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IO.select broken with output stream from Open3.popen3 #1783

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mhoran opened this issue Jul 3, 2014 · 3 comments
Closed

IO.select broken with output stream from Open3.popen3 #1783

mhoran opened this issue Jul 3, 2014 · 3 comments
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JRuby 1.7.x
Milestone
Won't Fix

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@mhoran
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mhoran commented Jul 3, 2014

IO.select does not wait for data to be readable, nor does it respect the timeout when reading from an output stream provided by `Open3.popen3'.

require 'open3'
path = "test.txt"
File.open(path, 'w') { |f| f.write('Z' * ARGV[0].to_i) }
STDOUT.write "Starting..."
Open3.popen3('cat', path) do |stdin, stdout, stderr|
  stdin.close
  while !stdout.eof? && IO.select([stdout], nil, nil, 1)
    stdout.read_nonblock 1
  end
end

This results in the following error on my Linux box, and Travis CI:

ruby -Xerrno.backtrace=true test.rb 1024
Starting...IO::EAGAINWaitReadable: Resource temporarily unavailable - 
  read_nonblock at org/jruby/RubyIO.java:2856
         (root) at test.rb:8
         popen3 at org/jruby/RubyIO.java:4433
         popen3 at /home/mhoran/.rbenv/versions/jruby-1.7.10/lib/ruby/1.9/open3.rb:74
         (root) at test.rb:5

If I first pipe the output of stderr to the write end of an IO.pipe, the program works as expected:

require 'open3'
path = "test.txt"
File.open(path, 'w') { |f| f.write('Z' * ARGV[0].to_i) }
STDOUT.write "Starting..."
Open3.popen3('cat', path) do |stdin, stdout, stderr|
  stdin.close
  read, write = IO.pipe
  Thread.new { IO.copy_stream(stdout, write); write.close }
  while !read.eof? && IO.select([read], nil, nil, 1)
    read.read_nonblock 1
  end
end

In actual usage, I'm just using IO.select to determine if there's content ready to be read, along with a timeout to ensure that the application quits if data is not available. However, there seems to be a fundamental issue with output forwarding given IO.pipe works as expected.
@mhoran
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mhoran commented Jul 3, 2014

See thoughtbot/capybara-webkit#651 for an example of actual usage, additional discussion, and failing tests.

@gucki
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gucki commented Sep 11, 2014

Was just hit by this bug as well. Is there any workaround? All I tried so far failed, because other methods like capture3 etc. are also buggy. Also IO#read_nonblock always retuns Errno::EWOULDBLOCK, even if there's data to read and even if the socket is already closed.

@kares
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kares commented Jun 22, 2017

works fine in 9K (checked >= 9.1.7.0), JRuby 1.7.x is EOL ... likely won't fix there.

@kares kares closed this as completed Jun 22, 2017
@kares kares added this to the Won't Fix milestone Jun 22, 2017
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@mhoran @kares @gucki