Fix string size for error message generated by windows_category (#966)

Due to the initial allocation of the error message, the returned std::string had a size of 4096. Since FormatMessageW already returns the number of characters written to the buffer an additional call to resize using the returned count will neatly trim the string.

Release/src/utilities/asyncrt_utils.cpp

@@ -265,8 +265,7 @@ std::string windows_category_impl::message(int errorCode) const CPPREST_NOEXCEPT
     }
 #endif
 
-    std::wstring buffer;
-    buffer.resize(buffer_size);
+    std::wstring buffer(buffer_size, 0);
 
     const auto result = ::FormatMessageW(
         dwFlags,
@@ -277,11 +276,15 @@ std::string windows_category_impl::message(int errorCode) const CPPREST_NOEXCEPT
         buffer_size,
         NULL);
 
+
     if (result == 0)
     {
         return "Unable to get an error message for error code: " + std::to_string(errorCode) + ".";
     }
 
+    // strip exceeding characters of the initial resize call
+    buffer.resize(result);
+
     return utility::conversions::to_utf8string(buffer);
 }
 

Release/tests/functional/utils/strings.cpp

@@ -373,6 +373,17 @@ TEST(scan_string_locale, "Ignore:Android", "Locale unsupported on Android")
     }
 }
 
+
+#ifdef _WIN32
+TEST(windows_category_message)
+{
+   // Ensure the error message string returned by windows_category doesn't contain trailing zeros.
+   std::string error_message = utility::details::windows_category().message( 0 );
+   std::string zero_terminated_copy = error_message.c_str();
+   VERIFY_ARE_EQUAL( zero_terminated_copy, error_message );
+}
+#endif // _WIN32
+
 }
     
 }}} //namespaces