Allow definition of a load and unload NIF callback

[brodeuralexis]

It is sometimes necessary for a NIF to have some global initialization logic in its load callback, like we already have for atom and resource registration.

This commit inlines the fine::__private__::load function in the FINE_INIT macro to delay template instantiation of a new fine::__private__::OnLoad struct that can be overriden by a user using the FINE_LOAD(env) macro to provide customized initialization logic, while still registering atoms and resources.

[jonatanklosko]

Hey @brodeuralexis! I dropped a comment regarding the API :)

That said, I wonder if there is a logic that the user would put on load, which couldn't be a typical initialization. In the docs example, couldn't they could as well do this:

static ThreadPool s_pool = ThreadPool(std::thread::hardware_concurrency());

And if that's the case, I would probably hold off with extra APIs, until there is a use case.

[brodeuralexis]

I've also added code to support the unload callback, as I believe both should be provided at the same time.

That said, I wonder if there is a logic that the user would put on load, which couldn't be a typical initialization. In the docs example, couldn't they could as well do this:

I've reworked the example to accept the number of threads from load_info, which might be obtained from an application configuration value and forwarded to :erlang.load_nif/2.

And if that's the case, I would probably hold off with extra APIs, until there is a use case.

In libbpf, ring buffers must be polled in a separate thread. I do something akin to the following in a separate thread:

while (!g_exiting) {
  err = ring_buffer__poll(rb, 100 /* timeout */);
  if (err < 0 && err != -EINTR) {
    perror("ring_buffer__poll");
    abort();
  }
}
g_exited.set();

For the ERTS to exit successfully, I need to modify the g_exiting atomic boolean and wait on the g_exited atomic flag to ensure that the thread has exited before unloading can complete. Failure to terminate a thread can cause a crash in the ERTS.

For now, I hard-code 100ms as a timeout for polling, but in the future, this value could be injected from the :erlang.load_nif/2 function as a keyword list load_info allowing users to use Mix.Config to select a timeout:

config :bpf,
  ring_buffer_polling_timeout: 100

[jonatanklosko]

The API looks great, I dropped one comment with alternative implementation :)

[jonatanklosko]

Small comments and looks good to me!

[jonatanklosko]

Fantastic, thank you! 🍵