tests: Add fuzzing harness for LoadExternalBlockFile(...) (validation.h)

src/test/fuzz/load_external_block_file.cpp

@@ -0,0 +1,31 @@
+// 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 <chainparams.h>
+#include <flatfile.h>
+#include <test/fuzz/FuzzedDataProvider.h>
+#include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
+#include <test/util/setup_common.h>
+#include <validation.h>
+
+#include <cstdint>
+#include <vector>
+
+void initialize()
+{
+    InitializeFuzzingContext();
+}
+
+void test_one_input(const std::vector<uint8_t>& buffer)
+{
+    FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
+    FuzzedFileProvider fuzzed_file_provider = ConsumeFile(fuzzed_data_provider);
+    FILE* fuzzed_block_file = fuzzed_file_provider.open();
+    if (fuzzed_block_file == nullptr) {
+        return;
+    }
+    FlatFilePos flat_file_pos;
+    LoadExternalBlockFile(Params(), fuzzed_block_file, fuzzed_data_provider.ConsumeBool() ? &flat_file_pos : nullptr);
+}