nio4r / non-blocking IO problem

DESCRIPTION.md

I'm trying to implement running ffmpeg process with timeout without resorting to Ruby's Timeout class or spawning a new Thread with counter inside (that's how streamio-ffmpeg does it and it's error-prone and it feels wrong)

Unfortunately when doing non-blocking reads with nio4r, the size of returned chunks of data is pretty much random. While doing synchronous reads result in block yielding every line, in case of nio4r the polling makes it unpredictable.

Is there a better way or should I just somehow glue together responses when I somehow detect the response is incomplete?

example_result.txt

Blocking
0: ffmpeg version 3.0.2 Copyright (c) 2000-2016 the FFmpeg developers
1:   built with Apple LLVM version 7.3.0 (clang-703.0.29)
2:   configuration: --prefix=/usr/local/Cellar/ffmpeg/3.0.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libxvid --enable-vda
3:   libavutil      55. 17.103 / 55. 17.103
4:   libavcodec     57. 24.102 / 57. 24.102
5:   libavformat    57. 25.100 / 57. 25.100
6:   libavdevice    57.  0.101 / 57.  0.101
7:   libavfilter     6. 31.100 /  6. 31.100
8:   libavresample   3.  0.  0 /  3.  0.  0
9:   libswscale      4.  0.100 /  4.  0.100
10:   libswresample   2.  0.101 /  2.  0.101
11:   libpostproc    54.  0.100 / 54.  0.100
12: [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f9219808600] moov atom not found
13: broken.mp4: Invalid data found when processing input

Non-blocking
0: ffmpeg version 3.0.2 Copyright (c) 2000-2016 the FFmpeg developers
  built with Apple LLVM version 7.3.0 (clang-703.0.29)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/3.0.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libxvid --enable-vda
  libavutil      55. 17.103 / 55. 17.103
  libavcodec     57. 24.102 / 57. 24.102
  libavformat    57. 25.100 / 57. 25.100
1:   libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 31.100 /  6. 31.100
  libavresample   3.  0.  0 /  3.  0.  0
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
2: [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f9cd1000000] moov atom not found
broken.mp4: Invalid data found when processing input

test.rb

require 'open3'
require 'nio'

file = 'broken.mp4'
output = 'output.mp4'

puts "Blocking"
stdin, _, stderr, thread = Open3.popen3("ffmpeg -i #{file} #{output}")
index = 0

stdin.close
while line = stderr.gets
  puts "#{index}: #{line}"
  index += 1
end

puts ""
puts "Non-blocking"

selector = NIO::Selector.new
stdin, _, stderr, thread = Open3.popen3("ffmpeg -i #{file} #{output}")

stderr_monitor = selector.register(stderr, :r)
index = 0

stderr_monitor.value = proc do
  line = stderr_monitor.io.read_nonblock(4096)
  puts "#{index}: #{line}"
  index += 1
end

stdin.close
while thread.alive?
  ready = selector.select(30)
  begin
    ready.each { |monitor| monitor.value.call }
  rescue EOFError
    break
  end
end