gh-144316: Fix missing exception in _remote_debugging with debug=False

Misc/NEWS.d/next/Library/2026-02-03-19-57-41.gh-issue-144316.wop870.rst

@@ -0,0 +1 @@
+Fix crash in ``_remote_debugging`` that caused ``test_external_inspection`` to intermittently fail. Patch by Taegyun Kim.

Modules/_remote_debugging/_remote_debugging.h

@@ -29,6 +29,7 @@ extern "C" {
 #include "internal/pycore_interpframe.h"    // FRAME_OWNED_BY_INTERPRETER
 #include "internal/pycore_llist.h"          // struct llist_node
 #include "internal/pycore_long.h"           // _PyLong_GetZero
+#include "internal/pycore_pyerrors.h"       // _PyErr_FormatFromCause
 #include "internal/pycore_stackref.h"       // Py_TAG_BITS
 #include "../../Python/remote_debug.h"
 
@@ -172,11 +173,25 @@ typedef enum _WIN32_THREADSTATE {
 #define THREAD_STATUS_GIL_REQUESTED       (1 << 3)
 #define THREAD_STATUS_HAS_EXCEPTION       (1 << 4)
 
-/* Exception cause macro */
-#define set_exception_cause(unwinder, exc_type, message) \
-    if (unwinder->debug) { \
-        _set_debug_exception_cause(exc_type, message); \
-    }
+/* 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 (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);                                           \
+                assert(PyErr_Occurred() && "function returned -1 without setting exception"); \
+            }                                                                                 \
+        }                                                                                     \
+    } while (0)
 
 /* ============================================================================
  * TYPE DEFINITIONS

Python/remote_debug.h

@@ -1263,6 +1263,7 @@ _Py_RemoteDebug_PagedReadRemoteMemory(proc_handle_t *handle,
             if (entry->data == NULL) {
                 entry->data = PyMem_RawMalloc(page_size);
                 if (entry->data == NULL) {
+                    PyErr_NoMemory();
                     _set_debug_exception_cause(PyExc_MemoryError,
                         "Cannot allocate %zu bytes for page cache entry "
                         "during read from PID %d at address 0x%lx",
@@ -1272,7 +1273,7 @@ _Py_RemoteDebug_PagedReadRemoteMemory(proc_handle_t *handle,
             }
 
             if (_Py_RemoteDebug_ReadRemoteMemory(handle, page_base, page_size, entry->data) < 0) {
-                // Try to just copy the exact ammount as a fallback
+                // Try to just copy the exact amount as a fallback
                 PyErr_Clear();
                 goto fallback;
             }