Merge #19415: net: Make DNS lookup mockable, add fuzzing harness

e528075189 tests: Add fuzzing harness for Lookup(...)/LookupHost(...)/LookupNumeric(...)/LookupSubNet(...) (practicalswift)
c6b4bfb4b3 net: Make DNS lookup code testable (practicalswift)

Pull request description:

  Make DNS lookup mockable/testable/fuzzable.

  Add fuzzing harness for `Lookup(…)`/`LookupHost(…)`/`LookupNumeric(…)`/`LookupSubNet(…)`.

  See [`doc/fuzzing.md`](https://github.com/bitcoin/bitcoin/blob/master/doc/fuzzing.md) for information on how to fuzz Bitcoin Core. Don't forget to contribute any coverage increasing inputs you find to the [Bitcoin Core fuzzing corpus repo](https://github.com/bitcoin-core/qa-assets).

  Happy fuzzing :)

ACKs for top commit:
  Crypt-iQ:
    cr ACK e528075189
  vasild:
    ACK e528075189

Tree-SHA512: 9984c2e2fedc3c1e1c3dbd701bb739ebd2f01766e6e83543dae5ae43eb8646c452bba0e101dd2f06079e5258bd5846c7d27a60ed5d77c1682b54c9544ffad443

src/test/fuzz/netbase_dns_lookup.cpp

@@ -0,0 +1,77 @@
+// Copyright (c) 2021 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 <netaddress.h>
+#include <netbase.h>
+#include <test/fuzz/FuzzedDataProvider.h>
+#include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace {
+FuzzedDataProvider* fuzzed_data_provider_ptr = nullptr;
+
+std::vector<CNetAddr> fuzzed_dns_lookup_function(const std::string& name, bool allow_lookup)
+{
+    std::vector<CNetAddr> resolved_addresses;
+    while (fuzzed_data_provider_ptr->ConsumeBool()) {
+        resolved_addresses.push_back(ConsumeNetAddr(*fuzzed_data_provider_ptr));
+    }
+    return resolved_addresses;
+}
+} // namespace
+
+FUZZ_TARGET(netbase_dns_lookup)
+{
+    FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
+    fuzzed_data_provider_ptr = &fuzzed_data_provider;
+    const std::string name = fuzzed_data_provider.ConsumeRandomLengthString(512);
+    const unsigned int max_results = fuzzed_data_provider.ConsumeIntegral<unsigned int>();
+    const bool allow_lookup = fuzzed_data_provider.ConsumeBool();
+    const int default_port = fuzzed_data_provider.ConsumeIntegral<int>();
+    {
+        std::vector<CNetAddr> resolved_addresses;
+        if (LookupHost(name, resolved_addresses, max_results, allow_lookup, fuzzed_dns_lookup_function)) {
+            for (const CNetAddr& resolved_address : resolved_addresses) {
+                assert(!resolved_address.IsInternal());
+            }
+        }
+        assert(resolved_addresses.size() <= max_results || max_results == 0);
+    }
+    {
+        CNetAddr resolved_address;
+        if (LookupHost(name, resolved_address, allow_lookup, fuzzed_dns_lookup_function)) {
+            assert(!resolved_address.IsInternal());
+        }
+    }
+    {
+        std::vector<CService> resolved_services;
+        if (Lookup(name, resolved_services, default_port, allow_lookup, max_results, fuzzed_dns_lookup_function)) {
+            for (const CNetAddr& resolved_service : resolved_services) {
+                assert(!resolved_service.IsInternal());
+            }
+        }
+        assert(resolved_services.size() <= max_results || max_results == 0);
+    }
+    {
+        CService resolved_service;
+        if (Lookup(name, resolved_service, default_port, allow_lookup, fuzzed_dns_lookup_function)) {
+            assert(!resolved_service.IsInternal());
+        }
+    }
+    {
+        CService resolved_service = LookupNumeric(name, default_port, fuzzed_dns_lookup_function);
+        assert(!resolved_service.IsInternal());
+    }
+    {
+        CSubNet resolved_subnet;
+        if (LookupSubNet(name, resolved_subnet, fuzzed_dns_lookup_function)) {
+            assert(resolved_subnet.IsValid());
+        }
+    }
+    fuzzed_data_provider_ptr = nullptr;
+}