kmsg

defmodule Kmsg do
  @moduledoc """
  A logger backend for the printk buffer.
  Documentation is available on [hex.pm](https://hexdocs.pm/kmsg)

  ## Usage
  Add Kmsg to your application by adding `{:kmsg, "~> #{Mix.Project.config()[:version]}"}` to your list of dependencies in `mix.exs`:
  ```elixir
  def deps do
    [
      # ...
      {:kmsg, "~> #{Mix.Project.config()[:version]}"}
    ]
  end
  ```

  And adding it to your `:logger` configuration in `config.exs`:
  ```elixir
  config :logger,
    backends: [
      :console,
      Kmsg
    ]
  ```

  ## Nerves
  Currently this is not compatible with nerves due to the `KmsgTailer` which creates a neverending circulation of Logentries.
  I'm working on fixing this.
  """

  # Copyright 2019 Hans Bernhard Goedeke
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.

  @behaviour :gen_event
  @kmsg_device "/dev/kmsg"
  # @kmsg_device "/dev/stdout"

  defstruct [
    :format,
    :metadata,
    :level
  ]

  def init(__MODULE__) do
    config = Application.get_env(:logger, __MODULE__, [])

    if File.exists?(@kmsg_device) do
      {:ok, init(config, %__MODULE__{})}
    else
      {:error, :ignore}
    end
  end

  def init({__MODULE__, opts}) when is_list(opts) do
    config = Keyword.merge(Application.get_env(:logger, __MODULE__, []), opts)
    {:ok, init(config, %__MODULE__{})}
  end

  def handle_call({:configure, options}, state) do
    {:ok, :ok, configure(options, state)}
  end

  def handle_event(
        {level, group_leader, {Logger, message, timestamp, metadata}},
        state
      ) do
    alias Logger.Formatter

    if meet_level?(level, state.level) and node(group_leader) == node() do
      formatted = Formatter.format(state.format, level, message, timestamp, metadata)
      File.write!(@kmsg_device, formatted)
    end

    {:ok, state}
  end

  def handle_event(_, state) do
    {:ok, state}
  end

  def handle_info(_, state) do
    {:ok, state}
  end

  def code_change(_old_vsn, state, _extra) do
    {:ok, state}
  end

  def terminate(_reason, _state) do
    :ok
  end

  ## Helpers

  defp meet_level?(_lvl, nil), do: true

  defp meet_level?(lvl, min) do
    Logger.compare_levels(lvl, min) != :lt
  end

  defp configure(options, state) do
    config = Keyword.merge(Application.get_env(:logger, __MODULE__, []), options)
    Application.put_env(:logger, __MODULE__, config)
    init(config, state)
  end

  defp init(config, state) do
    level = Keyword.get(config, :level)
    format = Logger.Formatter.compile(Keyword.get(config, :format))
    metadata = Keyword.get(config, :metadata, [])
    # max_buffer = Keyword.get(config, :max_buffer, 32)

    %__MODULE__{
      state
      | format: format,
        metadata: metadata,
        level: level
    }
  end
end