Save and restore the SEH state from the main greenlet into new greenl…

…ets.

If we just grab it the first time we're switched to, we get things from the callers stack, which will unwind when it gets switched back to, leading to a corrupt SEH chain.

src/greenlet/greenlet.cpp

@@ -13,9 +13,10 @@
 
 #include "greenlet_internal.hpp"
 #include "greenlet_refs.hpp"
+#include "greenlet_slp_switch.hpp"
 #include "greenlet_thread_state.hpp"
 #include "greenlet_thread_support.hpp"
-#include "greenlet_slp_switch.hpp"
+
 using std::swap;
 using std::cerr;
 using std::endl;
@@ -1258,7 +1259,10 @@ class SwitchingState {
         */
         if (err.status == 1) {
             /* in the new greenlet */
+            // TODO: Move this to its own 'noexcept' function:
+            // C++ exceptions cannot propagate to the parent greenlet from here.
             assert(this->thread_state.borrow_current() == this->target);
+            this->thread_state.restore_exception_state();
             /* stack variables from above are no good and also will not unwind! */
             // EXCEPT: That can't be true, we access run, among others, here.
 

src/greenlet/greenlet_thread_state.hpp

@@ -109,6 +109,9 @@ class ThreadState {
        them. */
     greenlet::g_deleteme_t deleteme;
 
+#ifdef GREENLET_NEEDS_EXCEPTION_STATE_SAVED
+    void* exception_state;
+#endif
 
     static ImmortalObject get_referrers_name;
     static PythonAllocator<ThreadState> allocator;
@@ -150,6 +153,21 @@ class ThreadState {
         if (!get_referrers_name) {
             ThreadState::get_referrers_name = Greenlet_Intern("get_referrers");
         }
+#ifdef GREENLET_NEEDS_EXCEPTION_STATE_SAVED
+        this->exception_state = slp_get_exception_state();
+#endif
+    }
+
+    inline void restore_exception_state()
+    {
+#ifdef GREENLET_NEEDS_EXCEPTION_STATE_SAVED
+        // temp debug
+        fprintf(stderr, "About to restore exception state in new greenlet. Current chain:\n");
+        slp_show_seh_chain();
+        slp_set_exception_state(this->exception_state);
+        fprintf(stderr, "Did restore exception state in new greenlet. Current chain:\n");
+        slp_show_seh_chain();
+#endif
     }
 
     inline bool has_main_greenlet()

src/greenlet/platform/switch_x86_msvc.h

@@ -79,7 +79,21 @@
  *
  * Help would be appreciated.
  */
-#define GREENLET_CANNOT_USE_EXCEPTIONS_NEAR_SWITCH 1
+//#define GREENLET_CANNOT_USE_EXCEPTIONS_NEAR_SWITCH 1
+
+#define GREENLET_NEEDS_EXCEPTION_STATE_SAVED
+
+static void*
+slp_get_exception_state()
+{
+    return (void*)__readfsdword(FIELD_OFFSET(NT_TIB, ExceptionList));
+}
+
+static void
+slp_set_exception_state(const void *const seh_state)
+{
+    __writefsdword(FIELD_OFFSET(NT_TIB, ExceptionList), seh_state);
+}
 
 typedef struct _GExceptionRegistration {
     struct _GExceptionRegistration* prev;