tests: Add fuzzing harness for IsRBFOptIn(...)

src/test/fuzz/rbf.cpp

@@ -0,0 +1,47 @@
+// Copyright (c) 2020 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <optional.h>
+#include <policy/rbf.h>
+#include <primitives/transaction.h>
+#include <sync.h>
+#include <test/fuzz/FuzzedDataProvider.h>
+#include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
+#include <txmempool.h>
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+void test_one_input(const std::vector<uint8_t>& buffer)
+{
+    FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
+    Optional<CMutableTransaction> mtx = ConsumeDeserializable<CMutableTransaction>(fuzzed_data_provider);
+    if (!mtx) {
+        return;
+    }
+    CTxMemPool pool;
+    while (fuzzed_data_provider.ConsumeBool()) {
+        const Optional<CMutableTransaction> another_mtx = ConsumeDeserializable<CMutableTransaction>(fuzzed_data_provider);
+        if (!another_mtx) {
+            break;
+        }
+        const CTransaction another_tx{*another_mtx};
+        if (fuzzed_data_provider.ConsumeBool() && !mtx->vin.empty()) {
+            mtx->vin[0].prevout = COutPoint{another_tx.GetHash(), 0};
+        }
+        LOCK2(cs_main, pool.cs);
+        pool.addUnchecked(ConsumeTxMemPoolEntry(fuzzed_data_provider, another_tx));
+    }
+    const CTransaction tx{*mtx};
+    if (fuzzed_data_provider.ConsumeBool()) {
+        LOCK2(cs_main, pool.cs);
+        pool.addUnchecked(ConsumeTxMemPoolEntry(fuzzed_data_provider, tx));
+    }
+    {
+        LOCK(pool.cs);
+        (void)IsRBFOptIn(tx, pool);
+    }
+}