do not chain exceptions to themselves

cpython actually makes sure the newly chained exception doesn't create
a cycle (even indirectly); see _PyErr_SetObject use of "Floyd's cycle
detection algo". We'll go for the simpler solution of just checking
one level deep until it's clear we need to do more.

Closes: #7414

Author: jepler

py/vm.c

@@ -1150,7 +1150,7 @@ unwind_jump:;
                     mp_obj_t obj = mp_make_raise_obj(TOP());
                     #if MICROPY_CPYTHON_EXCEPTION_CHAIN
                     mp_obj_t active_exception = get_active_exception(exc_sp, exc_stack);
-                    if (active_exception != MP_OBJ_NULL) {
+                    if (active_exception != MP_OBJ_NULL && active_exception != obj) {
                         mp_store_attr(obj, MP_QSTR___context__, active_exception);
                     }
                     #endif
@@ -1164,7 +1164,7 @@ unwind_jump:;
                     #if MICROPY_CPYTHON_EXCEPTION_CHAIN
                     // search for the inner-most previous exception, to chain it
                     mp_obj_t active_exception = get_active_exception(exc_sp, exc_stack);
-                    if (active_exception != MP_OBJ_NULL) {
+                    if (active_exception != MP_OBJ_NULL && active_exception != obj) {
                         mp_store_attr(obj, MP_QSTR___context__, active_exception);
                     }
                     mp_store_attr(obj, MP_QSTR___cause__, cause);
@@ -1463,7 +1463,7 @@ unwind_jump:;
                 exc_sp->prev_exc = nlr.ret_val;
                 mp_obj_t obj = MP_OBJ_FROM_PTR(nlr.ret_val);
                 #if MICROPY_CPYTHON_EXCEPTION_CHAIN
-                if (active_exception != MP_OBJ_NULL) {
+                if (active_exception != MP_OBJ_NULL && active_exception != obj) {
                     mp_store_attr(obj, MP_QSTR___context__, active_exception);
                 }
                 #endif

tests/circuitpython/traceback_test_chained.py

@@ -62,3 +62,13 @@ def print_exc_info(e, chain=True):
         1 / 0
 except Exception as e:
     print_exc_info(e)
+
+try:
+    try:
+        1 / 0
+    except Exception as inner:
+        raise inner
+except Exception as e:
+    print_exc_info(e, chain=False)
+    print_exc_info(e)
+print()

tests/circuitpython/traceback_test_chained.py.exp

@@ -59,3 +59,18 @@ Traceback (most recent call last):
 ZeroDivisionError: division by zero
 ------------------------------------------------------------------------
 
+------------------------------------------------------------------------
+Traceback (most recent call last):
+  File "circuitpython/traceback_test_chained.py", line 70, in <module>
+  File "circuitpython/traceback_test_chained.py", line 68, in <module>
+ZeroDivisionError: division by zero
+------------------------------------------------------------------------
+
+------------------------------------------------------------------------
+Traceback (most recent call last):
+  File "circuitpython/traceback_test_chained.py", line 70, in <module>
+  File "circuitpython/traceback_test_chained.py", line 68, in <module>
+ZeroDivisionError: division by zero
+------------------------------------------------------------------------
+
+