Environment

Ruby 2.3.3, 2.4.1, 2.5.0 for expected results
JRuby 9.0.5, 9.1.7, 9.1.16 for actual results
Windows 7 64 bit
java -version output

java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)

Other relevant info you may wish to add:

• Installed or activated gems - None
• Application/framework version (e.g. Rails, Sinatra) - None
• Environment variables - None that matter. These was done under vanilla installs of Ruby and JRuby.

Expected Behavior

The attached archive contains a script called test.rb.

child_process_io_select_demo.zip

When I run test.rb under Ruby >= 2.3 I get results similar to this.

stdout is #IO:0x00000002da0668
stderr is #IO:0x00000002da05c8
No IO Available - select timeout worked
No IO Available - select timeout worked
No IO Available - select timeout worked
Read 100 from stream #IO:0x00000002da0668
Read 100 from stream #IO:0x00000002da0668
Read 100 from stream #IO:0x00000002da0668
Read 100 from stream #IO:0x00000002da0668
Read 100 from stream #IO:0x00000002da0668
Read 100 from stream #IO:0x00000002da0668
Read 100 from stream #IO:0x00000002da0668
Read 100 from stream #IO:0x00000002da0668
Read 100 from stream #IO:0x00000002da0668
Read 100 from stream #IO:0x00000002da0668
No IO Available - select timeout worked
No IO Available - select timeout worked
Read 4 from stream #IO:0x00000002da0668
Read 54 from stream #IO:0x00000002da05c8
No IO Available - select timeout worked
EAGAINWaitReadable exceptions 0
EOFError exceptions 2
total duration 7.243
avg IO.Select time 0.4003888888888889

Actual Behavior

When I run the script under the JRuby versions listed at the top I get results similar to this.

stdout is #IO:0x553a3d88
stderr is #IO:0x5891e32e
EAGAINWaitReadable exceptions 912342
EOFError exceptions 0
total duration 12.085
avg IO.Select time 4.6999918890062625e-06

It seems that select is not working (avg select function time is 0.00000469 seconds). When read_nonblock is called it raises EAGAINWaitReadable every time it is called. It is then impossible to read from the stdout after that.