Fix 1-byte heap overrun

Summary: Fix 1-byte buffer overrun when parsing the string "\t".

Reviewed By: mhlakhani

Differential Revision: D22931605

fbshipit-source-id: a90d0a64b882ed8fef96285d47e11fa082178449

folly/Conv.cpp

@@ -397,9 +397,9 @@ Expected<Tgt, ConversionCode> str_to_floating(StringPiece* src) noexcept {
   auto* e = src->end();
   auto* b =
       std::find_if_not(src->begin(), e, [](char c) { return std::isspace(c); });
-
-  // There must be non-whitespace, otherwise we would have caught this above
-  assert(b < e);
+  if (b == e) {
+    return makeUnexpected(ConversionCode::EMPTY_INPUT_STRING);
+  }
   auto size = size_t(e - b);
 
   bool negative = false;

folly/test/ConvTest.cpp

@@ -969,6 +969,7 @@ TEST(Conv, ConversionErrorStrToFloat) {
   EXPECT_CONV_ERROR_STR_NOVAL(float, StringPiece(), EMPTY_INPUT_STRING);
   EXPECT_CONV_ERROR_STR_NOVAL(float, "", EMPTY_INPUT_STRING);
   EXPECT_CONV_ERROR_STR(float, "  ", EMPTY_INPUT_STRING);
+  EXPECT_CONV_ERROR_STR(float, "\t", EMPTY_INPUT_STRING);
   EXPECT_CONV_ERROR_STR(float, "  junk", STRING_TO_FLOAT_ERROR);
   EXPECT_CONV_ERROR(to<float>("  1bla"), NON_WHITESPACE_AFTER_END, "bla");
 }
@@ -1213,6 +1214,10 @@ TEST(Conv, TryStringToDouble) {
   auto rv2 = folly::tryTo<double>("3.14");
   EXPECT_TRUE(rv2.hasValue());
   EXPECT_NEAR(rv2.value(), 3.14, 1e-10);
+  // No trailing '\0' to expose 1-byte buffer over-read
+  char y = '\t';
+  auto rv4 = folly::tryTo<double>(folly::StringPiece(&y, 1));
+  EXPECT_FALSE(rv4.hasValue());
 }
 
 TEST(Conv, TryIntToInt) {