Merge bitcoin/bitcoin#30300: fuzz: have package_rbf always make small txns

4ccb3d6 fuzz: have package_rbf always make small txns (Greg Sanders)

Pull request description:

  hopefully resolves bitcoin/bitcoin#30241

  The fuzz target is generating a large amount of
  transactions, but the core of the logic is
  ConsumeTxMemPoolEntry making the mempool
  entries for adding to the mempool. Since
  ConsumeTxMemPoolEntry generates its own transaction "vsize",
  we can improve efficiency of the target
  by explicitly creating very small transactions,
  reducing the hashing and memory burden.

ACKs for top commit:
  maflcko:
    lgtm ACK 4ccb3d6
  hodlinator:
    ACK 4ccb3d6
  glozow:
    ACK 4ccb3d6

Tree-SHA512: 5d2e7e98460c6144dfe7deac554865e2e8e0e5f934dbdf5857dc4b4f471a64dc933297dc0dcf516f748a4348be6bd184808b7ece17ce073fdcc77f81b74c64de

Author: glozow

src/test/fuzz/rbf.cpp

@@ -91,8 +91,10 @@ FUZZ_TARGET(package_rbf, .init = initialize_package_rbf)
     FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
     SetMockTime(ConsumeTime(fuzzed_data_provider));
 
-    std::optional<CMutableTransaction> child = ConsumeDeserializable<CMutableTransaction>(fuzzed_data_provider, TX_WITH_WITNESS);
-    if (!child) return;
+    // "Real" virtual size is not important for this test since ConsumeTxMemPoolEntry generates its own virtual size values
+    // so we construct small transactions for performance reasons. Child simply needs an input for later to perhaps connect to parent.
+    CMutableTransaction child;
+    child.vin.resize(1);
 
     bilingual_str error;
     CTxMemPool pool{MemPoolOptionsForTest(g_setup->m_node), error};
@@ -113,15 +115,13 @@ FUZZ_TARGET(package_rbf, .init = initialize_package_rbf)
     LIMITED_WHILE(fuzzed_data_provider.ConsumeBool(), NUM_ITERS)
     {
         // Make sure txns only have one input, and that a unique input is given to avoid circular references
-        std::optional<CMutableTransaction> parent = ConsumeDeserializable<CMutableTransaction>(fuzzed_data_provider, TX_WITH_WITNESS);
-        if (!parent) {
-            return;
-        }
+        CMutableTransaction parent;
         assert(iter <= g_outpoints.size());
-        parent->vin.resize(1);
-        parent->vin[0].prevout = g_outpoints[iter++];
+        parent.vin.resize(1);
+        parent.vin[0].prevout = g_outpoints[iter++];
+        parent.vout.emplace_back(0, CScript());
 
-        mempool_txs.emplace_back(*parent);
+        mempool_txs.emplace_back(parent);
         const auto parent_entry = ConsumeTxMemPoolEntry(fuzzed_data_provider, mempool_txs.back());
         running_vsize_total += parent_entry.GetTxSize();
         if (running_vsize_total > std::numeric_limits<int32_t>::max()) {
@@ -130,10 +130,10 @@ FUZZ_TARGET(package_rbf, .init = initialize_package_rbf)
             break;
         }
         pool.addUnchecked(parent_entry);
-        if (fuzzed_data_provider.ConsumeBool() && !child->vin.empty()) {
-            child->vin[0].prevout = COutPoint{mempool_txs.back().GetHash(), 0};
+        if (fuzzed_data_provider.ConsumeBool()) {
+            child.vin[0].prevout = COutPoint{mempool_txs.back().GetHash(), 0};
         }
-        mempool_txs.emplace_back(*child);
+        mempool_txs.emplace_back(child);
         const auto child_entry = ConsumeTxMemPoolEntry(fuzzed_data_provider, mempool_txs.back());
         running_vsize_total += child_entry.GetTxSize();
         if (running_vsize_total > std::numeric_limits<int32_t>::max()) {