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419 lines
13 KiB
Python
419 lines
13 KiB
Python
"""Generic socket server classes.
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This module tries to capture the various aspects of defining a server:
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- address family:
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- AF_INET: IP (Internet Protocol) sockets (default)
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- AF_UNIX: Unix domain sockets
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- others, e.g. AF_DECNET are conceivable (see <socket.h>
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- socket type:
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- SOCK_STREAM (reliable stream, e.g. TCP)
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- SOCK_DGRAM (datagrams, e.g. UDP)
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- client address verification before further looking at the request
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(This is actually a hook for any processing that needs to look
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at the request before anything else, e.g. logging)
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- how to handle multiple requests:
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- synchronous (one request is handled at a time)
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- forking (each request is handled by a new process)
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- threading (each request is handled by a new thread)
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The classes in this module favor the server type that is simplest to
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write: a synchronous TCP/IP server. This is bad class design, but
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save some typing. (There's also the issue that a deep class hierarchy
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slows down method lookups.)
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There are four classes in an inheritance diagram that represent
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synchronous servers of four types:
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+-----------+ +------------------+
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| TCPServer |------->| UnixStreamServer |
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+-----------+ +------------------+
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v
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+-----------+ +--------------------+
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| UDPServer |------->| UnixDatagramServer |
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+-----------+ +--------------------+
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Note that UnixDatagramServer derives from UDPServer, not from
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UnixStreamServer -- the only difference between an IP and a Unix
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stream server is the address family, which is simply repeated in both
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unix server classes.
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Forking and threading versions of each type of server can be created
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using the ForkingServer and ThreadingServer mix-in classes. For
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instance, a threading UDP server class is created as follows:
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class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
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The Mix-in class must come first, since it overrides a method defined
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in UDPServer!
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To implement a service, you must derive a class from
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BaseRequestHandler and redefine its handle() method. You can then run
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various versions of the service by combining one of the server classes
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with your request handler class.
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The request handler class must be different for datagram or stream
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services. This can be hidden by using the mix-in request handler
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classes StreamRequestHandler or DatagramRequestHandler.
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Of course, you still have to use your head!
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For instance, it makes no sense to use a forking server if the service
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contains state in memory that can be modified by requests (since the
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modifications in the child process would never reach the initial state
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kept in the parent process and passed to each child). In this case,
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you can use a threading server, but you will probably have to use
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locks to avoid two requests that come in nearly simultaneous to apply
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conflicting changes to the server state.
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On the other hand, if you are building e.g. an HTTP server, where all
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data is stored externally (e.g. in the file system), a synchronous
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class will essentially render the service "deaf" while one request is
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being handled -- which may be for a very long time if a client is slow
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to reqd all the data it has requested. Here a threading or forking
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server is appropriate.
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In some cases, it may be appropriate to process part of a request
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synchronously, but to finish processing in a forked child depending on
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the request data. This can be implemented by using a synchronous
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server and doing an explicit fork in the request handler class's
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handle() method.
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Another approach to handling multiple simultaneous requests in an
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environment that supports neither threads nor fork (or where these are
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too expensive or inappropriate for the service) is to maintain an
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explicit table of partially finished requests and to use select() to
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decide which request to work on next (or whether to handle a new
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incoming request). This is particularly important for stream services
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where each client can potentially be connected for a long time (if
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threads or subprocesses can't be used).
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Future work:
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- Standard classes for Sun RPC (which uses either UDP or TCP)
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- Standard mix-in classes to implement various authentication
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and encryption schemes
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- Standard framework for select-based multiplexing
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XXX Open problems:
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- What to do with out-of-band data?
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"""
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__version__ = "0.2"
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import socket
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import sys
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import os
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class TCPServer:
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"""Base class for various socket-based server classes.
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Defaults to synchronous IP stream (i.e., TCP).
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Methods for the caller:
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- __init__(server_address, RequestHandlerClass)
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- serve_forever()
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- handle_request() # if you don't use serve_forever()
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- fileno() -> int # for select()
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Methods that may be overridden:
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- server_bind()
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- server_activate()
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- get_request() -> request, client_address
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- verify_request(request, client_address)
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- process_request(request, client_address)
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- handle_error()
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Methods for derived classes:
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- finish_request(request, client_address)
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Class variables that may be overridden by derived classes or
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instances:
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- address_family
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- socket_type
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- request_queue_size (only for stream sockets)
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Instance variables:
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- server_address
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- RequestHandlerClass
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- socket
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"""
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address_family = socket.AF_INET
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socket_type = socket.SOCK_STREAM
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request_queue_size = 5
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def __init__(self, server_address, RequestHandlerClass):
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"""Constructor. May be extended, do not override."""
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self.server_address = server_address
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self.RequestHandlerClass = RequestHandlerClass
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self.socket = socket.socket(self.address_family,
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self.socket_type)
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self.server_bind()
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self.server_activate()
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def server_bind(self):
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"""Called by constructor to bind the socket.
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May be overridden.
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"""
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self.socket.bind(self.server_address)
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def server_activate(self):
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"""Called by constructor to activate the server.
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May be overridden.
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"""
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self.socket.listen(self.request_queue_size)
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def fileno(self):
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"""Return socket file number.
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Interface required by select().
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"""
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return self.socket.fileno()
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def serve_forever(self):
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"""Handle one request at a time until doomsday."""
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while 1:
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self.handle_request()
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# The distinction between handling, getting, processing and
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# finishing a request is fairly arbitrary. Remember:
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#
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# - handle_request() is the top-level call. It calls
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# get_request(), verify_request() and process_request()
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# - get_request() is different for stream or datagram sockets
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# - process_request() is the place that may fork a new process
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# or create a new thread to finish the request
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# - finish_request() instantiates the request handler class;
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# this constructor will handle the request all by itself
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def handle_request(self):
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"""Handle one request, possibly blocking."""
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request, client_address = self.get_request()
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if self.verify_request(request, client_address):
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try:
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self.process_request(request, client_address)
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except:
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self.handle_error(request, client_address)
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def get_request(self):
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"""Get the request and client address from the socket.
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May be overridden.
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"""
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return self.socket.accept()
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def verify_request(self, request, client_address):
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"""Verify the request. May be overridden.
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Return true if we should proceed with this request.
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"""
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return 1
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def process_request(self, request, client_address):
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"""Call finish_request.
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Overridden by ForkingMixIn and ThreadingMixIn.
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"""
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self.finish_request(request, client_address)
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def finish_request(self, request, client_address):
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"""Finish one request by instantiating RequestHandlerClass."""
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self.RequestHandlerClass(request, client_address, self)
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def handle_error(self, request, client_address):
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"""Handle an error gracefully. May be overridden.
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The default is to print a traceback and continue.
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"""
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print '-'*40
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print 'Exception happened during processing of request from',
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print client_address
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import traceback
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traceback.print_exc()
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print '-'*40
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class UDPServer(TCPServer):
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"""UDP server class."""
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socket_type = socket.SOCK_DGRAM
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max_packet_size = 8192
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def get_request(self):
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data, client_addr = self.socket.recvfrom(self.max_packet_size)
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return (data, self.socket), client_addr
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def server_activate(self):
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# No need to call listen() for UDP.
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pass
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if hasattr(socket, 'AF_UNIX'):
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class UnixStreamServer(TCPServer):
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address_family = socket.AF_UNIX
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class UnixDatagramServer(UDPServer):
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address_family = socket.AF_UNIX
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class ForkingMixIn:
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"""Mix-in class to handle each request in a new process."""
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active_children = None
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def collect_children(self):
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"""Internal routine to wait for died children."""
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while self.active_children:
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pid, status = os.waitpid(0, os.WNOHANG)
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if not pid: break
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self.active_children.remove(pid)
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def process_request(self, request, client_address):
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"""Fork a new subprocess to process the request."""
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self.collect_children()
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pid = os.fork()
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if pid:
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# Parent process
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if self.active_children is None:
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self.active_children = []
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self.active_children.append(pid)
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return
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else:
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# Child process.
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# This must never return, hence os._exit()!
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try:
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self.finish_request(request, client_address)
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os._exit(0)
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except:
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try:
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self.handle_error(request,
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client_address)
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finally:
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os._exit(1)
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class ThreadingMixIn:
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"""Mix-in class to handle each request in a new thread."""
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def process_request(self, request, client_address):
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"""Start a new thread to process the request."""
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import thread
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thread.start_new_thread(self.finish_request,
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(request, client_address))
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class ForkingUDPServer(ForkingMixIn, UDPServer): pass
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class ForkingTCPServer(ForkingMixIn, TCPServer): pass
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class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
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class ThreadingTCPServer(ThreadingMixIn, TCPServer): pass
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class BaseRequestHandler:
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"""Base class for request handler classes.
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This class is instantiated for each request to be handled. The
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constructor sets the instance variables request, client_address
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and server, and then calls the handle() method. To implement a
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specific service, all you need to do is to derive a class which
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defines a handle() method.
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The handle() method can find the request as self.request, the
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client address as self.client_request, and the server (in case it
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needs access to per-server information) as self.server. Since a
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separate instance is created for each request, the handle() method
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can define arbitrary other instance variariables.
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"""
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def __init__(self, request, client_address, server):
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self.request = request
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self.client_address = client_address
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self.server = server
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try:
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self.setup()
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self.handle()
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self.finish()
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finally:
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sys.exc_traceback = None # Help garbage collection
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def setup(self):
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pass
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def __del__(self):
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pass
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def handle(self):
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pass
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def finish(self):
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pass
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# The following two classes make it possible to use the same service
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# class for stream or datagram servers.
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# Each class sets up these instance variables:
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# - rfile: a file object from which receives the request is read
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# - wfile: a file object to which the reply is written
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# When the handle() method returns, wfile is flushed properly
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class StreamRequestHandler(BaseRequestHandler):
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"""Define self.rfile and self.wfile for stream sockets."""
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def setup(self):
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self.connection = self.request
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self.rfile = self.connection.makefile('rb', 0)
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self.wfile = self.connection.makefile('wb', 0)
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def finish(self):
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self.wfile.flush()
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self.wfile.close()
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self.rfile.close()
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class DatagramRequestHandler(BaseRequestHandler):
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"""Define self.rfile and self.wfile for datagram sockets."""
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def setup(self):
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import StringIO
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self.packet, self.socket = self.request
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self.rfile = StringIO.StringIO(self.packet)
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self.wfile = StringIO.StringIO(self.packet)
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def finish(self):
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self.socket.sendto(self.wfile.getvalue(), self.client_address)
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