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.

1 files changed, 10 insertions, 0 deletions

diff --git a/py/objgenerator.c b/py/objgenerator.c
index d500dbd9d..a2ad490d6 100644
--- a/py/objgenerator.c
+++ b/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) {