py/objgenerator: Protect against reentering a generator.

Generators that are already executing cannot be reexecuted.  This patch
puts in a check for such a case.

Thanks to @jepler for finding the bug.

py/objgenerator.c

@@ -117,9 +117,19 @@ mp_vm_return_kind_t mp_obj_gen_resume(mp_obj_t self_in, mp_obj_t send_value, mp_
             *self->code_state.sp = send_value;
         }
     }
+
+    // We set self->globals=NULL while executing, for a sentinel to ensure the generator
+    // cannot be reentered during execution
+    if (self->globals == NULL) {
+        mp_raise_ValueError("generator already executing");
+    }
+
+    // Set up the correct globals context for the generator and execute it
     self->code_state.old_globals = mp_globals_get();
     mp_globals_set(self->globals);
+    self->globals = NULL;
     mp_vm_return_kind_t ret_kind = mp_execute_bytecode(&self->code_state, throw_value);
+    self->globals = mp_globals_get();
     mp_globals_set(self->code_state.old_globals);
 
     switch (ret_kind) {

tests/basics/gen_yield_from_executing.py

@@ -0,0 +1,15 @@
+# yielding from an already executing generator is not allowed
+
+def f():
+    yield 1
+    # g here is already executing so this will raise an exception
+    yield from g
+
+g = f()
+
+print(next(g))
+
+try:
+    next(g)
+except ValueError:
+    print('ValueError')

tests/run-tests

@@ -336,7 +336,7 @@ def run_tests(pyb, tests, args, base_path="."):
     # Some tests are known to fail with native emitter
     # Remove them from the below when they work
     if args.emit == 'native':
-        skip_tests.update({'basics/%s.py' % t for t in 'gen_yield_from gen_yield_from_close gen_yield_from_ducktype gen_yield_from_exc gen_yield_from_iter gen_yield_from_send gen_yield_from_stopped gen_yield_from_throw gen_yield_from_throw2 gen_yield_from_throw3 generator1 generator2 generator_args generator_close generator_closure generator_exc generator_pend_throw generator_return generator_send'.split()}) # require yield
+        skip_tests.update({'basics/%s.py' % t for t in 'gen_yield_from gen_yield_from_close gen_yield_from_ducktype gen_yield_from_exc gen_yield_from_executing gen_yield_from_iter gen_yield_from_send gen_yield_from_stopped gen_yield_from_throw gen_yield_from_throw2 gen_yield_from_throw3 generator1 generator2 generator_args generator_close generator_closure generator_exc generator_pend_throw generator_return generator_send'.split()}) # require yield
         skip_tests.update({'basics/%s.py' % t for t in 'bytes_gen class_store_class globals_del string_join'.split()}) # require yield
         skip_tests.update({'basics/async_%s.py' % t for t in 'def await await2 for for2 with with2'.split()}) # require yield
         skip_tests.update({'basics/%s.py' % t for t in 'try_reraise try_reraise2'.split()}) # require raise_varargs