Class: PhusionPassenger::MessageChannel

Inherits:
Object
  • Object
show all
Defined in:
lib/phusion_passenger/message_channel.rb

Overview

This class provides convenience methods for:

  • sending and receiving raw data over an IO channel.

  • sending and receiving messages over an IO channel.

  • file descriptor (IO object) passing over a Unix socket.

All of these methods use exceptions for error reporting.

There are two kinds of messages:

Array messages

These are just a list of strings, and the message itself has a specific length. The contained strings may not contain NUL characters ('\0'). Note that an array message must have at least one element.

Scalar messages

These are byte strings which may contain arbitrary binary data. Scalar messages also have a specific length.

The protocol is designed to be low overhead, easy to implement and easy to parse.

MessageChannel is to be wrapped around an IO object. For example:

a, b = IO.pipe
channel1 = MessageChannel.new(a)
channel2 = MessageChannel.new(b)

# Send an array message.
channel2.write("hello", "world !!")
channel1.read    # => ["hello", "world !!"]

# Send a scalar message.
channel2.write_scalar("some long string which can contain arbitrary binary data")
channel1.read_scalar

The life time of a MessageChannel is independent from that of the wrapped IO object. If a MessageChannel object is destroyed, the underlying IO object is not automatically closed. Call close() if you want to close the underlying IO object.

Note: Be careful with mixing the sending/receiving of array messages, scalar messages and IO objects. If you send a collection of any of these in a specific order, then the receiving side must receive them in the exact some order. So suppose you first send a message, then an IO object, then a scalar, then the receiving side must first receive a message, then an IO object, then a scalar. If the receiving side does things in the wrong order then bad things will happen.

Constant Summary collapse

HEADER_SIZE =

:nodoc:

2
DELIMITER =

:nodoc:

"\0"
DELIMITER_NAME =

:nodoc:

"null byte"

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(io) ⇒ MessageChannel

Create a new MessageChannel by wrapping the given IO object.



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# File 'lib/phusion_passenger/message_channel.rb', line 83

def initialize(io)
	@io = io
end

Instance Attribute Details

#io ⇒ Object (readonly)

The wrapped IO object.



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# File 'lib/phusion_passenger/message_channel.rb', line 80

def io
  @io
end

Instance Method Details

#close ⇒ Object

Close the underlying IO stream. Might raise SystemCallError or IOError when something goes wrong.



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# File 'lib/phusion_passenger/message_channel.rb', line 259

def close
	@io.close
end

#fileno ⇒ Object

Return the file descriptor of the underlying IO object.



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# File 'lib/phusion_passenger/message_channel.rb', line 253

def fileno
	return @io.fileno
end

#read ⇒ Object

Read an array message from the underlying file descriptor. Returns the array message as an array, or nil when end-of-stream has been reached.

Might raise SystemCallError, IOError or SocketError when something goes wrong.



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# File 'lib/phusion_passenger/message_channel.rb', line 93

def read
	buffer = ''
	while buffer.size < HEADER_SIZE
		buffer << @io.readpartial(HEADER_SIZE - buffer.size)
	end
	
	chunk_size = buffer.unpack('n')[0]
	buffer = ''
	while buffer.size < chunk_size
		buffer << @io.readpartial(chunk_size - buffer.size)
	end
	
	message = []
	offset = 0
	delimiter_pos = buffer.index(DELIMITER, offset)
	while !delimiter_pos.nil?
		if delimiter_pos == 0
			message << ""
		else
			message << buffer[offset .. delimiter_pos - 1]
		end
		offset = delimiter_pos + 1
		delimiter_pos = buffer.index(DELIMITER, offset)
	end
	return message
rescue Errno::ECONNRESET
	return nil
rescue EOFError
	return nil
end

#read_scalar(max_size = nil) ⇒ Object

Read a scalar message from the underlying IO object. Returns the read message, or nil on end-of-stream.

Might raise SystemCallError, IOError or SocketError when something goes wrong.

The max_size argument allows one to specify the maximum allowed size for the scalar message. If the received scalar message’s size is larger than max_size, then a SecurityError will be raised.



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# File 'lib/phusion_passenger/message_channel.rb', line 133

def read_scalar(max_size = nil)
	buffer = ''
	temp = ''
	while buffer.size < 4
		buffer << @io.readpartial(4 - buffer.size, temp)
	end
	size = buffer.unpack('N')[0]
	if size == 0
		return ''
	else
		if !max_size.nil? && size > max_size
			raise SecurityError, "Scalar message size (#{size}) " <<
				"exceeds maximum allowed size (#{max_size})."
		end
		buffer = ''
		while buffer.size < size
			temp = '' # JRuby doesn't clear the buffer. TODO: remove this when JRuby has been fixed.
			buffer << @io.readpartial(size - buffer.size, temp)
		end
		return buffer
	end
rescue Errno::ECONNRESET
	return nil
rescue EOFError
	return nil
end

#recv_io(klass = IO, negotiate = true) ⇒ Object

Receive an IO object (a file descriptor) from the channel. The other side must have sent an IO object by calling send_io(). Note that this only works on Unix sockets.

Might raise SystemCallError, IOError or SocketError when something goes wrong.



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# File 'lib/phusion_passenger/message_channel.rb', line 196

def recv_io(klass = IO, negotiate = true)
	write("pass IO") if negotiate
	io = @io.recv_io(klass)
	write("got IO") if negotiate
	return io
end

#send_io(io) ⇒ Object

Send an IO object (a file descriptor) over the channel. The other side must receive the IO object by calling recv_io(). Note that this only works on Unix sockets.

Might raise SystemCallError, IOError or SocketError when something goes wrong.



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# File 'lib/phusion_passenger/message_channel.rb', line 209

def send_io(io)
	# We read a message before actually calling #send_io
	# in order to prevent the other side from accidentally
	# read()ing past the normal data and reading our file
	# descriptor too.
	#
	# For example suppose that side A looks like this:
	#
	#   read(fd, buf, 1024)
	#   read_io(fd)
	#
	# and side B:
	#
	#   write(fd, buf, 100)
	#   send_io(fd_to_pass)
	#
	# If B completes both write() and send_io(), then A's read() call
	# reads past the 100 bytes that B sent. On some platforms, like
	# Linux, this will cause read_io() to fail. And it just so happens
	# that Ruby's IO#read method slurps more than just the given amount
	# of bytes.
	result = read
	if !result
		raise EOFError, "End of stream"
	elsif result != ["pass IO"]
		raise IOError, "IO passing pre-negotiation header expected"
	else
		@io.send_io(io)
		# Once you've sent the IO you expect to be able to close it on the
		# sender's side, even if the other side hasn't read the IO yet.
		# Not so: on some operating systems (I'm looking at you OS X) this
		# can cause the receiving side to receive a bad file descriptor.
		# The post negotiation protocol ensures that we block until the
		# other side has really received the IO.
		result = read
		if !result
			raise EOFError, "End of stream"
		elsif result != ["got IO"]
			raise IOError, "IO passing post-negotiation header expected"
		end
	end
end

#write(name, *args) ⇒ Object

Send an array message, which consists of the given elements, over the underlying file descriptor. name is the first element in the message, and args are the other elements. These arguments will internally be converted to strings by calling to_s().

Might raise SystemCallError, IOError or SocketError when something goes wrong.



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# File 'lib/phusion_passenger/message_channel.rb', line 167

def write(name, *args)
	check_argument(name)
	args.each do |arg|
		check_argument(arg)
	end
	
	message = "#{name}#{DELIMITER}"
	args.each do |arg|
		message << arg.to_s << DELIMITER
	end
	@io.write([message.size].pack('n') << message)
	@io.flush
end

#write_scalar(data) ⇒ Object

Send a scalar message over the underlying IO object.

Might raise SystemCallError, IOError or SocketError when something goes wrong.



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# File 'lib/phusion_passenger/message_channel.rb', line 185

def write_scalar(data)
	@io.write([data.size].pack('N') << data)
	@io.flush
end