fuzz: have package_rbf always make small txns

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.

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()) {