pipetool

#!/usr/bin/env ruby
# This tool reads from STDIN and forwards all data to a file.
# It reopens the file upon receiving the SIGHUP signal.
require 'fcntl'

class Pipetool
  def initialize(input, filename)
    @input = input
    @filename = filename
    @event_pipe = IO.pipe
    make_nonblock(@input)
    make_nonblock(@event_pipe[0])
    make_nonblock(@event_pipe[1])
    @input.binmode
    @event_pipe[1].sync = true
    reopen_file
    trap("HUP", &method(:on_sighup))
    debug "Event pipe on FDs #{@event_pipe[0].fileno}, #{@event_pipe[1].fileno}"
  end

  def main
    debug "Pipetool started on PID #{Process.pid}"
    done = false
    while !done
      ios = select([@input, @event_pipe[0]])[0]
      if ios[0] == @event_pipe[0]
        debug "Reopening #{@filename}"
        drain_io(@event_pipe[0])
        reopen_file
      else
        done = forward_block
      end
    end
  rescue SignalException => e
    if !is_exit_signal?(e)
      raise
    end
  end

private
  def make_nonblock(io)
    flags = io.fcntl(Fcntl::F_GETFL)
    io.fcntl(Fcntl::F_SETFL, flags | Fcntl::O_NONBLOCK)
  end

  def on_sighup(signo)
    debug "SIGHUP received"
    @event_pipe[1].write('x')
  end

  def drain_io(io)
    while true
      begin
        io.read_nonblock(1024 * 16)
      rescue Errno::EAGAIN, Errno::EWOULDBLOCK, Errno::EINTR
        break
      end
    end
  end

  def reopen_file
    @output.close if @output
    @output = File.open(@filename, "ab")
    @output.sync = true
  end

  def forward_block
    begin
      buf = @input.read_nonblock(1024 * 32)
    rescue EOFError
      return true
    rescue Errno::EAGAIN, Errno::EWOULDBLOCK, Errno::EINTR
      return false
    end

    @output.write(buf)
    false
  end

  def is_exit_signal?(e)
    e.signo == Signal.list['TERM'] || e.signo == Signal.list['INT']
  end

  def debug(message)
    #STDERR.puts(message)
  end
end

abort "Usage: pipetool <FILENAME>" if !ARGV[0]
Pipetool.new(STDIN, ARGV[0]).main