[PATCH] Cygwin: main exception handler (64-bit): continue GCC exceptions

[Ken Brown]

This is necessary in order to be consistent with the following comment
in the definition of _Unwind_RaiseException() in the GCC source file
libgcc/unwind-seh.c:

     The exception handler installed in crt0 will continue any GCC
     exception that reaches there (and isn't marked non-continuable).

Previously we failed to do this and, as a consequence, the C++ runtime
didn't call std::terminate after an unhandled exception.

This fixes the problem reported here:

  https://sourceware.org/pipermail/cygwin/2020-August/245897.html
---
 winsup/cygwin/exceptions.cc | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/winsup/cygwin/exceptions.cc b/winsup/cygwin/exceptions.cc
index 0d9e8f70e..f8da7529b 100644
--- a/winsup/cygwin/exceptions.cc
+++ b/winsup/cygwin/exceptions.cc
@@ -647,6 +647,17 @@ exception::handle (EXCEPTION_RECORD *e, exception_list *frame, CONTEXT *in,
   if (exit_state || e->ExceptionFlags)
     return ExceptionContinueSearch;
 
+/* From the GCC source file libgcc/unwind-seh.c. */
+#ifdef __x86_64__
+#define STATUS_USER_DEFINED		(1U << 29)
+#define GCC_MAGIC			(('G' << 16) | ('C' << 8) | 'C')
+#define GCC_EXCEPTION(TYPE)		\
+       (STATUS_USER_DEFINED | ((TYPE) << 24) | GCC_MAGIC)
+#define STATUS_GCC_THROW		GCC_EXCEPTION (0)
+#define STATUS_GCC_UNWIND		GCC_EXCEPTION (1)
+#define STATUS_GCC_FORCED		GCC_EXCEPTION (2)
+#endif
+
   siginfo_t si = {};
   si.si_code = SI_KERNEL;
   /* Coerce win32 value to posix value.  */
@@ -762,6 +773,16 @@ exception::handle (EXCEPTION_RECORD *e, exception_list *frame, CONTEXT *in,
 	 handling.  */
       return ExceptionContinueExecution;
 
+#ifdef __x86_64__
+      /* According to a comment in the GCC function
+	 _Unwind_RaiseException(), GCC expects us to continue all the
+	 (continuable) GCC exceptions that reach us. */
+    case STATUS_GCC_THROW:
+    case STATUS_GCC_UNWIND:
+    case STATUS_GCC_FORCED:
+      return ExceptionContinueExecution;
+#endif
+
     default:
       /* If we don't recognize the exception, we have to assume that
 	 we are doing structured exception handling, and we let
-- 
2.28.0