fuzz: initial compact block fuzz harness

Adds a fuzz harness cmpctblock to test BIP152 compact block relay.
Currently just sets mock time in a loop.

src/test/fuzz/CMakeLists.txt

@@ -26,6 +26,7 @@ add_executable(fuzz
   chain.cpp
   checkqueue.cpp
   cluster_linearize.cpp
+  cmpctblock.cpp
   coins_view.cpp
   coinscache_sim.cpp
   connman.cpp

src/test/fuzz/cmpctblock.cpp

@@ -0,0 +1,95 @@
+// Copyright (c) 2026 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 <chain.h>
+#include <chainparams.h>
+#include <consensus/consensus.h>
+#include <node/miner.h>
+#include <primitives/block.h>
+#include <primitives/transaction.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/random.h>
+#include <test/util/script.h>
+#include <test/util/setup_common.h>
+#include <test/util/time.h>
+#include <test/util/txmempool.h>
+#include <test/util/validation.h>
+#include <txmempool.h>
+#include <util/check.h>
+#include <util/time.h>
+#include <util/translation.h>
+#include <validation.h>
+#include <validationinterface.h>
+
+#include <cstdint>
+#include <functional>
+#include <memory>
+
+namespace {
+
+TestingSetup* g_setup;
+
+void ResetChainmanAndMempool(TestingSetup& setup)
+{
+    SetMockTime(Params().GenesisBlock().Time());
+
+    bilingual_str error{};
+    setup.m_node.mempool.reset();
+    setup.m_node.mempool = std::make_unique<CTxMemPool>(MemPoolOptionsForTest(setup.m_node), error);
+    Assert(error.empty());
+
+    setup.m_node.chainman.reset();
+    setup.m_make_chainman();
+    setup.LoadVerifyActivateChainstate();
+
+    node::BlockAssembler::Options options;
+    options.coinbase_output_script = P2WSH_OP_TRUE;
+    options.include_dummy_extranonce = true;
+
+    for (int i = 0; i < 2 * COINBASE_MATURITY; ++i) {
+        MineBlock(setup.m_node, options);
+    }
+}
+
+} // namespace
+
+void initialize_cmpctblock()
+{
+    static const auto testing_setup = MakeNoLogFileContext<TestingSetup>();
+    g_setup = testing_setup.get();
+    ResetChainmanAndMempool(*g_setup);
+}
+
+FUZZ_TARGET(cmpctblock, .init = initialize_cmpctblock)
+{
+    SeedRandomStateForTest(SeedRand::ZEROS);
+    FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
+
+    NodeClockContext clock_ctx{1610000000s};
+
+    auto setup = g_setup;
+    auto& chainman = static_cast<TestChainstateManager&>(*setup->m_node.chainman);
+    chainman.ResetIbd();
+    chainman.DisableNextWrite();
+
+    LIMITED_WHILE(fuzzed_data_provider.ConsumeBool(), 1000)
+    {
+        CallOneOf(
+            fuzzed_data_provider,
+            [&]() {
+                // Set mock time randomly or to tip's time.
+                if (fuzzed_data_provider.ConsumeBool()) {
+                    clock_ctx.set(ConsumeTime(fuzzed_data_provider));
+                } else {
+                    const NodeSeconds tip_time = WITH_LOCK(::cs_main, return chainman.ActiveChain().Tip()->Time());
+                    clock_ctx.set(tip_time);
+                }
+            });
+    }
+}