New Socket Module

TableOfContents(3)

Introduction

This page descibes the socket module for jython 2.2, which for the first time has support for [http://en.wikipedia.org/wiki/Non-blocking_I/O asynchronous or non-blocking operations]. Hopefully, this new support should make possible the use of event-based server frameworks on jython. Examples of cpython event-based frameworks are [http://twistedmatrix.com/ Twisted], [http://www.zope.org/ Zope] and [http://www.amk.ca/python/code/medusa.html Medusa]. It is considered by some that Asynchronous IO is the only way to address [http://www.kegel.com/c10k.html The C10K problem]

The socket module is pretty much feature complete. It is written to use java.nio apis as much as possible, i.e. all reads and writes are carried out through the sockets java.nio.channel based interface. However, there are some circumstances, specifically timeout operations, where the java.net.socket interfaces have to be used.

The socket module is quite stable; a [http://bugs.jython.org/issue?%40search_text=&title=&%40columns=title&id=&%40columns=id&creation=&creator=&activity=&%40columns=activity&%40sort=activity&actor=&nosy=&type=&components=&versions=&severity=&dependencies=&assignee=amak&keywords=&priority=&%40group=priority&status=&%40columns=status&resolution=&%40pagesize=50&%40startwith=0&%40action=search number of bugs in the functionality have been found and fixed].

There are necessarily some differences between the behaviour of the cpython and jython socket modules, because jython is implemented on the java socket model, which is more restrictive than the C language socket interface that cpython is based on. It is the purpose of this document to describe those differences. If you find a difference in behaviour between cpython and jython that is not documented here, and where jython is behaving differently to the cpython socket documentation, then [http://bugs.jython.org/ that should be considered a bug and reported, please].

Requirements

Non-blocking support

• JVM version with [http://java.sun.com/j2se/1.4.2/docs/guide/nio/ java.nio] support, i.e. >= version 1.4

• Any jython version >= 2.2

SSL support

• JVM version: any version on which jython runs.

• Any jython version >= 2.2

Cpython compatibility

The new socket module has been written to comply as closely as possible with the cpython 2.4 API for non-blocking sockets, therefore you should use the cpython socket documentation as your reference when writing code.

http://www.python.org/doc/2.4/lib/module-socket.html

If the new jython module exhibits behaviour that differs from that described in the cpython documentation, then that should be considered a bug and reported as such.

SSL Support

The module includes an implementation of client side SSL support, which is compatible with the cpython SSL API. The client side ssl example from the cpython documentation should just work.

http://docs.python.org/lib/socket-example.html

However, the cpython SSL API is extremely basic, and essentially only permits the formation of SSL wrapped sockets. It does NOT include support for any of the following

• Management of Certificates, i.e. loading, storing, manipulating certificates.
• Verification of Certificates, i.e. verifying the chain of trust.
• Non-blocking SSL support.
• Server side SSL support.

All of the above are possible, but since no other python version includes that support in the base distribution, I'm not going to do it for jython either; trying to design an API would complex enough; implementing would be a lot of work beyond that.

If you have serious SSL or crypto requirements, then I strongly recommend using the java crypto libraries, or one of the excellent third-party crypto libraries for java, such as that from the [http://www.bouncycastle.org Legion of the Bouncy Castle].

Certificate Checking

By default, the cpython socket library does not carry out certificate checking, which means that it will accept expired certificates, etc. This is acceptable when the validity of the remote certificate is not important, such as in testing.

In Java, by default, certificate checking is enabled. This means that the jython ssl routines *will* verify the validity of the remote certificate, and refuse to form the connection if it is not valid. If this is a problem for you, then you can get around as described here.

By configuring your JVM

If you are using self-generated certificates for testing, then you can import those certificates into the JVM running your jython scripts, and the certificate will then be recognised. See this article from Sun on [http://java.sun.com/j2ee/1.4/docs/tutorial/doc/Security6.html how to install certificates on a JVM].

BY installing your own SecurityManager

TODO: Need to write some code and documentation for how to install your own [http://java.sun.com/j2se/1.4.2/docs/api/java/lang/SecurityManager.html SecurityManager] so that it simply accepts all certificates. Here is a java article on [http://scv.bu.edu/Doc/Java/tutorial/networking/security/index.html Providing Your Own Security Manager]

Cpython versions

It is the intention that these modules be fully API compatible with cpython, as far as is possible or sensible. This means that any cpython socket code that is syntax compatible with your selected jython version should produce identical behaviour to the same code running on cpython.

The unit-tests provided with these modules should also run on all versions of cpython. See below under unit-tests for more details.

Other I/O

The design of the cpython non-blocking API is derived from the UNIX C api, which deals with FILE DESCRIPTORS. Since file descriptors can describe any type of I/O channel on unix OSes, cpython can deal with selecting and polling on multiple channel types, such non-blocking files, pipes and named-pipes, fifos, etc.

The java model for selecting on channels is much more restrictive. There is a java abstract class, java.nio.channels.SelectableChannel, which other channel classes must subclass if they are to be multiplexed. On 1.4 JVMs, only the following classes subclass [http://java.sun.com/j2se/1.4.2/docs/api/java/nio/channels/SelectableChannel.html SelectableChannel].

• [http://java.sun.com/j2se/1.4.2/docs/api/java/nio/channels/SocketChannel.html Socket channels]

• Pipe [http://java.sun.com/j2se/1.4.2/docs/api/java/nio/channels/Pipe.SourceChannel.html source] and [http://java.sun.com/j2se/1.4.2/docs/api/java/nio/channels/Pipe.SinkChannel.html sink] channels

Specifically, [http://java.sun.com/j2se/1.4.2/docs/api/java/nio/channels/FileChannel.html FileChannel's] do not subclass [http://java.sun.com/j2se/1.4.2/docs/api/java/nio/channels/SelectableChannel.html SelectableChannel], and thus it is not possible to include files in non-blocking multiplex operations on Java platforms.

Of the two channel types listed above, these modules only support socket channels. This is because it was necessary to rewrite all of the socket creation calls to return SelectableChannels. To do so for Pipes, which are used for communication with sub-processes, it would be necessary to rewrite the jython sub-process creation modules, i.e. popen, etc, to create SelectableChannels. Although it should be reasonably straightforward to implement this, I have no plans to do this work.

Differences between cpython and jython

Error handling

These modules have been coded so that the error handling is as close to the error handling of cpython as possible. So, ideally, cpython socket and select code run on jython should exhibit identical behaviour to cpython, in terms of exception types raised, error numbers returned, etc.

However, due to the different semantics of java and C, there will be differences in the error handling, which will be documented here as they are discovered.

Differences in the treatment of zero timeout values

On cpython, when you specify a zero (i.e. 0 or 0.0) timeout value, the socket should behave the same as if the socket had been placed in non-blocking mode. See the [http://www.python.org/doc/lib/socket-objects.html cpython socket object documentation] for details.

However, [http://java.sun.com/j2se/1.4.2/docs/api/java/net/Socket.html#setSoTimeout(int) Java interprets a zero timeout value as an infinite timeout], i.e. the socket is placed in blocking mode.

To solve this conflict, I decided that the best thing to do with zero timeouts is to adjust them to the smallest possible timeout in java, which is 1 millisecond. So if you do socket.settimeout(0) with the new jython socket module, what you will really get is equivalent to the cpython call socket.settimeout(0.001).

This means that you may get differing exception signatures when using zero timeouts under cpython and jython.

1. Cpython: socket operations with zero timeouts that fail will raise socket.error exceptions. This is equivalent to a -1 return from the C socket.recv call, with errno set to EAGAIN, meaning "The socket is marked non-blocking and the receive operation would block, or a receive timeout had been set and the timeout expired before data was received."
2. Jython: socket operations with zero timeouts that fail will generate socket.timeout exceptions.

Known issues and workarounds

Null address returned from UDP socket.recvfrom() in timeout mode

This bug is reported on the jython bug tracker: [http://bugs.jython.org/issue1018 UDP recvfrom fails to get the remote address].

Description

When an UDP socket is in timeout mode, the address returned from socket.recvfrom() is always (None, -1).

This only happens in timeout mode, because that code path uses java.net.DatagramSocket.receive() to receive packets. For some reason, DatagramPackets receive()d in this way either

• Return null from the DatagramPacket.getAddress() method, thus preventing us from obtaining the source address

• Cause an exception when DatagramPacket.getSocketAddress() is called.

This bug has been reported to Sun, but is not yet public on the Java bug database.

Workaround

1. Until the java bug is fixed, only use sockets in either blocking or non-blocking mode; the problem will not occur in this case.

2. If you need timeouts, then until the java bug is fixed, you should probably create your own DatagramSockets, directly through the java.net APIs.