fix #35708, GC seeing uninitialized struct in jl_new_struct* (#35737)

src/datatype.c

@@ -901,20 +901,16 @@ JL_DLLEXPORT jl_value_t *jl_new_structv(jl_datatype_t *type, jl_value_t **args,
  jl_type_error("new", (jl_value_t*)jl_datatype_type, (jl_value_t*)type);
  if (type->ninitialized > na || na > jl_datatype_nfields(type))
  jl_error("invalid struct allocation");
- if (type->instance != NULL) {
- for (size_t i = 0; i < na; i++) {
- jl_value_t *ft = jl_field_type(type, i);
- if (!jl_isa(args[i], ft))
- jl_type_error("new", ft, args[i]);
- }
- return type->instance;
- }
- jl_value_t *jv = jl_gc_alloc(ptls, jl_datatype_size(type), type);
- JL_GC_PUSH1(&jv);
  for (size_t i = 0; i < na; i++) {
  jl_value_t *ft = jl_field_type(type, i);
  if (!jl_isa(args[i], ft))
  jl_type_error("new", ft, args[i]);
+ }
+ if (type->instance != NULL)
+ return type->instance;
+ jl_value_t *jv = jl_gc_alloc(ptls, jl_datatype_size(type), type);
+ JL_GC_PUSH1(&jv);
+ for (size_t i = 0; i < na; i++) {
  set_nth_field(type, (void*)jv, i, args[i]);
  }
  init_struct_tail(type, jv, na);
@@ -946,6 +942,12 @@ JL_DLLEXPORT jl_value_t *jl_new_structt(jl_datatype_t *type, jl_value_t *tup)
  jl_value_t *jv = jl_gc_alloc(ptls, jl_datatype_size(type), type);
  jl_value_t *fi = NULL;
  JL_GC_PUSH2(&jv, &fi);
+ if (type->layout->npointers > 0) {
+ // if there are references, zero the space first to prevent the GC
+ // from seeing uninitialized references during jl_get_nth_field and jl_isa,
+ // which can allocate.
+ memset(jl_data_ptr(jv), 0, jl_datatype_size(type));
+ }
  for (size_t i = 0; i < nargs; i++) {
  jl_value_t *ft = jl_field_type(type, i);
  fi = jl_get_nth_field(tup, i);