fuzz: parse the command line arguments in fuzz tests

Retrieve the command line arguments from the fuzzer and save them for
later retrieval by `BasicTestingSetup` so that we gain extra flexibility
of passing any config options on the test command line, e.g.:

```
FUZZ=addrman ./src/test/fuzz/fuzz --checkaddrman=5
```

A fuzz test should call `MakeNoLogFileContext<>()` in its initialize
function in order to invoke the constructor of `BasicTestingSetup`,
which sets `gArgs`.

Author: vasild

doc/fuzzing.md

@@ -71,6 +71,15 @@ block^@M-^?M-^?M-^?M-^?M-^?nM-^?M-^?
 
 In this case the fuzzer managed to create a `block` message which when passed to `ProcessMessage(...)` increased coverage.
 
+It is possible to specify `bitcoind` arguments to the `fuzz` executable.
+Depending on the test, they may be ignored or consumed and alter the behavior
+of the test. Just make sure to use double-dash to distinguish them from the
+fuzzer's own arguments:
+
+```sh
+$ FUZZ=address_deserialize_v2 src/test/fuzz/fuzz -runs=1 fuzz_seed_corpus/address_deserialize_v2 --checkaddrman=5 --printtoconsole=1
+```
+
 ## Fuzzing corpora
 
 The project's collection of seed corpora is found in the [`bitcoin-core/qa-assets`](https://github.com/bitcoin-core/qa-assets) repo.

src/test/fuzz/fuzz.cpp

@@ -20,7 +20,28 @@
 
 const std::function<void(const std::string&)> G_TEST_LOG_FUN{};
 
-const std::function<std::vector<const char*>()> G_TEST_COMMAND_LINE_ARGUMENTS{};
+/**
+ * A copy of the command line arguments that start with `--`.
+ * First `LLVMFuzzerInitialize()` is called, which saves the arguments to `g_args`.
+ * Later, depending on the fuzz test, `G_TEST_COMMAND_LINE_ARGUMENTS()` may be
+ * called by `BasicTestingSetup` constructor to fetch those arguments and store
+ * them in `BasicTestingSetup::m_node::args`.
+ */
+static std::vector<const char*> g_args;
+
+static void SetArgs(int argc, char** argv) {
+    for (int i = 1; i < argc; ++i) {
+        // Only take into account arguments that start with `--`. The others are for the fuzz engine:
+        // `fuzz -runs=1 fuzz_seed_corpus/address_deserialize_v2 --checkaddrman=5`
+        if (strlen(argv[i]) > 2 && argv[i][0] == '-' && argv[i][1] == '-') {
+            g_args.push_back(argv[i]);
+        }
+    }
+}
+
+const std::function<std::vector<const char*>()> G_TEST_COMMAND_LINE_ARGUMENTS = []() {
+    return g_args;
+};
 
 std::map<std::string_view, std::tuple<TypeTestOneInput, TypeInitialize, TypeHidden>>& FuzzTargets()
 {
@@ -98,6 +119,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 // This function is used by libFuzzer
 extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv)
 {
+    SetArgs(*argc, *argv);
     initialize();
     return 0;
 }