Use @optional_callbacks in GenServer

[josevalim]

One of the features added to Elixir early on to help integration with Erlang code was the idea of overridable function definitions. This is what allowed our GenServer definition to be as simple as:

defmodule MyServer do
  use GenServer
end

Implementation-wise, use GenServer defines functions such as:

def terminate(reason, state) do
  :ok
end

and then mark them as overridable:

defoverridable terminate: 2

In some cases, the use of defoverridable seems to be unnecessary. For instance, we provide a default implementation for handle_call/3 and mark it as overridable, but the default implementation simply raises when invoked. That's counter-intuitive as it would be best to simply not define a default implementation in the first place, truly making the handle_call/3 callback optional.

Luckily, Erlang 18 added support for marking callbacks as optional, which we support on Elixir v1.4. We propose Elixir and libraries to leverage this feature and no longer define default implementations for the handle_* functions and instead mark them as optional.

Instead of the version we have today:

defmodule GenServer do
  @callback handle_call(message, from, state)

  defmacro __using__(_) do
    quote do
      @behaviour GenServer

      def handle_call(_message, _from, _state) do
        raise "handle_call/3 not implemented"
      end

      # ...

      defoverridable handle_call: 3
    end
  end
end

We propose:

defmodule GenServer do
  @callback handle_call(message, from, state)
  @optional_callbacks handle_call: 3

  defmacro __using__(_) do
    quote do
      @behaviour GenServer

      # ...
    end
  end
end

The proposed code is much simpler conceptually since we are using the @optional_callbacks feature instead of defoverridable to correctly mark optional callbacks as optional. defoverridable will still be used for functions such as terminate/2, which are truly required.

We should take account of backwards compatibility, as users may be calling super in their current GenServer implementations.

This depends on #5676.

[josevalim]

We are holding off #5676 for now in an attempt to get those changes in OTP instead. As a result, those changes won't make it to Elixir v1.5.

[josevalim]

Given optional callbacks were added as part of OTP 20, we can likely move forward with this without rolling our own implementation. The only concern is regarding init which should probably no longer be optional, which requires us to put the proper deprecation warnings in place.

[josevalim]

The decision on init should also align with how we want init to work on supervisors. Today we may use Supervisor for the child_spec function but that warns if init is missing. We need to decide how we want to act on this case.

[josevalim]

This is being broken in two parts. One is the discussion of init in the standard library and the other is the use of optional callbacks, which require OTP 20 to be done completely.

[vans163]

Is there the inverse of this? @force_callbacks?

The problem is recompile/0 unloads modules, if a gen_server receives a msg at the exact time a recompile is happening (100ms margin) the msg gets dropped with WARNING: handle_info/2 not defined.

This leads to inconsistent state as messages get lost.

The old behaviour would result in the gen_server crashing. Which is correct to maintain state.