swarm_dynamic_supervisor

defmodule Swarm.DynamicSupervisor do
  @moduledoc ~S"""
  A supervisor that starts children dynamically.

  The `Supervisor` module was designed to handle mostly static children
  that are started in the given order when the supervisor starts. A
  `DynamicSupervisor` starts with no children. Instead, children are
  started on demand via `start_child/2`. When a dynamic supervisor
  terminates, all children are shutdown at the same time, with no guarantee
  of ordering.

  ## Examples

  A dynamic supervisor is started with no children, often under a
  supervisor with the supervision strategy (the only strategy currently
  supported is `:one_for_one`) and a name:

      children = [
        {DynamicSupervisor, strategy: :one_for_one, name: MyApp.DynamicSupervisor}
      ]

      Supervisor.start_link(strategy: :one_for_one)

  The options given in the child specification are documented in `start_link/1`.

  Once the dynamic supervisor is running, we can start children
  with `start_child/2`, which receives a child specification:

      {:ok, agent1} = DynamicSupervisor.start_child(MyApp.DynamicSupervisor, {Agent, fn -> %{} end})
      Agent.update(agent1, &Map.put(&1, :key, "value"))
      Agent.get(agent1, & &1)
      #=> %{key: "value"}

      {:ok, agent2} = DynamicSupervisor.start_child(MyApp.DynamicSupervisor, {Agent, fn -> %{} end})
      Agent.get(agent2, & &1)
      #=> %{}

      DynamicSupervisor.count_children(sup)
      #=> %{active: 2, specs: 2, supervisors: 0, workers: 2}

  ## Module-based supervisors

  Similar to `Supervisor`, dynamic supervisors also support module-based
  supervisors.

      defmodule MyApp.DynamicSupervisor do
        # Automatically defines child_spec/1
        use DynamicSupervisor

        def start_link(arg) do
          DynamicSupervisor.start_link(__MODULE__, arg, name: __MODULE__)
        end

        def init(_arg) do
          DynamicSupervisor.init(strategy: :one_for_one)
        end
      end

  See the `Supervisor` docs for a discussion of when you may want to use
  module-based supervisors.

  ## Name registration

  A supervisor is bound to the same name registration rules as a `GenServer`.
  Read more about these rules in the documentation for `GenServer`.

  ## Migrating from Supervisor's :simple_one_for_one

  In case you were using the deprecated `:simple_one_for_one` strategy from
  the `Supervisor` module, you can migrate to the `DynamicSupervisor` in
  few steps.

  Imagine the given "old" code:

      defmodule MySupervisor do
        use Supervisor

        def start_link(arg) do
          Supervisor.start_link(__MODULE__, arg, name: __MODULE__)
        end

        def start_child(foo, bar, baz) do
          # This will start child by calling MyWorker.start_link(initial_arg, foo, bar, baz)
          Supervisor.start_child(__MODULE__, [foo, bar, baz])
        end

        def init(initial_arg) do
          children = [
            # Or the deprecated: worker(MyWorker, [initial_arg])
            %{id: MyWorker, start: {MyWorker, :start_link, [initial_arg]})
          ]

          Supervisor.init(children, strategy: :simple_one_for_one)
        end
      end

  It can be upgraded to the DynamicSupervisor like this:

      defmodule MySupervisor do
        use DynamicSupervisor

        def start_link(arg) do
          DynamicSupervisor.start_link(__MODULE__, arg, name: __MODULE__)
        end

        def start_child(foo, bar, baz) do
          # If MyWorker is not using the new child specs, we need to pass a map:
          # spec = %{id: MyWorker, start: {MyWorker, :start_link, [foo, bar, baz]}}
          spec = MyWorker
          DynamicSupervisor.start_child(__MODULE__, spec)
        end

        def init(initial_arg) do
          DynamicSupervisor.init(
            strategy: :one_for_one,
            extra_arguments: [initial_arg]
          )
        end
      end

  The difference is that the `DynamicSupervisor` expects the child specification
  at the moment `start_child/2` is called, and no longer on the init callback.
  If there are any initial arguments given on initialization, such as `[initial_arg]`,
  it can be given in the `:extra_arguments` flag on `DynamicSupervisor.init/1`.
  """

  @behaviour GenServer

  @doc """
  Callback invoked to start the supervisor and during hot code upgrades.

  Developers typically invoke `DynamicSupervisor.init/1` at the end of
  their init callback to return the proper supervision flags.
  """
  @callback init(args :: term) :: {:ok, sup_flags()} | :ignore

  @opaque sup_flags() :: %{
            strategy: strategy(),
            intensity: non_neg_integer(),
            period: pos_integer(),
            max_children: non_neg_integer() | :infinity,
            extra_arguments: [term()]
          }

  @typedoc "Option values used by the `start*` functions"
  @type option :: {:name, Supervisor.name()} | init_option()

  @typedoc "Options used by the `start*` functions"
  @type options :: [option, ...]

  @typedoc "Options given to `start_link/2` and `init/1`"
  @type init_option ::
          {:strategy, strategy()}
          | {:max_restarts, non_neg_integer()}
          | {:max_seconds, pos_integer()}
          | {:max_children, non_neg_integer() | :infinity}
          | {:extra_arguments, [term()]}

  @typedoc "Supported strategies"
  @type strategy :: :one_for_one

  defstruct [
    :args,
    :extra_arguments,
    :mod,
    :name,
    :strategy,
    :max_children,
    :max_restarts,
    :max_seconds,
    children: %{},
    dynamic: 0,
    restarts: []
  ]

  @doc false
  def child_spec(arg) do
    %{
      id: Swarm.DynamicSupervisor,
      start: {Swarm.DynamicSupervisor, :start_link, [arg]},
      type: :supervisor
    }
  end

  @doc false
  defmacro __using__(opts) do
    quote location: :keep, bind_quoted: [opts: opts] do
      @behaviour Swarm.DynamicSupervisor

      @doc false
      def child_spec(arg) do
        default = %{
          id: __MODULE__,
          start: {__MODULE__, :start_link, [arg]},
          type: :supervisor
        }

        Supervisor.child_spec(default, unquote(Macro.escape(opts)))
      end

      defoverridable child_spec: 1

      @doc false
      def init(arg)
    end
  end

  @doc """
  Starts a supervisor with the given options.

  The `:strategy` is a required option and the currently supported
  value is `:one_for_one`. The remaining options can be found in the
  `init/1` docs.

  The `:name` option can also be used to register a supervisor name.
  The supported values are described under the "Name registration"
  section in the `GenServer` module docs.

  If the supervisor is successfully spawned, this function returns
  `{:ok, pid}`, where `pid` is the PID of the supervisor. If the supervisor
  is given a name and a process with the specified name already exists,
  the function returns `{:error, {:already_started, pid}}`, where `pid`
  is the PID of that process.

  Note that a supervisor started with this function is linked to the parent
  process and exits not only on crashes but also if the parent process exits
  with `:normal` reason.
  """
  @since "1.6.0"
  @spec start_link(options) :: Supervisor.on_start()
  def start_link(options) when is_list(options) do
    keys = [:extra_arguments, :max_children, :max_seconds, :max_restarts, :strategy]
    {sup_opts, start_opts} = Keyword.split(options, keys)
    start_link(Supervisor.Default, init(sup_opts), start_opts)
  end

  @doc """
  Starts a module-based supervisor process with the given `module` and `arg`.

  To start the supervisor, the `c:init/1` callback will be invoked in the given
  `module`, with `arg` as its argument. The `c:init/1` callback must return a
  supervisor specification which can be created with the help of the `init/1`
  function.

  If the `c:init/1` callback returns `:ignore`, this function returns
  `:ignore` as well and the supervisor terminates with reason `:normal`.
  If it fails or returns an incorrect value, this function returns
  `{:error, term}` where `term` is a term with information about the
  error, and the supervisor terminates with reason `term`.

  The `:name` option can also be given in order to register a supervisor
  name, the supported values are described in the "Name registration"
  section in the `GenServer` module docs.
  """
  @since "1.6.0"
  @spec start_link(module, term, GenServer.options()) :: Supervisor.on_start()
  def start_link(mod, args, opts \\ []) do
    GenServer.start_link(__MODULE__, {mod, args, opts[:name]}, opts)
  end

  @doc """
  Dynamically adds a child specification to `supervisor` and starts that child.

  `child_spec` should be a valid child specification. The child process will
  be started as defined in the child specification.

  If the child process start function returns `{:ok, child}` or `{:ok, child,
  info}`, then child specification and PID are added to the supervisor and
  this function returns the same value.

  If the child process start function returns `:ignore`, then no child is added
  to the supervision tree and this function returns `:ignore` too.

  If the child process start function returns an error tuple or an erroneous
  value, or if it fails, the child specification is discarded and this function
  returns `{:error, error}` where `error` is a term containing information about
  the error and child specification.

  If the supervisor already has N children in a way that N exceeds the amount
  of `:max_children` set on the supervisor initialization (see `init/1`), then
  this function returns `{:error, :max_children}`.
  """
  @since "1.6.0"
  @spec start_child(Supervisor.supervisor(), :supervisor.child_spec() | {module, term} | module) ::
          Supervisor.on_start_child()
  def start_child(supervisor, {_, _, _, _, _, _} = child_spec) do
    validate_and_start_child(supervisor, child_spec)
    |> Task.await
  end

  def start_child(supervisor, child_spec) do
    validate_and_start_child(supervisor, Supervisor.child_spec(child_spec, []))
    |> Task.await
  end

  defp register_name(supervisor, {id, _, _, _, _, _} = child) do
    Task.async(fn -> Swarm.register_name(id, GenServer, :call, [supervisor, {:start_child, child}, :infinity]) end)
  end

  defp validate_and_start_child(supervisor, child_spec) do
    case validate_child(child_spec) do
      {:ok, child} -> register_name(supervisor, child)
      error -> Task.async(fn -> {:error, error} end)
    end
  end

  defp validate_child(%{id: id, start: {mod, _, _} = start} = child) do
    restart = Map.get(child, :restart, :permanent)
    type = Map.get(child, :type, :worker)
    modules = Map.get(child, :modules, [mod])

    shutdown =
      case type do
        :worker -> Map.get(child, :shutdown, 5_000)
        :supervisor -> Map.get(child, :shutdown, :infinity)
      end

    validate_child(id, start, restart, shutdown, type, modules)
  end

  defp validate_child({id, start, restart, shutdown, type, modules}) do
    validate_child(id, start, restart, shutdown, type, modules)
  end

  defp validate_child(other) do
    {:invalid_child_spec, other}
  end

  defp validate_child(id, start, restart, shutdown, type, modules) do
    with :ok <- validate_id(id),
         :ok <- validate_start(start),
         :ok <- validate_restart(restart),
         :ok <- validate_shutdown(shutdown),
         :ok <- validate_type(type),
         :ok <- validate_modules(modules) do
      {:ok, {id, start, restart, shutdown, type, modules}}
    end
  end

  defp validate_id(id) do
    case Swarm.whereis_name(id) do
      nil -> :ok
      :undefined -> :ok
      pid -> {:already_started, pid}
    end
  end

  defp validate_start({m, f, args}) when is_atom(m) and is_atom(f) and is_list(args), do: :ok
  defp validate_start(mfa), do: {:invalid_mfa, mfa}

  defp validate_type(type) when type in [:supervisor, :worker], do: :ok
  defp validate_type(type), do: {:invalid_child_type, type}

  defp validate_restart(restart) when restart in [:permanent, :temporary, :transient], do: :ok
  defp validate_restart(restart), do: {:invalid_restart_type, restart}

  defp validate_shutdown(shutdown) when is_integer(shutdown) and shutdown > 0, do: :ok
  defp validate_shutdown(shutdown) when shutdown in [:infinity, :brutal_kill], do: :ok
  defp validate_shutdown(shutdown), do: {:invalid_shutdown, shutdown}

  defp validate_modules(:dynamic), do: :ok

  defp validate_modules(mods) do
    if is_list(mods) and Enum.all?(mods, &is_atom/1) do
      :ok
    else
      {:invalid_modules, mods}
    end
  end

  @doc """
  Terminates the given child identified by child id.

  If successful, this function returns `:ok`. If there is no process with
  the given PID, this function returns `{:error, :not_found}`.
  """
  @since "1.6.0"
  @spec terminate_child(Supervisor.supervisor(), pid) :: :ok | {:error, :not_found}
  def terminate_child(supervisor, pid) when is_pid(pid) do
    call(supervisor, {:terminate_child, pid})
  end

  @doc """
  Returns a list with information about all children.

  Note that calling this function when supervising a large number
  of children under low memory conditions can cause an out of memory
  exception.

  This function returns a list of tuples containing:

    * `id` - it is always `:undefined` for dynamic supervisors

    * `child` - the pid of the corresponding child process or the
      atom `:restarting` if the process is about to be restarted

    * `type` - `:worker` or `:supervisor` as defined in the child
      specification

    * `modules` - as defined in the child specification

  """
  @since "1.6.0"
  @spec which_children(Supervisor.supervisor()) :: [
          {:undefined, pid | :restarting, :worker | :supervisor, :supervisor.modules()}
        ]
  def which_children(supervisor) do
    call(supervisor, :which_children)
  end

  @doc """
  Returns a map containing count values for the supervisor.

  The map contains the following keys:

    * `:specs` - always 1 as dynamic supervisors have a single specification

    * `:active` - the count of all actively running child processes managed by
      this supervisor

    * `:supervisors` - the count of all supervisors whether or not the child
      process is still alive

    * `:workers` - the count of all workers, whether or not the child process
      is still alive

  """
  @since "1.6.0"
  @spec count_children(Supervisor.supervisor()) :: %{
          specs: non_neg_integer,
          active: non_neg_integer,
          supervisors: non_neg_integer,
          workers: non_neg_integer
        }
  def count_children(supervisor) do
    call(supervisor, :count_children) |> :maps.from_list()
  end

  @doc """
  Receives a set of options that initializes a dynamic supervisor.

  This is typically invoked at the end of the `c:init/1` callback of
  module-based supervisors. See the sections "Module-based supervisors"
  in the module documentation for more information.

  The options received by this function are also supported by `start_link/2`.

  This function returns a tuple containing the supervisor options.

  ## Examples

      def init(_arg) do
        DynamicSupervisor.init(max_children: 1000, strategy: :one_for_one)
      end

  ## Options

    * `:strategy` - the restart strategy option. The only supported
      value is `:one_for_one` which means that no other child is
      terminate if a child process terminates. You can learn more
      about strategies in the `Supervisor` module docs.

    * `:max_restarts` - the maximum number of restarts allowed in
      a time frame. Defaults to `3`.

    * `:max_seconds` - the time frame in which `:max_restarts` applies.
      Defaults to `5`.

    * `:max_children` - the maximum amount of children to be running
      under this supervisor at the same time. When `:max_children` is
      exceeded, `start_child/2` returns `{:error, :dynamic}`. Defaults
      to `:infinity`.

    * `:extra_arguments` - arguments that are prepended to the arguments
      specified in the child spec given to `start_child/2`. Defaults to
      an empty list.

  """
  @since "1.6.0"
  @spec init([init_option]) :: {:ok, map()}
  def init(options) when is_list(options) do
    unless strategy = options[:strategy] do
      raise ArgumentError, "expected :strategy option to be given"
    end

    intensity = Keyword.get(options, :max_restarts, 3)
    period = Keyword.get(options, :max_seconds, 5)
    max_children = Keyword.get(options, :max_children, :infinity)
    extra_arguments = Keyword.get(options, :extra_arguments, [])

    flags = %{
      strategy: strategy,
      intensity: intensity,
      period: period,
      max_children: max_children,
      extra_arguments: extra_arguments
    }

    {:ok, flags}
  end

  ## Callbacks

  @impl true
  def init({mod, args, name}) do
    Process.put(:"$initial_call", {:supervisor, mod, 1})
    Process.flag(:trap_exit, true)

    case mod.init(args) do
      {:ok, flags} when is_map(flags) ->
        state = %Swarm.DynamicSupervisor{mod: mod, args: args, name: name || {self(), mod}}

        case init(state, flags) do
          {:ok, state} -> {:ok, state}
          {:error, reason} -> {:stop, {:supervisor_data, reason}}
        end

      :ignore ->
        :ignore

      other ->
        {:stop, {:bad_return, {mod, :init, other}}}
    end
  end

  defp init(state, flags) do
    extra_arguments = Map.get(flags, :extra_arguments, [])
    max_children = Map.get(flags, :max_children, :infinity)
    max_restarts = Map.get(flags, :intensity, 1)
    max_seconds = Map.get(flags, :period, 5)
    strategy = Map.get(flags, :strategy, :one_for_one)

    with :ok <- validate_strategy(strategy),
         :ok <- validate_restarts(max_restarts),
         :ok <- validate_seconds(max_seconds),
         :ok <- validate_dynamic(max_children),
         :ok <- validate_extra_arguments(extra_arguments) do
      {:ok,
       %{
         state
         | extra_arguments: extra_arguments,
           max_children: max_children,
           max_restarts: max_restarts,
           max_seconds: max_seconds,
           strategy: strategy
       }}
    end
  end

  defp validate_strategy(strategy) when strategy in [:one_for_one], do: :ok
  defp validate_strategy(strategy), do: {:error, {:invalid_strategy, strategy}}

  defp validate_restarts(restart) when is_integer(restart) and restart >= 0, do: :ok
  defp validate_restarts(restart), do: {:error, {:invalid_intensity, restart}}

  defp validate_seconds(seconds) when is_integer(seconds) and seconds > 0, do: :ok
  defp validate_seconds(seconds), do: {:error, {:invalid_period, seconds}}

  defp validate_dynamic(:infinity), do: :ok
  defp validate_dynamic(dynamic) when is_integer(dynamic) and dynamic >= 0, do: :ok
  defp validate_dynamic(dynamic), do: {:error, {:invalid_max_children, dynamic}}

  defp validate_extra_arguments(list) when is_list(list), do: :ok
  defp validate_extra_arguments(extra), do: {:error, {:invalid_extra_arguments, extra}}

  @impl true
  def handle_call(:which_children, _from, state) do
    %{children: children} = state

    reply =
      for {pid, args} <- children do
        case args do
          {:restarting, {id, _, _, _, type, modules}} ->
            {id, :restarting, type, modules}

          {id, _, _, _, type, modules} ->
            {id, pid, type, modules}
        end
      end

    {:reply, reply, state}
  end

  def handle_call(:count_children, _from, state) do
    %{children: children} = state
    specs = map_size(children)

    {active, workers, supervisors} =
      Enum.reduce(children, {0, 0, 0}, fn
        {_pid, {:restarting, {_, _, _, _, :worker, _}}}, {active, worker, supervisor} ->
          {active, worker + 1, supervisor}

        {_pid, {:restarting, {_, _, _, _, :supervisor, _}}}, {active, worker, supervisor} ->
          {active, worker, supervisor + 1}

        {_pid, {_, _, _, _, :worker, _}}, {active, worker, supervisor} ->
          {active + 1, worker + 1, supervisor}

        {_pid, {_, _, _, _, :supervisor, _}}, {active, worker, supervisor} ->
          {active + 1, worker, supervisor + 1}
      end)

    reply = [specs: specs, active: active, supervisors: supervisors, workers: workers]
    {:reply, reply, state}
  end

  def handle_call({:terminate_child, pid}, _from, %{children: children} = state) do
    case children do
      %{^pid => info} ->
        :ok = terminate_children(%{pid => info}, state)
        {:reply, :ok, delete_child(pid, state)}

      %{} ->
        {:reply, {:error, :not_found}, state}
    end
  end

  def handle_call({:start_child, child}, _from, state) do
    %{dynamic: dynamic, max_children: max_children} = state

    if dynamic < max_children do
      handle_start_child(child, %{state | dynamic: dynamic + 1})
    else
      {:reply, {:error, :max_children}, state}
    end
  end

  defp handle_start_child({id, {m, f, args} = mfa, restart, shutdown, type, modules}, state) do
    %{extra_arguments: extra} = state

    case reply = do_start_child(m, f, extra ++ args) do
      {:ok, pid, _} ->
        {:reply, reply, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}

      {:ok, pid} ->
        {:reply, reply, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}

      _ ->
        {:reply, reply, update_in(state.dynamic, &(&1 - 1))}
    end
  end

  defp do_start_child(m, f, a) do
    try do
      apply(m, f, a)
    catch
      kind, reason ->
        {:error, exit_reason(kind, reason, System.stacktrace())}
    else
      {:ok, pid, extra} when is_pid(pid) -> {:ok, pid, extra}
      {:ok, pid} when is_pid(pid) -> {:ok, pid}
      :ignore -> :ignore
      {:error, _} = error -> error
      other -> {:error, other}
    end
  end

  defp save_child(pid, id, {m, f, _}, :temporary, shutdown, type, modules, state) do
    put_in(state.children[pid], {id, {m, f, :undefined}, :temporary, shutdown, type, modules})
  end

  defp save_child(pid, id, mfa, restart, shutdown, type, modules, state) do
    put_in(state.children[pid], {id, mfa, restart, shutdown, type, modules})
  end

  defp exit_reason(:exit, reason, _), do: reason
  defp exit_reason(:error, reason, stack), do: {reason, stack}
  defp exit_reason(:throw, value, stack), do: {{:nocatch, value}, stack}

  @impl true
  def handle_cast(_msg, state) do
    {:noreply, state}
  end

  @impl true
  def handle_info({:EXIT, pid, reason}, state) do
    case maybe_restart_child(pid, reason, state) do
      {:ok, state} -> {:noreply, state}
      {:shutdown, state} -> {:stop, :shutdown, state}
    end
  end

  def handle_info({:"$gen_restart", pid}, state) do
    %{children: children} = state

    case children do
      %{^pid => restarting_args} ->
        {:restarting, child} = restarting_args

        case restart_child(pid, child, state) do
          {:ok, state} -> {:noreply, state}
          {:shutdown, state} -> {:stop, :shutdown, state}
        end

      # We may hit clause if we send $gen_restart and then
      # someone calls terminate_child, removing the child.
      %{} ->
        {:noreply, state}
    end
  end

  # Ignore Task monitors
  def handle_info({_ref, _result}, state) do
    {:noreply, state}
  end
  def handle_info({:DOWN, _ref, :process, pid, :normal}, state) when is_pid(pid) do
    {:noreply, state}
  end

  def handle_info(msg, state) do
    :error_logger.error_msg('Swarm.DynamicSupervisor received unexpected message: ~p~n', [msg])
    {:noreply, state}
  end

  @impl true
  def code_change(_, %{mod: mod, args: args} = state, _) do
    case mod.init(args) do
      {:ok, flags} when is_map(flags) ->
        case init(state, flags) do
          {:ok, state} -> {:ok, state}
          {:error, reason} -> {:error, {:supervisor_data, reason}}
        end

      :ignore ->
        {:ok, state}

      error ->
        error
    end
  end

  @impl true
  def terminate(_, %{children: children} = state) do
    :ok = terminate_children(children, state)
  end

  defp terminate_children(children, state) do
    {pids, times, stacks} = monitor_children(children)
    size = map_size(pids)

    timers =
      Enum.reduce(times, %{}, fn {time, pids}, acc ->
        Map.put(acc, :erlang.start_timer(time, self(), :kill), pids)
      end)

    stacks = wait_children(pids, size, timers, stacks)

    for {pid, {child, reason}} <- stacks do
      report_error(:shutdown_error, reason, pid, child, state)
    end

    :ok
  end

  defp monitor_children(children) do
    Enum.reduce(children, {%{}, %{}, %{}}, fn
      {_, {:restarting, _}}, acc ->
        acc

      {pid, {_, _, restart, _, _, _} = child}, {pids, times, stacks} ->
        case monitor_child(pid) do
          :ok ->
            times = exit_child(pid, child, times)
            {Map.put(pids, pid, child), times, stacks}

          {:error, :normal} when restart != :permanent ->
            {pids, times, stacks}

          {:error, reason} ->
            {pids, times, Map.put(stacks, pid, {child, reason})}
        end
    end)
  end

  defp monitor_child(pid) do
    ref = Process.monitor(pid)
    Process.unlink(pid)

    receive do
      {:EXIT, ^pid, reason} ->
        receive do
          {:DOWN, ^ref, :process, ^pid, _} -> {:error, reason}
        end
    after
      0 -> :ok
    end
  end

  defp exit_child(pid, {_, _, _, shutdown, _, _}, times) do
    case shutdown do
      :brutal_kill ->
        Process.exit(pid, :kill)
        times

      :infinity ->
        Process.exit(pid, :shutdown)
        times

      time ->
        Process.exit(pid, :shutdown)
        Map.update(times, time, [pid], &[pid | &1])
    end
  end

  defp wait_children(_pids, 0, timers, stacks) do
    for {timer, _} <- timers do
      _ = :erlang.cancel_timer(timer)

      receive do
        {:timeout, ^timer, :kill} -> :ok
      after
        0 -> :ok
      end
    end

    stacks
  end

  defp wait_children(pids, size, timers, stacks) do
    receive do
      {:DOWN, _ref, :process, pid, reason} ->
        case pids do
          %{^pid => child} ->
            stacks = wait_child(pid, child, reason, stacks)
            wait_children(pids, size - 1, timers, stacks)

          %{} ->
            wait_children(pids, size, timers, stacks)
        end

      {:timeout, timer, :kill} ->
        for pid <- Map.fetch!(timers, timer), do: Process.exit(pid, :kill)
        wait_children(pids, size, Map.delete(timers, timer), stacks)
    end
  end

  defp wait_child(pid, {_, _, _, :brutal_kill, _, _} = child, reason, stacks) do
    case reason do
      :killed -> stacks
      _ -> Map.put(stacks, pid, {child, reason})
    end
  end

  defp wait_child(pid, {_, _, restart, _, _, _} = child, reason, stacks) do
    case reason do
      {:shutdown, _} -> stacks
      :shutdown -> stacks
      :normal when restart != :permanent -> stacks
      reason -> Map.put(stacks, pid, {child, reason})
    end
  end

  defp maybe_restart_child(pid, reason, %{children: children} = state) do
    case children do
      %{^pid => {_, _, restart, _, _, _} = child} ->
        maybe_restart_child(restart, reason, pid, child, state)

      %{} ->
        {:ok, state}
    end
  end

  defp maybe_restart_child(:permanent, reason, pid, child, %{name: supervisor} = state) do
    report_error(:child_terminated, reason, pid, child, state)
    register_name(supervisor, child)
    {:ok, delete_child(pid, state)}
  end

  defp maybe_restart_child(_, :normal, pid, _child, state) do
    {:ok, delete_child(pid, state)}
  end

  defp maybe_restart_child(_, :shutdown, pid, _child, state) do
    {:ok, delete_child(pid, state)}
  end

  defp maybe_restart_child(_, {:shutdown, _}, pid, _child, state) do
    {:ok, delete_child(pid, state)}
  end

  defp maybe_restart_child(:transient, reason, pid, child, %{name: supervisor} = state) do
    report_error(:child_terminated, reason, pid, child, state)
    register_name(supervisor, child)
    {:ok, delete_child(pid, state)}
  end

  defp maybe_restart_child(:temporary, reason, pid, child, state) do
    report_error(:child_terminated, reason, pid, child, state)
    {:ok, delete_child(pid, state)}
  end

  defp delete_child(pid, state) do
    %{children: children, dynamic: dynamic} = state
    %{state | children: Map.delete(children, pid), dynamic: dynamic - 1}
  end

  defp restart_child(pid, child, state) do
    case add_restart(state) do
      {:ok, %{strategy: strategy} = state} ->
        case restart_child(strategy, pid, child, state) do
          {:ok, state} ->
            {:ok, state}

          {:try_again, state} ->
            send(self(), {:"$gen_restart", pid})
            {:ok, state}
        end

      {:shutdown, state} ->
        report_error(:shutdown, :reached_max_restart_intensity, pid, child, state)
        {:shutdown, delete_child(pid, state)}
    end
  end

  defp add_restart(state) do
    %{max_seconds: max_seconds, max_restarts: max_restarts, restarts: restarts} = state

    # The below is equivalent to 1 second. We avoid
    # :second because of incompatibilties with OTP < 20
    now = :erlang.monotonic_time(1)
    restarts = add_restart([now | restarts], now, max_seconds)
    state = %{state | restarts: restarts}

    if length(restarts) <= max_restarts do
      {:ok, state}
    else
      {:shutdown, state}
    end
  end

  defp add_restart(restarts, now, period) do
    for then <- restarts, now <= then + period, do: then
  end

  defp restart_child(:one_for_one, current_pid, child, state) do
    {id, {m, f, args} = mfa, restart, shutdown, type, modules} = child

    case do_start_child(m, f, args) do
      {:ok, pid, _} ->
        state = delete_child(current_pid, state)
        {:ok, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}

      {:ok, pid} ->
        state = delete_child(current_pid, state)
        {:ok, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}

      :ignore ->
        {:ok, delete_child(current_pid, state)}

      {:error, reason} ->
        report_error(:start_error, reason, {:restarting, current_pid}, child, state)
        state = put_in(state.children[current_pid], {:restarting, child})
        {:try_again, state}
    end
  end

  defp report_error(error, reason, pid, child, %{name: name}) do
    :error_logger.error_report(
      :supervisor_report,
      supervisor: name,
      errorContext: error,
      reason: reason,
      offender: extract_child(pid, child)
    )
  end

  defp extract_child(pid, {id, mfa, restart, shutdown, type, _modules}) do
    [
      pid: pid,
      id: id,
      mfargs: mfa,
      restart_type: restart,
      shutdown: shutdown,
      child_type: type
    ]
  end

  @impl true
  def format_status(:terminate, [_pdict, state]) do
    state
  end

  def format_status(_, [_pdict, %{mod: mod} = state]) do
    [data: [{~c"State", state}], supervisor: [{~c"Callback", mod}]]
  end

  ## Helpers

  @compile {:inline, call: 2}

  defp call(supervisor, req) do
    GenServer.call(supervisor, req, :infinity)
  end
end