Merge bitcoin/bitcoin#26930: fuzz: Actually use mocked mempool in tx_pool target

9ab62d71fb [fuzz] Actually use mocked mempool in tx_pool target (dergoegge)

Pull request description:

  The current tx_pool target uses the default mempool, making the target non-deterministic. This PR replaces the active chainstate's mempool (i.e. the node's default mempool) with the already present mocked mempool in the target.

ACKs for top commit:
  fanquake:
    ACK 9ab62d71fb

Tree-SHA512: fe9af3dbdd13cb569fdc2ddbb4290b5ce94206ae83d94267c6365ed0ee9bbe072fcfe7fd632a1a8522dce44608e89aba2f398c1e20bd250484bbadb78143320c

src/test/fuzz/tx_pool.cpp

@@ -311,7 +311,7 @@ FUZZ_TARGET_INIT(tx_pool, initialize_tx_pool)
 {
     FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
     const auto& node = g_setup->m_node;
-    auto& chainstate = node.chainman->ActiveChainstate();
+    auto& chainstate{static_cast<DummyChainState&>(node.chainman->ActiveChainstate())};
 
     MockTime(fuzzed_data_provider, chainstate);
 
@@ -329,6 +329,8 @@ FUZZ_TARGET_INIT(tx_pool, initialize_tx_pool)
     CTxMemPool tx_pool_{MakeMempool(fuzzed_data_provider, node)};
     MockedTxPool& tx_pool = *static_cast<MockedTxPool*>(&tx_pool_);
 
+    chainstate.SetMempool(&tx_pool);
+
     LIMITED_WHILE(fuzzed_data_provider.ConsumeBool(), 300)
     {
         const auto mut_tx = ConsumeTransaction(fuzzed_data_provider, txids);