module Backticks
  # Represents a running process; provides mechanisms for capturing the process's
  # output, passing input, waiting for the process to end, and learning its
  # exitstatus.
  #
  # Interactive commands print their output to Ruby's STDOUT and STDERR
  # in realtime, and also pass input from Ruby's STDIN to the command's stdin.
  class Command
    # Duration that we use when a caller is willing to wait "forever" for
    # a command to finish. This means that `#join` is buggy when used with
    # commands that take longer than a year to complete. You have been
    # warned!
    FOREVER = 86_400 * 365

    # Number of bytes to read from the command in one "chunk".
    CHUNK = 1_024

    # @return [Integer] child process ID
    attr_reader :pid

    # @return [nil,Process::Status] result of command if it has ended; nil if still running
    attr_reader :status

    # @return [String] all input that has been captured so far
    attr_reader :captured_input

    # @return [String] all output that has been captured so far
    attr_reader :captured_output

    # @return [String] all output to stderr that has been captured so far
    attr_reader :captured_error

    # Watch a running command by taking ownership of the IO objects that
    # are passed in.
    #
    # @param [Integer] pid
    # @param [IO] stdin
    # @param [IO] stdout
    # @param [IO] stderr
    def initialize(pid, stdin, stdout, stderr, interactive:false)
      @pid = pid
      @stdin = stdin
      @stdout = stdout
      @stderr = stderr
      @interactive = !!interactive
      @tap = nil
      @status = nil

      @captured_input  = String.new.force_encoding(Encoding::BINARY)
      @captured_output = String.new.force_encoding(Encoding::BINARY)
      @captured_error  = String.new.force_encoding(Encoding::BINARY)
    end

    # @return [String] a basic string representation of this command
    def to_s
      "#<Backticks::Command(@pid=#{pid},@status=#{@status || 'nil'})>"
    end

    # @return [Boolean] true if this command is tied to STDIN/STDOUT
    def interactive?
      @interactive
    end

    # Block until the command completes; return true if its status
    # was zero, false if nonzero.
    #
    # @return [Boolean]
    def success?
      join
      status.success?
    end

    # Determine whether output has been exhausted.
    def eof?
      @stdout.eof? && @stderr.eof?
    rescue Errno::EIO
      # The result of read operation when pty slave is closed is platform
      # dependent.
      # @see https://stackoverflow.com/questions/10238298/ruby-on-linux-pty-goes-away-without-eof-raises-errnoeio
      true
    end

    # Provide a callback to monitor input and output in real time. This method
    # saves a reference to block for later use; whenever the command generates
    # output or receives input, the block is called back with the name of the
    # stream on which I/O occurred and the actual data that was read or written.
    # @yield
    # @yieldparam [Symbol] stream one of :stdin, :stdout or :stderr
    # @yieldparam [String] data fresh input from the designated stream
    def tap(&block)
      raise StandardError.new("Tap is already set (#{@tap}); cannot set twice") if @tap && @tap != block
      @tap = block
    end

    # Block until the command exits, or until limit seconds have passed. If
    # interactive is true, proxy STDIN to the command and print its output
    # to STDOUT. If the time limit expires, return `nil`; otherwise, return
    # self.
    #
    # If the command has already exited when this method is called, return
    # self immediately.
    #
    # @param [Float,Integer] limit number of seconds to wait before returning
    def join(limit=FOREVER)
      return self if @status

      tf = Time.now + limit
      until (t = Time.now) >= tf
        capture(tf-t)
        res = Process.waitpid(@pid, Process::WNOHANG)
        if res
          @status = $?
          capture(nil) until eof?
          return self
        end
      end

      return nil
    end

    # Block until one of the following happens:
    #  - the command produces fresh output on stdout or stderr
    #  - the user passes some input to the command (if interactive)
    #  - the process exits
    #  - the time limit elapses (if provided) OR 60 seconds pass
    #
    # Return up to CHUNK bytes of fresh output from the process, or return nil
    # if no fresh output was produced
    #
    # @param [Float,Integer] number of seconds to wait before returning nil
    # @return [String,nil] fresh bytes from stdout/stderr, or nil if no output
    def capture(limit=nil)
      streams = [@stdout, @stderr]
      streams << STDIN if STDIN.tty? && interactive?

      ready, _, _ = IO.select(streams, [], [], limit)

      # proxy STDIN to child's stdin
      if ready && ready.include?(STDIN)
        data = STDIN.readpartial(CHUNK) rescue nil
        if data
          data = @tap.call(:stdin, data) if @tap
          if data
            @captured_input << data
            @stdin.write(data)
          end
        else
          @tap.call(:stdin, nil) if @tap
          # our own STDIN got closed; proxy this fact to the child
          @stdin.close unless @stdin.closed?
        end
      end

      # capture child's stdout and maybe proxy to STDOUT
      if ready && ready.include?(@stdout)
        data = @stdout.readpartial(CHUNK) rescue nil
        if data
          data = @tap.call(:stdout, data) if @tap
          if data
            @captured_output << data
            STDOUT.write(data) if interactive?
            fresh_output = data
          end
        end
      end

      # capture child's stderr and maybe proxy to STDERR
      if ready && ready.include?(@stderr)
        data = @stderr.readpartial(CHUNK) rescue nil
        if data
          data = @tap.call(:stderr, data) if @tap
          if data
            @captured_error << data
            STDERR.write(data) if interactive?
          end
        end
      end
      fresh_output
    rescue Interrupt
      # Proxy Ctrl+C to the child
      (Process.kill('INT', @pid) rescue nil) if @interactive
      raise
    end
  end
end