napi: zero length typedarray in 12 vs 13

[mafintosh]

We just hit this bug, which may be a feature, but at least not documented where in Node.js
13, the napi_get_typedarray_info returns NULL as the data pointer for 0 length typedarrays but Node.js 12 returns a pointer.

This C program shows the difference:

#include <node_api.h>
#include <stdio.h>

napi_value print (napi_env env, napi_callback_info info) {
  napi_value argv[1];
  size_t argc = 1;

  napi_get_cb_info(env, info, &argc, argv, NULL, NULL);

  // https://nodejs.org/dist/latest-v13.x/docs/api/n-api.html#n_api_napi_get_typedarray_info

  napi_typedarray_type type;
  napi_value array_buffer;
  size_t len;
  void *data;
  size_t offset;
  napi_get_typedarray_info(env, argv[0], &type, &len, &data, &array_buffer, &offset);

  printf("buffer is %zu\n", data);

  return NULL;
}

napi_value init_all (napi_env env, napi_value exports) {
  napi_value print_fn;
  napi_create_function(env, NULL, 0, print, NULL, &print_fn);
  napi_set_named_property(env, exports, "print", print_fn);
  return exports;
}

NAPI_MODULE(NODE_GYP_MODULE_NAME, init_all)

Compile it and run it with

require('./binding...').print(Buffer.alloc(0))

Node.js 12 returns a pointer and 13 NULL.

[addaleax]

… what is the (un)expected behaviour here? It seems perfectly fine to me to return any pointer for a zero-length ArrayBuffer, including NULL.

(I’m guessing that this is happening because of V8’s BackingStore migration, but in this case that should just be an implementation detail.)

[legendecas]

It seems perfectly fine to me to return any pointer for a zero-length ArrayBuffer, including NULL.

In the case, the length shall always be respected even if the pointer is not NULL. But that is an elementary check over the return values, documenting it might increase the verbosity of the document.

[mafintosh]

We had logic there we checked if a pointer was NULL to see we had load it from the typed_array method, which this broke. As mentioned, it might be a feature but having it documented would have saved us debugging.