Captcha.get() can fail due to Port.open message split

[roylez]

This thing drives me nuts. The same Dockerfile compiles okay locally, but the gives me the following when compiled by CI remotely.

In short, Port.open for some unknown reason could return two messages instead of one.

Erlang/OTP 20 [erts-9.3.3.3] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:10] [hipe] [kernel-poll:false]             

Interactive Elixir (1.6.6) - press Ctrl+C to exit (type h() ENTER for help)
iex(xxxx@127.0.0.1)3> Port.open({:spawn, Path.join(:code.priv_dir(:captcha), "captcha")}, [:binary])         
#Port<0.7418>
iex(xxxx@127.0.0.1)4> flush                                                                         
{#Port<0.7418>, {:data, "upadm"}}
{#Port<0.7418>,
 {:data,
  <<71, 73, 70, 56, 57, 97, 200, 0, 70, 0, 131, 0, 0, 233, 30, 99, 233, 30, 99,
    233, 30, 99, 233, 30, 99, 233, 30, 99, 233, 30, 99, 233, 30, 99, 233, 30,
    99, 233, 30, 99, 233, 30, 99, 233, 30, 99, ...>>}}

[roylez]

I find System.cmd works with better consistency. It seems to give more reliable output, and internally, it still uses Port.open.

iex(xxxx@127.0.0.1)17> System.cmd(Path.join(:code.priv_dir(:captcha), "captcha"), [])                                    
{<<119, 117, 121, 102, 115, 71, 73, 70, 56, 57, 97, 200, 0, 70, 0, 131, 0, 0,                                                  
   63, 81, 181, 63, 81, 181, 63, 81, 181, 63, 81, 181, 63, 81, 181, 63, 81, 181,                                               
   63, 81, 181, 63, 81, 181, 63, 81, 181, 63, 81, 181, 63, ...>>, 0}  

[Sebisnow]

Will this pull request be accepted or does Port.open work "more reliably" by now?