From 49544ea9432a13285846aca6d191f01e8ed00655 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Tue, 3 Nov 2020 19:34:35 -0800
Subject: [PATCH] Fuzz fallback formatter

---
 test/fuzzing/float.cc | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/test/fuzzing/float.cc b/test/fuzzing/float.cc
index a9a347ef..073e4bcd 100644
--- a/test/fuzzing/float.cc
+++ b/test/fuzzing/float.cc
@@ -9,26 +9,31 @@
 
 #include "fuzzer-common.h"
 
-extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
-  if (size <= sizeof(double) || !std::numeric_limits<double>::is_iec559)
-    return 0;
-
-  auto value = assign_from_buf<double>(data);
+void check_round_trip(fmt::string_view format_str, double value) {
   auto buffer = fmt::memory_buffer();
-  fmt::format_to(buffer, "{}", value);
+  fmt::format_to(buffer, format_str, value);
 
-  // Check a round trip.
   if (std::isnan(value)) {
     auto nan = std::signbit(value) ? "-nan" : "nan";
     if (fmt::string_view(buffer.data(), buffer.size()) != nan)
       throw std::runtime_error("round trip failure");
-    return 0;
+    return;
   }
+
   buffer.push_back('\0');
   char* ptr = nullptr;
   if (std::strtod(buffer.data(), &ptr) != value)
     throw std::runtime_error("round trip failure");
   if (ptr + 1 != buffer.end())
     throw std::runtime_error("unparsed output");
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  if (size <= sizeof(double) || !std::numeric_limits<double>::is_iec559)
+    return 0;
+  check_round_trip("{}", assign_from_buf<double>(data));
+  // A larger than necessary precision is used to trigger the fallback
+  // formatter.
+  check_round_trip("{:.50g}", assign_from_buf<double>(data));
   return 0;
 }