Update macro to crash in debug mode when error is not set

Author: taegyunkim

Modules/_remote_debugging/_remote_debugging.h

@@ -173,14 +173,22 @@ typedef enum _WIN32_THREADSTATE {
 #define THREAD_STATUS_GIL_REQUESTED       (1 << 3)
 #define THREAD_STATUS_HAS_EXCEPTION       (1 << 4)
 
-/* Exception cause macro */
+/* Exception cause macro - chains context to existing exceptions in debug mode.
+ * This macro assumes an exception has already been set by the failing function.
+ * If no exception exists, this indicates a bug that should be fixed. */
 #define set_exception_cause(unwinder, exc_type, message)          \
     do {                                                          \
         if (!PyErr_ExceptionMatches(PyExc_PermissionError)) {     \
-            if (!PyErr_Occurred()) {                              \
+            if (PyErr_Occurred()) {                               \
+                if (unwinder->debug) {                            \
+                    /* Chain exception with context */            \
+                    _PyErr_FormatFromCause(exc_type, "%s", message);  \
+                }                                                 \
+            } else {                                              \
+                /* BUG: Exception should have been set by caller */ \
+                /* Fallback prevents crash; assert catches bug in debug builds */ \
                 PyErr_SetString(exc_type, message);               \
-            } else if (unwinder->debug) {                         \
-                _PyErr_FormatFromCause(exc_type, "%s", message);  \
+                assert(PyErr_Occurred() && "function returned -1 without setting exception"); \
             }                                                     \
         }                                                         \
     } while (0)