federator: add publisher module defining a contract for publishing behaviours

lib/pleroma/web/federator/publisher.ex

+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Federator.Publisher do
+  @moduledoc """
+  Defines the contract used by federation implementations to publish messages to
+  their peers.
+  """
+
+  @doc """
+  Determine whether an activity can be relayed using the federation module.
+  """
+  @callback is_representable?(Pleroma.Activity.t()) :: boolean()
+
+  @doc """
+  Relays an activity to a specified peer, determined by the parameters.  The
+  parameters used are controlled by the federation module.
+  """
+  @callback publish_one(Map.t()) :: {:ok, Map.t()} | {:error, any()}
+
+  @doc """
+  Relays an activity to all specified peers.
+  """
+  @callback publish(Pleroma.User.t(), Pleroma.Activity.t()) :: :ok | {:error, any()}
+
+  @doc """
+  Enqueues work generated by the federation module.
+  """
+  @spec enqueue(module(), keyword()) :: :ok
+  def enqueue(module, args), do: PleromaJobQueue.enqueue(:federation_outgoing, module, args)
+
+  @doc """
+  Enqueue publishing a single activity.
+  """
+  @spec enqueue_one(module(), Map.t()) :: :ok
+  def enqueue_one(module, %{} = args), do: enqueue(module, [:publish_one, args])
+end