[Threading][TSan] Fix linkage issue.

We need to pick up the `_swift_tsan_xxx` symbols from libswiftCore in
most cases, but sometimes we're statically linked and in that case we
want to use a local copy.

rdar://1106655213

Author: al45tair

cmake/modules/AddSwiftUnittests.cmake

@@ -58,7 +58,8 @@ function(add_swift_unittest test_dirname)
 
   string(TOUPPER "${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_THREADING_PACKAGE}" _threading_package)
   target_compile_definitions("${test_dirname}" PRIVATE
-    "SWIFT_THREADING_${_threading_package}")
+    "SWIFT_THREADING_${_threading_package}"
+    "SWIFT_THREADING_STATIC")
 
   if(NOT SWIFT_COMPILER_IS_MSVC_LIKE)
     if(SWIFT_USE_LINKER)

include/swift/Threading/ThreadSanitizer.h

@@ -34,11 +34,20 @@ template <typename T> T *release(T *ptr) { return ptr; }
 } // namespace tsan
 #else
 
+// If we're static linking to libswiftThreading.a, these symbols can come
+// from there.  If, on the other hand, we're dynamically linked, we want
+// to get them from libswiftCore.dylib instead.
+#if SWIFT_THREADING_STATIC
+#define SWIFT_THREADING_EXPORT extern "C"
+#else
+#define SWIFT_THREADING_EXPORT SWIFT_RUNTIME_EXPORT
+#endif
+
 namespace threading_impl {
 
-SWIFT_RUNTIME_EXPORT bool _swift_tsan_enabled;
-SWIFT_RUNTIME_EXPORT void (*_swift_tsan_acquire)(const void *ptr);
-SWIFT_RUNTIME_EXPORT void (*_swift_tsan_release)(const void *ptr);
+SWIFT_THREADING_EXPORT bool _swift_tsan_enabled;
+SWIFT_THREADING_EXPORT void (*_swift_tsan_acquire)(const void *ptr);
+SWIFT_THREADING_EXPORT void (*_swift_tsan_release)(const void *ptr);
 
 } // namespace threading_impl
 

lib/Threading/ThreadSanitizer.cpp

@@ -25,12 +25,9 @@
 namespace swift {
 namespace threading_impl {
 
-extern "C" SWIFT_ATTRIBUTE_FOR_EXPORTS
-bool _swift_tsan_enabled = false;
-extern "C" SWIFT_ATTRIBUTE_FOR_EXPORTS
-void (*_swift_tsan_acquire)(const void *) = nullptr;
-extern "C" SWIFT_ATTRIBUTE_FOR_EXPORTS
-void (*_swift_tsan_release)(const void *) = nullptr;
+SWIFT_THREADING_EXPORT bool _swift_tsan_enabled = false;
+SWIFT_THREADING_EXPORT void (*_swift_tsan_acquire)(const void *) = nullptr;
+SWIFT_THREADING_EXPORT void (*_swift_tsan_release)(const void *) = nullptr;
 
 #if __has_include(<dlfcn.h>)
 #include <dlfcn.h>