Provide a way to intercept Win32 structured exceptions in async tasks

Something about the stacks created by boost::context prevents global structured exception handlers
from being called.  This allows the user to register a handler which will be called when there
is an unhandled structured exception in async task.

CMakeLists.txt

@@ -174,8 +174,8 @@ IF(WIN32)
     # Needed to disable MSVC autolinking feature (#pragma comment)
     BOOST_ALL_NO_LIB
   )
-  # Activate C++ exception handling inc. SEH to catch GPFs
+  # Activate C++ exception handling, assume extern C calls don't throw
-  target_compile_options(fc PUBLIC /EHa)
+  target_compile_options(fc PUBLIC /EHsc)
 ELSE()
   SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99 -Wall" )
 

include/fc/thread/task.hpp

@@ -33,6 +33,8 @@ namespace fc {
       void*         _functor;
       void          (*_destroy_functor)(void*);
       void          (*_run_functor)(void*, void* );
+
+      void          run_impl(); 
   };
 
   namespace detail {

include/fc/thread/thread.hpp

@@ -175,5 +175,29 @@ namespace fc {
    auto async( Functor&& f, const char* desc ="", priority prio = priority()) -> fc::future<decltype(f())> {
       return fc::thread::current().async( fc::forward<Functor>(f), desc, prio );
    }
+
+} // end namespace fc
+
+#ifdef _MSC_VER
+struct _EXCEPTION_POINTERS;
+
+namespace fc {
+   /* There's something about the setup of the stacks created for fc::async tasks
+    * that screws up the global structured exception filters installed by
+    * SetUnhandledExceptionFilter().  The only way I've found to catch an 
+    * unhaldned structured exception thrown in an async task is to put a 
+    * __try/__except block inside the async task.
+    * We do just that, and if a SEH escapes outside the function running 
+    * in the async task, fc will call an exception filter privided by 
+    * set_unhandled_structured_exception_filter(), passing as arguments
+    * the result of GetExceptionCode() and GetExceptionInformation().
+    *
+    * Right now there is only one global exception filter, used for any 
+    * async task.
+    */
+   typedef int (*unhandled_exception_filter_type)(unsigned, _EXCEPTION_POINTERS*);
+   void set_unhandled_structured_exception_filter(unhandled_exception_filter_type new_filter);
+   unhandled_exception_filter_type get_unhandled_structured_exception_filter();
+#endif
 }
 

src/thread/task.cpp

@@ -7,6 +7,11 @@
 #include <fc/log/logger.hpp>
 #include <boost/exception/all.hpp>
 
+#ifdef _MSC_VER
+# include <fc/thread/thread.hpp>
+# include <Windows.h>
+#endif
+
 namespace fc {
   task_base::task_base(void* func)
   :
@@ -18,6 +23,17 @@ namespace fc {
   }
 
   void task_base::run() {
+#ifdef _MSC_VER
+    __try {
+#endif
+      run_impl();
+#ifdef _MSC_VER
+    } __except (get_unhandled_structured_exception_filter() ? get_unhandled_structured_exception_filter()(GetExceptionCode(), GetExceptionInformation()) : EXCEPTION_CONTINUE_SEARCH) {
+       ExitProcess(1);
+    }
+#endif
+  }
+  void task_base::run_impl() {
     try {
       _run_functor( _functor, _promise_impl  );
     } 

src/thread/thread.cpp

@@ -381,5 +381,19 @@ namespace fc {
       return this == &current();
     }
 
-
+#ifdef _MSC_VER
-}
+    /* support for providing a structured exception handler for async tasks */
+    namespace detail
+    {
+      unhandled_exception_filter_type unhandled_structured_exception_filter = nullptr;
+    }
+    void set_unhandled_structured_exception_filter(unhandled_exception_filter_type new_filter)
+    {
+       detail::unhandled_structured_exception_filter = new_filter;
+    }
+    unhandled_exception_filter_type get_unhandled_structured_exception_filter()
+    {
+       return detail::unhandled_structured_exception_filter;
+    }
+#endif // _MSC_VER
+} // end namespace fc