add fuzzers for chrono timepoint and localtime,gmtime (#2469)

* add fuzzers for chrono timepoint and localtime,gmtime

* reorder cases to improve throughput

mutating the first byte of the input now always results in picking
on of the two paths. Before, two of the four possible outcomes resulted
in doit() not being invoked.

* drop the localtime fuzzer

* inline aliases

* only fuzz std::chrono::system_clock

test/fuzzing/CMakeLists.txt

@@ -25,6 +25,6 @@ function(add_fuzzer source)
   target_compile_features(${name} PRIVATE cxx_generic_lambdas)
 endfunction()
 
-foreach (source chrono-duration.cc float.cc named-arg.cc one-arg.cc two-args.cc)
+foreach (source chrono-duration.cc chrono-timepoint.cc float.cc named-arg.cc one-arg.cc two-args.cc)
   add_fuzzer(${source})
 endforeach ()

test/fuzzing/chrono-timepoint.cc

@@ -0,0 +1,32 @@
+// Copyright (c) 2021, Paul Dreik
+// For license information refer to format.h.
+#include <fmt/chrono.h>
+
+#include "fuzzer-common.h"
+
+/*
+ * a fuzzer for the chrono timepoints formatters
+ * C is a clock (std::chrono::system_clock etc)
+ */
+template <typename C> void doit(const uint8_t* data, size_t size) {
+  using Rep = typename C::time_point::rep;
+  constexpr auto N = sizeof(Rep);
+  if (size < N) return;
+
+  const auto x = assign_from_buf<Rep>(data);
+  typename C::duration dur{x};
+  typename C::time_point timepoint{dur};
+  data += N;
+  size -= N;
+  data_to_string format_str(data, size);
+
+  std::string message = fmt::format(format_str.get(), timepoint);
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  try {
+    doit<std::chrono::system_clock>(data, size);
+  } catch (...) {
+  }
+  return 0;
+}