Merge #17777: tests: Add fuzzing harness for DecodeHexTx(…)

3f95fb085e build: Sort fuzzing harnesses to avoid future merge conflicts (practicalswift)
bcad0144ef tests: Add fuzzing harness for DecodeHexTx(...) (practicalswift)

Pull request description:

  Add fuzzing harness for `DecodeHexTx(…)`.

  To test this PR:

  ```
  $ make distclean
  $ ./autogen.sh
  $ CC=clang CXX=clang++ ./configure --enable-fuzz \
        --with-sanitizers=address,fuzzer,undefined
  $ make
  $ src/test/fuzz/decode_tx
  …
  ```

ACKs for top commit:
  jonatack:
    ACK 3f95fb0

Tree-SHA512: 0f476d0cc26f1e03812664373118754042074bdab6c1e3a57c721f863feb82ca2986cceeaceb03192d893b9aa1d4ad8a5fb4c74824b9547fd8567805931a9ebd

src/test/fuzz/decode_tx.cpp

@@ -0,0 +1,31 @@
+// Copyright (c) 2019 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 <core_io.h>
+#include <primitives/transaction.h>
+#include <test/fuzz/fuzz.h>
+#include <util/strencodings.h>
+
+#include <cassert>
+#include <cstdint>
+#include <string>
+#include <vector>
+
+void test_one_input(const std::vector<uint8_t>& buffer)
+{
+    const std::string tx_hex = HexStr(std::string{buffer.begin(), buffer.end()});
+    CMutableTransaction mtx;
+    const bool result_none = DecodeHexTx(mtx, tx_hex, false, false);
+    const bool result_try_witness = DecodeHexTx(mtx, tx_hex, false, true);
+    const bool result_try_witness_and_maybe_no_witness = DecodeHexTx(mtx, tx_hex, true, true);
+    const bool result_try_no_witness = DecodeHexTx(mtx, tx_hex, true, false);
+    assert(!result_none);
+    if (result_try_witness_and_maybe_no_witness) {
+        assert(result_try_no_witness || result_try_witness);
+    }
+    // if (result_try_no_witness) { // Uncomment when https://github.com/bitcoin/bitcoin/pull/17775 is merged
+    if (result_try_witness) { // Remove stop-gap when https://github.com/bitcoin/bitcoin/pull/17775 is merged
+        assert(result_try_witness_and_maybe_no_witness);
+    }
+}