Non-blocking STDIN read

Help & Support
1

I doing this:

unless STDIN.tty?
    content = STDIN.gets_to_end
end

when there is no stdin, this works in terminal, but stale when running in docker compose run - because docker not close stdin.

if I do something like this:

unless STDIN.tty?
  STDIN.read_timeout = 0.001.seconds
  content = begin
    STDIN.gets_to_end
  rescue 
    nil
  end
end

it adds timeout of 1sec.

Is there any solution?

2

On POSIX systems use LibC.select with zero timeout to check if STDIN is readable before reading.

def read_stdin : String?
 fds = LibC::FdSet.new
 fds.fds_bits[0] = 1
 LibC.select(1, pointerof(fds), nil, nil, out tv) > 0 ? STDIN.gets_to_end : nil
end

HIH

3

Similarly, if the STDIN is opened in nonblocking mode one can just issue `LibC.read` on it until it returns EAGAIN or EOF (and collect the results). Naqvis’ solution seems cleaner.

I do wonder if there are portable solutions though.

4

It actually was not just docker bug, it was pipeline, ruby with backticks calls docker which run crystal.
when I change backticks to system, bug gone. but still there is some problem in crystal, because no any other soft with stdin capabilities hangs, except crystal.