Replace custom fifo implementation with Exile

This is for streaming media to ffmpeg thumbnailer. The existing implementation relies on undocumented behavior.

Erlang open_port/2 does not officially support passing a string of a file path for opening. The specs clearly state you are to provide one of the following for open_port/2:

    {spawn, Command :: string() | binary()} |
    {spawn_driver, Command :: string() | binary()} |
    {spawn_executable, FileName :: file:name_all()} |
    {fd, In :: integer() >= 0, Out :: integer() >= 0}

Our method technically works but is strongly discouraged as it can block the scheduler and dialyzer throws errors as it recognizes we're breaking the contract and some of the functions we wrote may never return.

This is indirectly covered by the Erlang FAQ section "9.12 Why can't I open devices (e.g. a serial port) like normal files?"
https://www.erlang.org/faq/problems#idm1127

lib/pleroma/helpers/media_helper.ex

@@ -43,89 +43,28 @@ defmodule Pleroma.Helpers.MediaHelper do
   def video_framegrab(url) do
     with executable when is_binary(executable) <- System.find_executable("ffmpeg"),
          {:ok, env} <- HTTP.get(url, [], pool: :media),
-         {:ok, fifo_path} <- mkfifo(),
-         args = [
-           "-y",
+         {:ok, pid} <- StringIO.open(env.body) do
+      body_stream = IO.binstream(pid, 1)
+
+      Exile.stream!(
+        [
+          executable,
           "-i",
-           fifo_path,
+          "pipe:0",
           "-vframes",
           "1",
           "-f",
           "mjpeg",
-           "-loglevel",
-           "error",
-           "-"
-         ] do
-      run_fifo(fifo_path, env, executable, args)
+          "pipe:1"
+        ],
+        input: body_stream,
+        ignore_epipe: true,
+        stderr: :disable
+      )
+      |> Enum.into(<<>>)
     else
       nil -> {:error, {:ffmpeg, :command_not_found}}
       {:error, _} = error -> error
     end
   end
-
-  defp run_fifo(fifo_path, env, executable, args) do
-    pid =
-      Port.open({:spawn_executable, executable}, [
-        :use_stdio,
-        :stream,
-        :exit_status,
-        :binary,
-        args: args
-      ])
-
-    fifo = Port.open(to_charlist(fifo_path), [:eof, :binary, :stream, :out])
-    fix = Pleroma.Helpers.QtFastStart.fix(env.body)
-    true = Port.command(fifo, fix)
-    :erlang.port_close(fifo)
-    loop_recv(pid)
-  after
-    File.rm(fifo_path)
-  end
-
-  defp mkfifo do
-    path = Path.join(System.tmp_dir!(), "pleroma-media-preview-pipe-#{Ecto.UUID.generate()}")
-
-    case System.cmd("mkfifo", [path]) do
-      {_, 0} ->
-        spawn(fifo_guard(path))
-        {:ok, path}
-
-      {_, err} ->
-        {:error, {:fifo_failed, err}}
-    end
-  end
-
-  defp fifo_guard(path) do
-    pid = self()
-
-    fn ->
-      ref = Process.monitor(pid)
-
-      receive do
-        {:DOWN, ^ref, :process, ^pid, _} ->
-          File.rm(path)
-      end
-    end
-  end
-
-  defp loop_recv(pid) do
-    loop_recv(pid, <<>>)
-  end
-
-  defp loop_recv(pid, acc) do
-    receive do
-      {^pid, {:data, data}} ->
-        loop_recv(pid, acc <> data)
-
-      {^pid, {:exit_status, 0}} ->
-        {:ok, acc}
-
-      {^pid, {:exit_status, status}} ->
-        {:error, status}
-    after
-      5000 ->
-        :erlang.port_close(pid)
-        {:error, :timeout}
-    end
-  end
 end

mix.exs

@@ -184,6 +184,7 @@ defmodule Pleroma.Mixfile do
       {:vix, "~> 0.26.0"},
       {:elixir_make, "~> 0.7.7", override: true},
       {:blurhash, "~> 0.1.0", hex: :rinpatch_blurhash},
+      {:exile, "~> 0.8.0"},
 
       ## dev & test
       {:ex_doc, "~> 0.22", only: :dev, runtime: false},

mix.lock

@@ -46,6 +46,7 @@
   "ex_doc": {:hex, :ex_doc, "0.29.4", "6257ecbb20c7396b1fe5accd55b7b0d23f44b6aa18017b415cb4c2b91d997729", [:mix], [{:earmark_parser, "~> 1.4.31", [hex: :earmark_parser, repo: "hexpm", optional: false]}, {:makeup_elixir, "~> 0.14", [hex: :makeup_elixir, repo: "hexpm", optional: false]}, {:makeup_erlang, "~> 0.1", [hex: :makeup_erlang, repo: "hexpm", optional: false]}], "hexpm", "2c6699a737ae46cb61e4ed012af931b57b699643b24dabe2400a8168414bc4f5"},
   "ex_machina": {:hex, :ex_machina, "2.7.0", "b792cc3127fd0680fecdb6299235b4727a4944a09ff0fa904cc639272cd92dc7", [:mix], [{:ecto, "~> 2.2 or ~> 3.0", [hex: :ecto, repo: "hexpm", optional: true]}, {:ecto_sql, "~> 3.0", [hex: :ecto_sql, repo: "hexpm", optional: true]}], "hexpm", "419aa7a39bde11894c87a615c4ecaa52d8f107bbdd81d810465186f783245bf8"},
   "ex_syslogger": {:hex, :ex_syslogger, "1.5.2", "72b6aa2d47a236e999171f2e1ec18698740f40af0bd02c8c650bf5f1fd1bac79", [:mix], [{:poison, ">= 1.5.0", [hex: :poison, repo: "hexpm", optional: true]}, {:syslog, "~> 1.1.0", [hex: :syslog, repo: "hexpm", optional: false]}], "hexpm", "ab9fab4136dbc62651ec6f16fa4842f10cf02ab4433fa3d0976c01be99398399"},
+  "exile": {:hex, :exile, "0.8.0", "7287f76add343c5dcf5e5b46066ebb93b02b7e643cce7a1a1c5eab3e7d9eb5d8", [:make, :mix], [{:elixir_make, "~> 0.6", [hex: :elixir_make, repo: "hexpm", optional: false]}], "hexpm", "63309c36590f3a524f513255601404e6672be4c8bea0cc4420fbae02e7ffffac"},
   "expo": {:hex, :expo, "0.4.1", "1c61d18a5df197dfda38861673d392e642649a9cef7694d2f97a587b2cfb319b", [:mix], [], "hexpm", "2ff7ba7a798c8c543c12550fa0e2cbc81b95d4974c65855d8d15ba7b37a1ce47"},
   "fast_html": {:hex, :fast_html, "2.0.5", "c61760340606c1077ff1f196f17834056cb1dd3d5cb92a9f2cabf28bc6221c3c", [:make, :mix], [{:elixir_make, "~> 0.4", [hex: :elixir_make, repo: "hexpm", optional: false]}, {:nimble_pool, "~> 0.2.0", [hex: :nimble_pool, repo: "hexpm", optional: false]}], "hexpm", "605f4f4829443c14127694ebabb681778712ceecb4470ec32aa31012330e6506"},
   "fast_sanitize": {:hex, :fast_sanitize, "0.2.3", "67b93dfb34e302bef49fec3aaab74951e0f0602fd9fa99085987af05bd91c7a5", [:mix], [{:fast_html, "~> 2.0", [hex: :fast_html, repo: "hexpm", optional: false]}, {:plug, "~> 1.8", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm", "e8ad286d10d0386e15d67d0ee125245ebcfbc7d7290b08712ba9013c8c5e56e2"},