From aabec3cbca699f238e9b78226c834b9789fdbbb6 Mon Sep 17 00:00:00 2001
From: Christian <43907599+chris0x44@users.noreply.github.com>
Date: Sun, 11 Nov 2018 01:35:07 +0100
Subject: [PATCH] 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    |  7 +++++--
 Release/tests/functional/utils/strings.cpp | 11 +++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/Release/src/utilities/asyncrt_utils.cpp b/Release/src/utilities/asyncrt_utils.cpp
index 9f9e7f852f..0bafacd514 100644
--- a/Release/src/utilities/asyncrt_utils.cpp
+++ b/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);
 }
 
diff --git a/Release/tests/functional/utils/strings.cpp b/Release/tests/functional/utils/strings.cpp
index 19d1b43cfd..8f6957f61a 100644
--- a/Release/tests/functional/utils/strings.cpp
+++ b/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