py/vm: Don't add traceback info for exc's propagated through a finally.

With this patch exception tracebacks that go through a finally are improved
(less confusing, match CPython), and it makes finally's slightly more
efficient (in time and RAM) because they no longer need to add a traceback.

Partially fixes issue #2928.

1 files changed, 5 insertions, 4 deletions

diff --git a/py/vm.c b/py/vm.c
index 6a4634d00..64bddd6b5 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -1359,10 +1359,11 @@ exception_handler:
 #if MICROPY_STACKLESS
 unwind_loop:
 #endif
-            // set file and line number that the exception occurred at
-            // TODO: don't set traceback for exceptions re-raised by END_FINALLY.
-            // But consider how to handle nested exceptions.
-            if (nlr.ret_val != &mp_const_GeneratorExit_obj) {
+            // Set traceback info (file and line number) where the exception occurred, but not for:
+            // - constant GeneratorExit object, because it's const
+            // - exceptions re-raised by END_FINALLY
+            if (nlr.ret_val != &mp_const_GeneratorExit_obj
+                && *code_state->ip != MP_BC_END_FINALLY) {
                 const byte *ip = code_state->fun_bc->bytecode;
                 ip = mp_decode_uint_skip(ip); // skip n_state
                 ip = mp_decode_uint_skip(ip); // skip n_exc_stack