Work around MSVC issue with std::atomic initialization (#4777)

* Work around MSVC issue with std::atomic initialization

MSVC seems to have a bug where it does not use constant initialization
for std::atomic, which ends up causing crashes during initialization.
This change introduces a workaround by putting the std::atomic inside a
union, which causes the compiler to use constant initialization for it.

* Added an AppVeyor test for static linking with MSVC

1 files changed, 9 insertions, 0 deletions

diff --git a/src/google/protobuf/generated_message_util.h b/src/google/protobuf/generated_message_util.h
index c26e3e52..706df383 100644
--- a/src/google/protobuf/generated_message_util.h
+++ b/src/google/protobuf/generated_message_util.h
@@ -335,7 +335,16 @@ struct LIBPROTOBUF_EXPORT SCCInfoBase {
     kRunning = 1,
     kUninitialized = -1,  // initial state
   };
+#ifndef _MSC_VER
   std::atomic<int> visit_status;
+#else
+  // MSVC doesnt make std::atomic constant initialized. This union trick
+  // makes it so.
+  union {
+    int visit_status_to_make_linker_init;
+    std::atomic<int> visit_status;
+  };
+#endif
   int num_deps;
   void (*init_func)();
   // This is followed by an array  of num_deps