test/fuzz: Invoke LoadMempool via CChainState

Not only does this increase coverage, it is also more correct in that
when ::LoadMempool is called with a mempool and chainstate, it calls
AcceptToMemoryPool with just the chainstate.

AcceptToMemoryPool will then act on the chainstate's mempool via
CChainState::GetMempool, which may be different from the mempool
originally passed to ::LoadMempool. (In this fuzz test's case, it
definitely is different)

Also, move DummyChainstate to its own file since it's now used by the
validation_load_mempool fuzz test to replace CChainState's m_mempool.

src/Makefile.test_fuzz.include

@@ -10,6 +10,7 @@ EXTRA_LIBRARIES += \
 TEST_FUZZ_H = \
     test/fuzz/fuzz.h \
     test/fuzz/FuzzedDataProvider.h \
+    test/fuzz/mempool_utils.h \
     test/fuzz/util.h
 
 libtest_fuzz_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(MINIUPNPC_CPPFLAGS) $(NATPMP_CPPFLAGS) $(EVENT_CFLAGS) $(EVENT_PTHREADS_CFLAGS)

src/test/fuzz/mempool_utils.h

@@ -0,0 +1,19 @@
+// Copyright (c) 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.
+
+#ifndef BITCOIN_TEST_FUZZ_MEMPOOL_UTILS_H
+#define BITCOIN_TEST_FUZZ_MEMPOOL_UTILS_H
+
+#include <validation.h>
+
+class DummyChainState final : public CChainState
+{
+public:
+    void SetMempool(CTxMemPool* mempool)
+    {
+        m_mempool = mempool;
+    }
+};
+
+#endif // BITCOIN_TEST_FUZZ_MEMPOOL_UTILS_H

src/test/fuzz/tx_pool.cpp

@@ -8,6 +8,7 @@
 #include <node/miner.h>
 #include <test/fuzz/FuzzedDataProvider.h>
 #include <test/fuzz/fuzz.h>
+#include <test/fuzz/mempool_utils.h>
 #include <test/fuzz/util.h>
 #include <test/util/mining.h>
 #include <test/util/script.h>
@@ -34,15 +35,6 @@ struct MockedTxPool : public CTxMemPool {
     }
 };
 
-class DummyChainState final : public CChainState
-{
-public:
-    void SetMempool(CTxMemPool* mempool)
-    {
-        m_mempool = mempool;
-    }
-};
-
 void initialize_tx_pool()
 {
     static const auto testing_setup = MakeNoLogFileContext<const TestingSetup>();

src/test/fuzz/validation_load_mempool.cpp

@@ -7,6 +7,7 @@
 #include <node/mempool_persist_args.h>
 #include <test/fuzz/FuzzedDataProvider.h>
 #include <test/fuzz/fuzz.h>
+#include <test/fuzz/mempool_utils.h>
 #include <test/fuzz/util.h>
 #include <test/util/setup_common.h>
 #include <txmempool.h>
@@ -36,9 +37,12 @@ FUZZ_TARGET_INIT(validation_load_mempool, initialize_validation_load_mempool)
 
     CTxMemPool pool{MemPoolOptionsForTest(g_setup->m_node)};
 
+    auto& chainstate{static_cast<DummyChainState&>(g_setup->m_node.chainman->ActiveChainstate())};
+    chainstate.SetMempool(&pool);
+
     auto fuzzed_fopen = [&](const fs::path&, const char*) {
         return fuzzed_file_provider.open();
     };
-    (void)LoadMempool(pool, g_setup->m_node.chainman->ActiveChainstate(), fuzzed_fopen);
+    (void)chainstate.LoadMempool(g_setup->m_args, fuzzed_fopen);
     (void)DumpMempool(pool, MempoolPath(g_setup->m_args), fuzzed_fopen, true);
 }

src/validation.cpp

@@ -3865,11 +3865,11 @@ void PruneBlockFilesManual(CChainState& active_chainstate, int nManualPruneHeigh
     }
 }
 
-void CChainState::LoadMempool(const ArgsManager& args)
+void CChainState::LoadMempool(const ArgsManager& args, FopenFn mockable_fopen_function)
 {
     if (!m_mempool) return;
     if (args.GetBoolArg("-persistmempool", DEFAULT_PERSIST_MEMPOOL)) {
-        ::LoadMempool(*m_mempool, *this);
+        ::LoadMempool(*m_mempool, *this, mockable_fopen_function);
     }
     m_mempool->SetLoadTried(!ShutdownRequested());
 }

src/validation.h

@@ -679,7 +679,7 @@ public:
     void CheckBlockIndex();
 
     /** Load the persisted mempool from disk */
-    void LoadMempool(const ArgsManager& args);
+    void LoadMempool(const ArgsManager& args, fsbridge::FopenFn mockable_fopen_function = fsbridge::fopen);
 
     /** Update the chain tip based on database information, i.e. CoinsTip()'s best block. */
     bool LoadChainTip() EXCLUSIVE_LOCKS_REQUIRED(cs_main);