[test util] to populate mempool with random transactions/packages

src/test/util/setup_common.cpp

@@ -43,6 +43,7 @@
 #include <validationinterface.h>
 #include <walletinitinterface.h>
 
+#include <algorithm>
 #include <functional>
 #include <stdexcept>
 
@@ -357,6 +358,52 @@ CMutableTransaction TestChain100Setup::CreateValidMempoolTransaction(CTransactio
     return mempool_txn;
 }
 
+std::vector<CTransactionRef> TestChain100Setup::PopulateMempool(FastRandomContext& det_rand, size_t num_transactions, bool submit)
+{
+    std::vector<CTransactionRef> mempool_transactions;
+    std::deque<std::pair<COutPoint, CAmount>> unspent_prevouts;
+    std::transform(m_coinbase_txns.begin(), m_coinbase_txns.end(), std::back_inserter(unspent_prevouts),
+        [](const auto& tx){ return std::make_pair(COutPoint(tx->GetHash(), 0), tx->vout[0].nValue); });
+    while (num_transactions > 0 && !unspent_prevouts.empty()) {
+        // The number of inputs and outputs are random, between 1 and 24.
+        CMutableTransaction mtx = CMutableTransaction();
+        const size_t num_inputs = det_rand.randrange(24) + 1;
+        CAmount total_in{0};
+        for (size_t n{0}; n < num_inputs; ++n) {
+            if (unspent_prevouts.empty()) break;
+            const auto& [prevout, amount] = unspent_prevouts.front();
+            mtx.vin.push_back(CTxIn(prevout, CScript()));
+            total_in += amount;
+            unspent_prevouts.pop_front();
+        }
+        const size_t num_outputs = det_rand.randrange(24) + 1;
+        // Approximately 1000sat "fee," equal output amounts.
+        const CAmount amount_per_output = (total_in - 1000) / num_outputs;
+        for (size_t n{0}; n < num_outputs; ++n) {
+            CScript spk = CScript() << CScriptNum(num_transactions + n);
+            mtx.vout.push_back(CTxOut(amount_per_output, spk));
+        }
+        CTransactionRef ptx = MakeTransactionRef(mtx);
+        mempool_transactions.push_back(ptx);
+        if (amount_per_output > 2000) {
+            // If the value is high enough to fund another transaction + fees, keep track of it so
+            // it can be used to build a more complex transaction graph. Insert randomly into
+            // unspent_prevouts for extra randomness in the resulting structures.
+            for (size_t n{0}; n < num_outputs; ++n) {
+                unspent_prevouts.push_back(std::make_pair(COutPoint(ptx->GetHash(), n), amount_per_output));
+                std::swap(unspent_prevouts.back(), unspent_prevouts[det_rand.randrange(unspent_prevouts.size())]);
+            }
+        }
+        if (submit) {
+            LOCK2(m_node.mempool->cs, cs_main);
+            LockPoints lp;
+            m_node.mempool->addUnchecked(CTxMemPoolEntry(ptx, 1000, 0, 1, false, 4, lp));
+        }
+        --num_transactions;
+    }
+    return mempool_transactions;
+}
+
 CTxMemPoolEntry TestMemPoolEntryHelper::FromTx(const CMutableTransaction& tx) const
 {
     return FromTx(MakeTransactionRef(tx));

src/test/util/setup_common.h

@@ -165,6 +165,19 @@ struct TestChain100Setup : public TestingSetup {
                                                       CAmount output_amount = CAmount(1 * COIN),
                                                       bool submit = true);
 
+    /** Create transactions spending from m_coinbase_txns. These transactions will only spend coins
+     * that exist in the current chain, but may be premature coinbase spends, have missing
+     * signatures, or violate some other consensus rules. They should only be used for testing
+     * mempool consistency. All transactions will have some random number of inputs and outputs
+     * (between 1 and 24). Transactions may or may not be dependent upon each other; if dependencies
+     * exit, every parent will always be somewhere in the list before the child so each transaction
+     * can be submitted in the same order they appear in the list.
+     * @param[in]   submit      When true, submit transactions to the mempool.
+     *                          When false, return them but don't submit them.
+     * @returns A vector of transactions that can be submitted to the mempool.
+     */
+    std::vector<CTransactionRef> PopulateMempool(FastRandomContext& det_rand, size_t num_transactions, bool submit);
+
     std::vector<CTransactionRef> m_coinbase_txns; // For convenience, coinbase transactions
     CKey coinbaseKey; // private/public key needed to spend coinbase transactions
 };