From 370d532a00581ca79c87af7d7811e56de0ca52a8 Mon Sep 17 00:00:00 2001
From: Chris Mumford <cmumford@google.com>
Date: Tue, 1 Oct 2019 13:00:35 -0700
Subject: [PATCH] Using CMake's check_cxx_compiler_flag to check support for
 -Wthread-safety.
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Previously used check_cxx_source_compiles to attempt a
build to determine support for clang thread safety checks.

This change is to support static analysis of the leveldb source by
lgtm.com (using Semmle). It failed to build with the following error:

```
[2019-07-04 22:29:58] [build] c++: error: unrecognized command line option ‘-Wthread-safety’; did you mean ‘-fthread-jumps’?
[2019-07-04 22:30:02] [build] make[2]: *** [CMakeFiles/leveldb.dir/build.make:66: CMakeFiles/leveldb.dir/db/builder.cc.o] Error 1
```

PiperOrigin-RevId: 272275528
---
 CMakeLists.txt | 21 ++++-----------------
 1 file changed, 4 insertions(+), 17 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 13ebbc9..a65afbf 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -73,25 +73,12 @@ else(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-rtti")
 endif(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
 
-include(CheckCXXSourceCompiles)
-
 # Test whether -Wthread-safety is available. See
 # https://clang.llvm.org/docs/ThreadSafetyAnalysis.html
-# -Werror is necessary because unknown attributes only generate warnings.
-set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-list(APPEND CMAKE_REQUIRED_FLAGS -Werror -Wthread-safety)
-check_cxx_source_compiles("
-struct __attribute__((lockable)) Lock {
-  void Acquire() __attribute__((exclusive_lock_function()));
-  void Release() __attribute__((unlock_function()));
-};
-struct ThreadSafeType {
-  Lock lock_;
-  int data_ __attribute__((guarded_by(lock_)));
-};
-int main() { return 0; }
-"  HAVE_CLANG_THREAD_SAFETY)
-set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+include(CheckCXXCompilerFlag)
+check_cxx_compiler_flag(-Wthread-safety HAVE_CLANG_THREAD_SAFETY)
+
+include(CheckCXXSourceCompiles)
 
 # Test whether C++17 __has_include is available.
 check_cxx_source_compiles("