2SH_OP_TRUE unlock script */
+static const CScript P2SH_OP_TRUE_UNLOCK = CScript() << MakeUCharSpan(CScript() << OP_TRUE);
+/** Static TAPROOT_OP_TRUE script and its witness */
+static CScript TAPROOT_OP_TRUE;
+static std::vector<std::vector<uint8_t>> TAPROOT_OP_TRUE_WITNESS;
+
+/**
+ * Initialize TAPROOT_OP_TRUE and TAPROOT_OP_TRUE_WITNESS static variables.
+ */
+static void InitTaprootScript()
+{
+    uint256 merkle_tree_hash = ComputeTapleafHash(TAPROOT_LEAF_TAPSCRIPT, MakeUCharSpan(CScript() << OP_TRUE));
+    uint256 internal_key{std::vector<uint8_t>(32, 1)};
+    auto res = XOnlyPubKey(internal_key).CreateTapTweak(&merkle_tree_hash);
+    Assert(res.has_value());
+    auto control = ToByteVector(internal_key);
+    control.insert(control.begin(), TAPROOT_LEAF_TAPSCRIPT | (res->second ? 1 : 0));
+
+    TAPROOT_OP_TRUE = CScript() << OP_1 << ToByteVector(res->first);
+    TAPROOT_OP_TRUE_WITNESS.clear();
+    TAPROOT_OP_TRUE_WITNESS.emplace_back(ToByteVector(CScript() << OP_TRUE));
+    TAPROOT_OP_TRUE_WITNESS.emplace_back(std::move(control));
+}
+
+/**
+ * Given a transaction and an output index, create a CTxIn that can be
+ * used to spend it.
+ */
+static CTxIn GetSpendingScript(const CTransaction& tx, uint32_t vout_index)
+{
+    Assert(vout_index < tx.vout.size());
+    const CTxOut& output = tx.vout[vout_index];
+
+    CTxIn res{COutPoint(tx.GetHash(), vout_index)};
+    if (output.scriptPubKey == P2WSH_OP_TRUE) {
+        res.scriptSig = CScript();
+        res.scriptWitness.stack.push_back(WITNESS_STACK_ELEM_OP_TRUE);
+    } else if (output.scriptPubKey == P2SH_OP_TRUE) {
+        res.scriptSig = P2SH_OP_TRUE_UNLOCK;
+    } else if (output.scriptPubKey == CScript()) {
+        res.scriptSig = CScript() << OP_TRUE;
+    } else if (output.scriptPubKey == TAPROOT_OP_TRUE) {
+        res.scriptSig = CScript();
+        res.scriptWitness.stack = TAPROOT_OP_TRUE_WITNESS;
+    }
+
+    return res;
+}
+
+/**
+ * Add a spend candidate CTxIn unless the output is unspendable.
+ */
+static void MaybeAddSpendCandidate(std::vector<CTxIn>& pool, const CTransaction& tx, uint32_t vout_index)
+{
+    Assert(vout_index < tx.vout.size());
+    if (tx.vout[vout_index].scriptPubKey.IsUnspendable()) return;
+    pool.push_back(GetSpendingScript(tx, vout_index));
+}
+
+
+/**
+ * Read the block from the BlockManager and add it to g_blocks.
+ */
+static void LoadCurrentBlock(Chainstate& chainstate, CBlockIndex* current_block)
+{
+    // Read the block from the BlockManager.
+    Assert(current_block->nHeight >= 0);
+    // Resize g_blocks if needed.
+    if (g_blocks.size() <= (size_t)current_block->nHeight) {
+        g_blocks.resize(current_block->nHeight + 1);
+    }
+
+    g_blocks[current_block->nHeight] = std::make_shared<CBlock>();
+    Assert(chainstate.m_blockman.ReadBlock(*g_blocks[current_block->nHeight], *current_block));
+
+    // Iterate all transaction outputs.
+    for (const auto& tx : g_blocks[current_block->nHeight]->vtx) {
+        for (uint32_t vout_index{0}; vout_index < tx->vout.size(); ++vout_index) {
+            MaybeAddSpendCandidate(g_spend_candidate_txins, *tx, vout_index);
+        }
+    }
+}
+
+/**
+ * Read the Chainstate object into g_blocks.
+ * Then fill g_spend_candidate_txins with inputs that can be tried by the target.
+ */
+static void LoadCurrentChain()
+{
+    // Clear existing data.
+    g_blocks.clear();
+    g_spend_candidate_txins.clear();
+
+    {
+        LOCK(::cs_main);
+        // Retrieve the current chainstate.
+        auto& chainstate = Assert(g_setup->m_node.chainman)->ActiveChainstate();
+        // Make sure it contains a valid mempool.
+        Assert(chainstate.GetMempool());
+
+        // Traverse the chain from tip to genesis.
+        auto current_block = chainstate.m_chain.Tip();
+
+        while (current_block != nullptr) {
+            LoadCurrentBlock(chainstate, current_block);
+            // Move to previous block.
+            current_block = current_block->pprev;
+        }
+    }
+
+    // Reverse the order of g_spend_candidate_txins to have them in ascending order of
+    // block height.
+    std::reverse(g_spend_candidate_txins.begin(), g_spend_candidate_txins.end());
+}
+
+
+/**
+ * Reset the chainman in the testing setup object.
+ * Mine 2*COINBASE_MATURITY blocks to have spendable UTXOs.
+ * It is called once in the initialization function.
+ */
+void ResetChainman(TestingSetup& setup)
+{
+    SetMockTime(setup.m_node.chainman->GetParams().GenesisBlock().Time());
+    setup.m_node.chainman.reset();
+    setup.m_node.notifications->m_shutdown_on_fatal_error = false;
+    setup.m_make_chainman();
+    setup.LoadVerifyActivateChainstate();
+
+    for (int i = 0; i < 2 * COINBASE_MATURITY; i++) {
+        node::BlockCreateOptions options;
+        options.coinbase_output_script = P2WSH_OP_TRUE;
+        MineBlock(setup.m_node, options);
+    }
+    setup.m_node.validation_signals->SyncWithValidationInterfaceQueue();
+}
+
+/** Create additional transactions in the mempool that spend
+ * coins from mature blocks. Otherwise the mined chain only contains
+ * coinbase transactions.
+ */
+void AddExtraTxsToMempool(TestingSetup& setup)
+{
+    Assert(setup.m_node.chainman->ActiveChainstate().GetMempool()->size() == 0);
+    for (size_t i = 1; i <= 10; i++) {
+        CMutableTransaction ctx;
+        ctx.version = CTransaction::CURRENT_VERSION;
+        ctx.vin.resize(1);
+        // CTxIn is spendable as g_spend_candidate_txins comes from early blocks whose
+        // coinbases are mature.
+        ctx.vin[0] = g_spend_candidate_txins[i];
+        ctx.vout.resize(4);
+        // Arbitrarily create various outputs of different kinds in the same tx.
+        // P2WSH
+        ctx.vout[0].nValue = CAmount(15 * COIN);
+        ctx.vout[0].scriptPubKey = P2WSH_OP_TRUE;
+        // P2SH
+        ctx.vout[1].nValue = CAmount(15 * COIN);
+        ctx.vout[1].scriptPubKey = P2SH_OP_TRUE;
+        // Taproot
+        ctx.vout[2].nValue = CAmount(10 * COIN);
+        ctx.vout[2].scriptPubKey = TAPROOT_OP_TRUE;
+        // Empty script
+        ctx.vout[3].nValue = CAmount(10 * COIN);
+        ctx.vout[3].scriptPubKey = CScript();
+
+        LOCK(::cs_main);
+        // Add transaction to the mempool.
+        const MempoolAcceptResult ctx_result = setup.m_node.chainman->ProcessTransaction(MakeTransactionRef(ctx));
+        Assert(ctx_result.m_result_type == MempoolAcceptResult::ResultType::VALID);
+
+        Assert(setup.m_node.chainman->ActiveChainstate().GetMempool()->size() == i);
+        // Force the mempool to select this transaction even though its fee is zero.
+        setup.m_node.chainman->ActiveChainstate().GetMempool()->PrioritiseTransaction(ctx.GetHash(), COIN);
+    }
+}
+
+/** Initialize the chain for this target. */
+static void initialize_connect_block()
+{
+    // Instantiate REGTEST chain.
+    static auto testing_setup = MakeNoLogFileContext<TestingSetup>(
+        /*chain_type=*/ChainType::REGTEST, TestOpts{
+                                               .extra_args = {
+                                                   "-minrelaytxfee=0",
+                                                   "-acceptnonstdtxn",
+                                               },
+                                           });
+    g_setup = testing_setup.get();
+
+    // Reset the chainman in the testing setup object.
+    ResetChainman(*g_setup);
+
+    // Initialize Taproot script declared as static variables.
+    InitTaprootScript();
+
+    // Load the chain mined in ResetChainman in global variables g_blocks and
+    // g_spend_candidate_txins, to make them available to pick by the target.
+    LoadCurrentChain();
+
+    // Prepare multiple transactions for block 201. They spend coins
+    // from various coinbases that are now mature enough.
+    AddExtraTxsToMempool(*g_setup);
+    // Mine block 201, which contains the transactions added to the mempool.
+    node::BlockCreateOptions options;
+    options.coinbase_output_script = P2WSH_OP_TRUE;
+    MineBlock(g_setup->m_node, options);
+    Assert(g_setup->m_node.chainman->ActiveChainstate().GetMempool()->size() == 0);
+
+    // Load the 201st block into g_blocks.
+    LOCK(::cs_main);
+    auto& chainstate = Assert(g_setup->m_node.chainman)->ActiveChainstate();
+    auto current_block = chainstate.m_chain.Tip();
+    LoadCurrentBlock(chainstate, current_block);
+}
+
+/**
+ * Read one transaction from the fuzzing input through the FuzzedDataProvider.
+ * It is intended to leave more space to craft complex transactions, especially
+ * with various script types (P2SH, P2WSH, TAPROOT, NOSCRIPT).
+ * It is exclusively used by ConsumeBlock to read transactions inside a block.
+ */
+CTransactionRef ConsumeTransaction(FuzzedDataProvider& fuzzed_data_provider,
+                                   std::vector<CTxIn>& additional_txins,
+                                   bool coinbase = false,
+                                   int target_height = 0)
+{
+    CMutableTransaction tx;
+    tx.version = fuzzed_data_provider.ConsumeBool() ?
+                     CTransaction::CURRENT_VERSION :
+                     fuzzed_data_provider.ConsumeIntegral<uint32_t>();
+    tx.nLockTime = fuzzed_data_provider.ConsumeBool() ?
+                       0 :
+                       fuzzed_data_provider.ConsumeIntegral<uint32_t>();
+
+    // Some harnesses want to explicitly read coinbase transactions from input.
+    if (coinbase) {
+        // vin size is hardcoded.
+        tx.vin.resize(1);
+        tx.vin[0].prevout.SetNull();
+        if (fuzzed_data_provider.ConsumeBool()) {
+            // 1/2 probability of a valid vin.
+            tx.vin[0].scriptSig = CScript() << target_height;
+        } else {
+            // Read arbitrary data from input as scriptSig.
+            auto script_sig = ConsumeRandomLengthByteVector<unsigned char>(fuzzed_data_provider, 100);
+            tx.vin[0].scriptSig.assign(script_sig.begin(), script_sig.end());
+        }
+    } else {
+        // Read a normal transaction, with up to 10 inputs.
+        int num_inputs = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, 10);
+        tx.vin.resize(num_inputs);
+        for (int i = 0; i < num_inputs; i++) {
+            // Read an integer to choose a CTxIn or reuse one generated by the
+            // input. The content of the CTxIn is not read from the input per se.
+            uint32_t input_index = fuzzed_data_provider.ConsumeIntegralInRange<uint32_t>(0, g_spend_candidate_txins.size() + additional_txins.size() - 1);
+            if (input_index < g_spend_candidate_txins.size()) {
+                // Pick it from the spend candidates.
+                tx.vin[i] = g_spend_candidate_txins[input_index];
+            } else {
+                // Pick it in the additional_txins set.
+                Assert((input_index - g_spend_candidate_txins.size()) < additional_txins.size());
+                tx.vin[i] = additional_txins[input_index - g_spend_candidate_txins.size()];
+            }
+
+            // Enable the fuzzer to mutate every CTxIn field after it is taken
+            // from the spend candidates.
+            if (fuzzed_data_provider.ConsumeBool()) {
+                tx.vin[i].nSequence = ConsumeSequence(fuzzed_data_provider);
+            }
+            if (fuzzed_data_provider.ConsumeBool()) {
+                tx.vin[i].prevout.n = fuzzed_data_provider.ConsumeIntegral<uint32_t>();
+            }
+            if (fuzzed_data_provider.ConsumeBool()) {
+                tx.vin[i].prevout.hash = Txid::FromUint256(ConsumeUInt256(fuzzed_data_provider));
+            }
+            if (fuzzed_data_provider.ConsumeBool()) {
+                tx.vin[i].scriptSig = ConsumeScript(fuzzed_data_provider);
+            }
+            if (fuzzed_data_provider.ConsumeBool()) {
+                tx.vin[i].scriptWitness.stack.clear();
+                int num_wit = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, 10);
+                for (int j = 0; j < num_wit; j++) {
+                    tx.vin[i].scriptWitness.stack.push_back(ConsumeRandomLengthByteVector<unsigned char>(fuzzed_data_provider, 100));
+                }
+            }
+        }
+    }
+
+    // Read outputs.
+    int num_outputs = fuzzed_data_provider.ConsumeIntegralInRange<int>(1, 10);
+    tx.vout.resize(num_outputs);
+    for (int i = 0; i < num_outputs; i++) {
+        // Read CAmount to spend.
+        tx.vout[i].nValue = fuzzed_data_provider.ConsumeIntegralInRange<CAmount>(-10, 50 * COIN + 10);
+
+        // Read scriptPubKey type into one of the valid types.
+        CallOneOf(
+            fuzzed_data_provider,
+            [&] {
+                // P2WSH
+                tx.vout[i].scriptPubKey = P2WSH_OP_TRUE;
+            },
+            [&] {
+                // P2SH
+                tx.vout[i].scriptPubKey = P2SH_OP_TRUE;
+            },
+            [&] {
+                // Taproot
+                tx.vout[i].scriptPubKey = TAPROOT_OP_TRUE;
+            },
+            [&] {
+                // Empty script
+                tx.vout[i].scriptPubKey = CScript();
+            },
+            [&] {
+                // Read arbitrary scriptPubKey.
+                tx.vout[i].scriptPubKey = ConsumeScript(fuzzed_data_provider);
+            });
+    }
+
+    // Create the shared pointer to the CTransaction object.
+    auto res = MakeTransactionRef(tx);
+
+    if (!coinbase) {
+        // Create spending scripts for CTxOuts so they can be spent in later
+        // transactions. Do it here as the transaction hash is definitive.
+        for (int i = 0; i < num_outputs; i++) {
+            MaybeAddSpendCandidate(additional_txins, *res, i);
+        }
+    }
+
+    return res;
+}
+
+/**
+ * Consume a block from the fuzzing input.
+ * It builds a block on top of the given prev_block.
+ */
+CBlock ConsumeBlock(FuzzedDataProvider& fuzzed_data_provider, const CBlock& prev_block, int target_height,
+                    std::vector<CTxIn>& additional_txins)
+{
+    CBlock block;
+
+    // Initialize header fields.
+    block.nVersion = g_blocks.back()->nVersion;
+    block.hashPrevBlock = prev_block.GetHash();
+    block.nTime = g_blocks.back()->nTime + 2;
+    block.nBits = g_blocks.back()->nBits;
+
+    // Give the fuzzer input the ability to mutate block header fields.
+    if (fuzzed_data_provider.ConsumeBool()) {
+        block.nVersion = fuzzed_data_provider.ConsumeIntegral<int32_t>();
+    }
+    if (fuzzed_data_provider.ConsumeBool()) {
+        block.hashPrevBlock = ConsumeUInt256(fuzzed_data_provider);
+    }
+
+    if (fuzzed_data_provider.ConsumeBool()) {
+        block.nTime = fuzzed_data_provider.ConsumeIntegral<uint32_t>();
+    }
+    if (fuzzed_data_provider.ConsumeBool()) {
+        block.nBits = fuzzed_data_provider.ConsumeIntegral<uint32_t>();
+    }
+
+    // Read the coinbase transaction from the input.
+    block.vtx.push_back(ConsumeTransaction(fuzzed_data_provider, additional_txins, true, target_height));
+
+    // Read up to num_tx transactions from the input.
+    int num_tx = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, 5);
+    for (int i = 0; i < num_tx; i++) {
+        block.vtx.push_back(ConsumeTransaction(fuzzed_data_provider, additional_txins));
+    }
+
+    // Commit witness.
+    if (fuzzed_data_provider.ConsumeBool()) {
+        g_setup->m_node.chainman->GenerateCoinbaseCommitment(block, nullptr);
+    }
+
+    // Set hashMerkleRoot to expected value.
+    block.hashMerkleRoot = BlockMerkleRoot(block);
+    // Let the fuzzer mutate hashMerkleRoot.
+    if (fuzzed_data_provider.ConsumeBool()) {
+        block.hashMerkleRoot = ConsumeUInt256(fuzzed_data_provider);
+    }
+
+    // Read the nonce from the input.
+    block.nNonce = fuzzed_data_provider.ConsumeIntegral<uint32_t>();
+
+    return block;
+}
+
+
+FUZZ_TARGET(connect_block, .init = initialize_connect_block)
+{
+    SeedRandomStateForTest(SeedRand::ZEROS);
+    FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
+    FakeNodeClock clock{g_blocks.back()->Time() + 2s};
+
+    LOCK(::cs_main);
+    g_setup->m_node.chainman->m_validation_cache.m_script_execution_cache.TestOnlyReset();
+    Chainstate& active_chainstate = g_setup->m_node.chainman->ActiveChainstate();
+    CBlockIndex* active_tip = active_chainstate.m_chain.Tip();
+    Assert(active_tip->GetBlockHash() == g_blocks.back()->GetHash());
+    CCoinsViewCache active_coins(&active_chainstate.CoinsTip());
+
+    // Read a new block from the data provider.
+    std::vector<CTxIn> additional_txins;
+    CBlock block = ConsumeBlock(fuzzed_data_provider, *g_blocks.back(), active_tip->nHeight + 1, additional_txins);
+
+    // Duplicate a transaction (not the coinbase) from the previous block
+    // to hit the BIP30 check.
+    if (fuzzed_data_provider.ConsumeBool()) {
+        const auto& duplicates = g_blocks.back()->vtx;
+        block.vtx.push_back(duplicates[fuzzed_data_provider.ConsumeIntegralInRange<size_t>(1, duplicates.size() - 1)]);
+    }
+
+    // Compute new CBlockIndex object.
+    uint256 current_hash = block.GetHash();
+    CBlockIndex new_index(block);
+    new_index.pprev = active_tip;
+    new_index.nHeight = active_tip->nHeight + 1;
+    new_index.phashBlock = &current_hash;
+
+    // Try to connect the block.
+    BlockValidationState state;
+    bool connected = active_chainstate.ConnectBlock(block,
+                                                    state,
+                                                    &new_index,
+                                                    active_coins,
+                                                    /*fJustCheck=*/true);
+    Assert(connected == state.IsValid());
+}
+
+} // namespace