Rewrite MP4/MOV binaries to be faststart

In some cases, MP4/MOV files can have the data _before_ the meta-data.

Thus, ffmpeg (and all similar tools) cannot really process the input if
it's given over stdin/streaming/pipes.

BUT I REALLY DON'T WANT TO MAKE TEMPORARY FILES

so here we go, an implementation of qtfaststart in elixir.

lib/pleroma/helpers/media_helper.ex

@@ -14,8 +14,7 @@ defmodule Pleroma.Helpers.MediaHelper do
          {:ok, args} <- prepare_image_resize_args(options),
          url = Pleroma.Web.MediaProxy.url(url),
          {:ok, env} <- Pleroma.HTTP.get(url),
-         {:ok, fifo_path} <- mkfifo()
-    do
+         {:ok, fifo_path} <- mkfifo() do
       args = List.flatten([fifo_path, args])
       run_fifo(fifo_path, env, executable, args)
     else
@@ -27,12 +26,17 @@ defmodule Pleroma.Helpers.MediaHelper do
   defp prepare_image_resize_args(%{max_width: max_width, max_height: max_height} = options) do
     quality = options[:quality] || 85
     resize = Enum.join([max_width, "x", max_height, ">"])
+
     args = [
-    "-interlace", "Plane",
-    "-resize", resize,
-    "-quality", to_string(quality),
+      "-interlace",
+      "Plane",
+      "-resize",
+      resize,
+      "-quality",
+      to_string(quality),
       "jpg:-"
     ]
+
     {:ok, args}
   end
 
@@ -45,11 +49,15 @@ defmodule Pleroma.Helpers.MediaHelper do
          {:ok, fifo_path} <- mkfifo(),
          args = [
            "-y",
-           "-i", fifo_path,
-           "-vframes", "1",
-           "-f", "mjpeg",
-           "-loglevel", "error",
-           "pipe:"
+           "-i",
+           fifo_path,
+           "-vframes",
+           "1",
+           "-f",
+           "mjpeg",
+           "-loglevel",
+           "error",
+           "-"
          ] do
       run_fifo(fifo_path, env, executable, args)
     else
@@ -59,9 +67,18 @@ defmodule Pleroma.Helpers.MediaHelper do
   end
 
   defp run_fifo(fifo_path, env, executable, args) do
-    pid = Port.open({:spawn_executable, executable}, [:use_stdio, :stream, :exit_status, :binary, args: args])
+    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])
-    true = Port.command(fifo, env.body)
+    fix = Pleroma.Helpers.QtFastStart.fix(env.body)
+    true = Port.command(fifo, fix)
     :erlang.port_close(fifo)
     loop_recv(pid)
   after
@@ -70,10 +87,12 @@ defmodule Pleroma.Helpers.MediaHelper do
 
   defp mkfifo() do
     path = "#{@tmp_base}#{to_charlist(:erlang.phash2(self()))}"
+
     case System.cmd("mkfifo", [path]) do
       {_, 0} ->
         spawn(fifo_guard(path))
         {:ok, path}
+
       {_, err} ->
         {:error, {:fifo_failed, err}}
     end
@@ -81,8 +100,10 @@ defmodule Pleroma.Helpers.MediaHelper do
 
   defp fifo_guard(path) do
     pid = self()
-    fn() ->
+
+    fn ->
       ref = Process.monitor(pid)
+
       receive do
         {:DOWN, ^ref, :process, ^pid, _} ->
           File.rm(path)
@@ -98,8 +119,10 @@ defmodule Pleroma.Helpers.MediaHelper do
     receive do
       {^pid, {:data, data}} ->
         loop_recv(pid, acc <> data)
+
       {^pid, {:exit_status, 0}} ->
         {:ok, acc}
+
       {^pid, {:exit_status, status}} ->
         {:error, status}
     after

lib/pleroma/helpers/qt_fast_start.ex

@@ -0,0 +1,131 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Helpers.QtFastStart do
+  @moduledoc """
+  (WIP) Converts a "slow start" (data before metadatas) mov/mp4 file to a "fast start" one (metadatas before data).
+  """
+
+  # TODO: Cleanup and optimizations
+  # Inspirations: https://www.ffmpeg.org/doxygen/3.4/qt-faststart_8c_source.html
+  #               https://github.com/danielgtaylor/qtfaststart/blob/master/qtfaststart/processor.py
+  #               ISO/IEC 14496-12:2015, ISO/IEC 15444-12:2015
+  #               Paracetamol
+
+  def fix(binary = <<0x00, 0x00, 0x00, _, 0x66, 0x74, 0x79, 0x70, _::binary>>) do
+    index = fix(binary, binary, 0, [])
+
+    case index do
+      [{"ftyp", _, _, _, _}, {"mdat", _, _, _, _} | _] -> faststart(index)
+      [{"ftyp", _, _, _, _}, {"free", _, _, _, _}, {"mdat", _, _, _, _} | _] -> faststart(index)
+      _ -> binary
+    end
+  end
+
+  def fix(binary) do
+    binary
+  end
+
+  defp fix(<<>>, _bin, _pos, acc) do
+    :lists.reverse(acc)
+  end
+
+  defp fix(
+         <<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), rest::binary>>,
+         bin,
+         pos,
+         acc
+       ) do
+    if fourcc == "mdat" && size == 0 do
+      # mdat with 0 size means "seek to the end" -- also, in that case the file is probably OK.
+      acc = [
+        {fourcc, pos, byte_size(bin) - pos, byte_size(bin) - pos,
+         <<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), rest::binary>>}
+        | acc
+      ]
+
+      fix(<<>>, bin, byte_size(bin), acc)
+    else
+      full_size = size - 8
+      <<data::binary-size(full_size), rest::binary>> = rest
+
+      acc = [
+        {fourcc, pos, pos + size, size,
+         <<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), data::binary>>}
+        | acc
+      ]
+
+      fix(rest, bin, pos + size, acc)
+    end
+  end
+
+  defp faststart(index) do
+    {{_ftyp, _, _, _, ftyp}, index} = List.keytake(index, "ftyp", 0)
+
+    # Skip re-writing the free fourcc as it's kind of useless. Why stream useless bytes when you can do without?
+    {free_size, index} =
+      case List.keytake(index, "free", 0) do
+        {{_, _, _, size, _}, index} -> {size, index}
+        _ -> {0, index}
+      end
+
+    {{_moov, _, _, moov_size, moov}, index} = List.keytake(index, "moov", 0)
+    offset = -free_size + moov_size
+    rest = for {_, _, _, _, data} <- index, do: data, into: <<>>
+    <<moov_head::binary-size(8), moov_data::binary>> = moov
+    new_moov = fix_moov(moov_data, offset)
+    <<ftyp::binary, moov_head::binary, new_moov::binary, rest::binary>>
+  end
+
+  defp fix_moov(moov, offset) do
+    fix_moov(moov, offset, <<>>)
+  end
+
+  defp fix_moov(<<>>, _, acc), do: acc
+
+  defp fix_moov(
+         <<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), rest::binary>>,
+         offset,
+         acc
+       ) do
+    full_size = size - 8
+    <<data::binary-size(full_size), rest::binary>> = rest
+
+    data =
+      cond do
+        fourcc in ["trak", "mdia", "minf", "stbl"] ->
+          # Theses contains sto or co64 part
+          <<size::integer-big-size(4)-unit(8), fourcc::binary-size(4),
+            fix_moov(data, offset, <<>>)::binary>>
+
+        fourcc in ["stco", "co64"] ->
+          # fix the damn thing
+          <<version::integer-big-size(4)-unit(8), count::integer-big-size(4)-unit(8),
+            rest::binary>> = data
+
+          entry_size =
+            case fourcc do
+              "stco" -> 4
+              "co64" -> 8
+            end
+
+          {_, result} =
+            Enum.reduce(1..count, {rest, <<>>}, fn _,
+                                                   {<<pos::integer-big-size(entry_size)-unit(8),
+                                                      rest::binary>>, acc} ->
+              {rest, <<acc::binary, pos + offset::integer-big-size(entry_size)-unit(8)>>}
+            end)
+
+          <<size::integer-big-size(4)-unit(8), fourcc::binary-size(4),
+            version::integer-big-size(4)-unit(8), count::integer-big-size(4)-unit(8),
+            result::binary>>
+
+        true ->
+          <<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), data::binary>>
+      end
+
+    acc = <<acc::binary, data::binary>>
+    fix_moov(rest, offset, acc)
+  end
+end