py: Prevent segfault for operations on closed StringIO.

Addresses issue #1067.

py/objstringio.c

@@ -42,15 +42,26 @@ typedef struct _mp_obj_stringio_t {
     mp_uint_t pos;
 } mp_obj_stringio_t;
 
+#if MICROPY_CPYTHON_COMPAT
+STATIC void check_stringio_is_open(const mp_obj_stringio_t *o) {
+    if (o->vstr == NULL) {
+        nlr_raise(mp_obj_new_exception_msg(&mp_type_ValueError, "I/O operation on closed file"));
+    }
+}
+#else
+#define check_stringio_is_open(o)
+#endif
+
 STATIC void stringio_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t self_in, mp_print_kind_t kind) {
     (void)kind;
     mp_obj_stringio_t *self = self_in;
-    print(env, self->base.type == &mp_type_stringio ? "<io.StringIO 0x%x>" : "<io.BytesIO 0x%x>", self->vstr);
+    print(env, self->base.type == &mp_type_stringio ? "<io.StringIO 0x%x>" : "<io.BytesIO 0x%x>", self);
 }
 
 STATIC mp_uint_t stringio_read(mp_obj_t o_in, void *buf, mp_uint_t size, int *errcode) {
     (void)errcode;
     mp_obj_stringio_t *o = o_in;
+    check_stringio_is_open(o);
     mp_uint_t remaining = o->vstr->len - o->pos;
     if (size > remaining) {
         size = remaining;
@@ -63,6 +74,7 @@ STATIC mp_uint_t stringio_read(mp_obj_t o_in, void *buf, mp_uint_t size, int *er
 STATIC mp_uint_t stringio_write(mp_obj_t o_in, const void *buf, mp_uint_t size, int *errcode) {
     (void)errcode;
     mp_obj_stringio_t *o = o_in;
+    check_stringio_is_open(o);
     mp_uint_t remaining = o->vstr->alloc - o->pos;
     if (size > remaining) {
         // Take all what's already allocated...
@@ -82,14 +94,22 @@ STATIC mp_uint_t stringio_write(mp_obj_t o_in, const void *buf, mp_uint_t size,
 
 STATIC mp_obj_t stringio_getvalue(mp_obj_t self_in) {
     mp_obj_stringio_t *self = self_in;
+    check_stringio_is_open(self);
     return mp_obj_new_str_of_type(STREAM_TO_CONTENT_TYPE(self), (byte*)self->vstr->buf, self->vstr->len);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(stringio_getvalue_obj, stringio_getvalue);
 
 STATIC mp_obj_t stringio_close(mp_obj_t self_in) {
     mp_obj_stringio_t *self = self_in;
+#if MICROPY_CPYTHON_COMPAT
     vstr_free(self->vstr);
     self->vstr = NULL;
+#else
+    vstr_clear(self->vstr);
+    self->vstr->alloc = 0;
+    self->vstr->len = 0;
+    self->pos = 0;
+#endif
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(stringio_close_obj, stringio_close);

tests/io/stringio1.py

@@ -28,3 +28,14 @@ print(a.getvalue())
 a = io.StringIO()
 a.write("foo")
 print(a.read())
+
+a = io.StringIO()
+a.close()
+for f in [a.read, a.getvalue, lambda:a.write("")]:
+    # CPython throws for operations on closed I/O, micropython makes
+    # the underlying string empty unless MICROPY_CPYTHON_COMPAT defined
+    try:
+        f()
+        print("ValueError")
+    except ValueError:
+        print("ValueError")