"""Generic socket server classes.

This module tries to capture the various aspects of defining a server:

For socket-based servers:

- address family:

        - AF_INET{,6}: IP (Internet Protocol) sockets (default)

        - AF_UNIX: Unix domain sockets

        - others, e.g. AF_DECNET are conceivable (see <socket.h>

- socket type:

        - SOCK_STREAM (reliable stream, e.g. TCP)

        - SOCK_DGRAM (datagrams, e.g. UDP)

For request-based servers (including socket-based):

- client address verification before further looking at the request

        (This is actually a hook for any processing that needs to look

         at the request before anything else, e.g. logging)

- how to handle multiple requests:

        - synchronous (one request is handled at a time)

        - forking (each request is handled by a new process)

        - threading (each request is handled by a new thread)

The classes in this module favor the server type that is simplest to

write: a synchronous TCP/IP server.  This is bad class design, but

save some typing.  (There's also the issue that a deep class hierarchy

slows down method lookups.)

There are five classes in an inheritance diagram, four of which represent

synchronous servers of four types:

        +------------+

        | BaseServer |

        +------------+

              |

              v

        +-----------+        +------------------+

        | TCPServer |------->| UnixStreamServer |

        +-----------+        +------------------+

              |

              v

        +-----------+        +--------------------+

        | UDPServer |------->| UnixDatagramServer |

        +-----------+        +--------------------+

Note that UnixDatagramServer derives from UDPServer, not from

UnixStreamServer -- the only difference between an IP and a Unix

stream server is the address family, which is simply repeated in both

unix server classes.

Forking and threading versions of each type of server can be created

using the ForkingMixIn and ThreadingMixIn mix-in classes.  For

instance, a threading UDP server class is created as follows:

        class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass

The Mix-in class must come first, since it overrides a method defined

in UDPServer! Setting the various member variables also changes

the behavior of the underlying server mechanism.

To implement a service, you must derive a class from

BaseRequestHandler and redefine its handle() method.  You can then run

various versions of the service by combining one of the server classes

with your request handler class.

The request handler class must be different for datagram or stream

services.  This can be hidden by using the request handler

subclasses StreamRequestHandler or DatagramRequestHandler.

Of course, you still have to use your head!

For instance, it makes no sense to use a forking server if the service

contains state in memory that can be modified by requests (since the

modifications in the child process would never reach the initial state

kept in the parent process and passed to each child).  In this case,

you can use a threading server, but you will probably have to use

locks to avoid two requests that come in nearly simultaneous to apply

conflicting changes to the server state.

On the other hand, if you are building e.g. an HTTP server, where all

data is stored externally (e.g. in the file system), a synchronous

class will essentially render the service "deaf" while one request is

being handled -- which may be for a very long time if a client is slow

to reqd all the data it has requested.  Here a threading or forking

server is appropriate.

In some cases, it may be appropriate to process part of a request

synchronously, but to finish processing in a forked child depending on

the request data.  This can be implemented by using a synchronous

server and doing an explicit fork in the request handler class

handle() method.

Another approach to handling multiple simultaneous requests in an

environment that supports neither threads nor fork (or where these are

too expensive or inappropriate for the service) is to maintain an

explicit table of partially finished requests and to use select() to

decide which request to work on next (or whether to handle a new

incoming request).  This is particularly important for stream services

where each client can potentially be connected for a long time (if

threads or subprocesses cannot be used).

Future work:

- Standard classes for Sun RPC (which uses either UDP or TCP)

- Standard mix-in classes to implement various authentication

  and encryption schemes

- Standard framework for select-based multiplexing

XXX Open problems:

- What to do with out-of-band data?

BaseServer:

- split generic "request" functionality out into BaseServer class.

  Copyright (C) 2000  Luke Kenneth Casson Leighton <lkcl@samba.org>

  example: read entries from a SQL database (requires overriding

  get_request() to return a table entry from the database).

  entry is processed by a RequestHandlerClass.

"""

# Author of the BaseServer patch: Luke Kenneth Casson Leighton

# XXX Warning!

# There is a test suite for this module, but it cannot be run by the

# standard regression test.

# To run it manually, run Lib/test/test_socketserver.py.

__version__ = "0.4"

import socket

import select

import sys

import os

try:

    import threading

except ImportError:

    import dummy_threading as threading

__all__ = ["TCPServer","UDPServer","ForkingUDPServer","ForkingTCPServer",

           "ThreadingUDPServer","ThreadingTCPServer","BaseRequestHandler",

           "StreamRequestHandler","DatagramRequestHandler",

           "ThreadingMixIn", "ForkingMixIn"]

if hasattr(socket, "AF_UNIX"):

    __all__.extend(["UnixStreamServer","UnixDatagramServer",

                    "ThreadingUnixStreamServer",

                    "ThreadingUnixDatagramServer"])

class BaseServer:

    """Base class for server classes.

    Methods for the caller:

    - __init__(server_address, RequestHandlerClass)

    - serve_forever(poll_interval=0.5)

    - shutdown()

    - handle_request()  # if you do not use serve_forever()

    - fileno() -> int   # for select()

    Methods that may be overridden:

    - server_bind()

    - server_activate()

    - get_request() -> request, client_address

    - handle_timeout()

    - verify_request(request, client_address)

    - server_close()

    - process_request(request, client_address)

    - close_request(request)

    - handle_error()

    Methods for derived classes:

    - finish_request(request, client_address)

    Class variables that may be overridden by derived classes or

    instances:

    - timeout

    - address_family

    - socket_type

    - allow_reuse_address

    Instance variables:

    - RequestHandlerClass

    - socket

    """

    timeout = None

    def __init__(self, server_address, RequestHandlerClass):

        """Constructor.  May be extended, do not override."""

        self.server_address = server_address

        self.RequestHandlerClass = RequestHandlerClass

        self.__is_shut_down = threading.Event()

        self.__serving = False

    def server_activate(self):

        """Called by constructor to activate the server.

        May be overridden.

        """

        pass

    def serve_forever(self, poll_interval=0.5):

        """Handle one request at a time until shutdown.

        Polls for shutdown every poll_interval seconds. Ignores

        self.timeout. If you need to do periodic tasks, do them in

        another thread.

        """

        self.__serving = True

        self.__is_shut_down.clear()

        while self.__serving:

            # XXX: Consider using another file descriptor or

            # connecting to the socket to wake this up instead of

            # polling. Polling reduces our responsiveness to a

            # shutdown request and wastes cpu at all other times.

            r, w, e = select.select([self], [], [], poll_interval)

            if r:

                self._handle_request_noblock()

        self.__is_shut_down.set()

    def shutdown(self):

        """Stops the serve_forever loop.

        Blocks until the loop has finished. This must be called while

        serve_forever() is running in another thread, or it will

        deadlock.

        """

        self.__serving = False

        self.__is_shut_down.wait()

    # The distinction between handling, getting, processing and

    # finishing a request is fairly arbitrary.  Remember:

    #

    # - handle_request() is the top-level call.  It calls

    #   select, get_request(), verify_request() and process_request()

    # - get_request() is different for stream or datagram sockets

    # - process_request() is the place that may fork a new process

    #   or create a new thread to finish the request

    # - finish_request() instantiates the request handler class;

    #   this constructor will handle the request all by itself

    def handle_request(self):

        """Handle one request, possibly blocking.

        Respects self.timeout.

        """

        # Support people who used socket.settimeout() to escape

        # handle_request before self.timeout was available.

        timeout = self.socket.gettimeout()

        if timeout is None:

            timeout = self.timeout

        elif self.timeout is not None:

            timeout = min(timeout, self.timeout)

        fd_sets = select.select([self], [], [], timeout)

        if not fd_sets[0]:

            self.handle_timeout()

            return

        self._handle_request_noblock()

    def _handle_request_noblock(self):

        """Handle one request, without blocking.

        I assume that select.select has returned that the socket is

        readable before this function was called, so there should be

        no risk of blocking in get_request().

        """

        try:

            request, client_address = self.get_request()

        except socket.error:

            return

        if self.verify_request(request, client_address):

            try:

                self.process_request(request, client_address)

            except:

                self.handle_error(request, client_address)

                self.close_request(request)

    def handle_timeout(self):

        """Called if no new request arrives within self.timeout.

        Overridden by ForkingMixIn.

        """

        pass

    def verify_request(self, request, client_address):

        """Verify the request.  May be overridden.

        Return True if we should proceed with this request.

        """

        return True

    def process_request(self, request, client_address):

        """Call finish_request.

        Overridden by ForkingMixIn and ThreadingMixIn.

        """

        self.finish_request(request, client_address)

        self.close_request(request)

    def server_close(self):

        """Called to clean-up the server.

        May be overridden.

        """

        pass

    def finish_request(self, request, client_address):

        """Finish one request by instantiating RequestHandlerClass."""

        self.RequestHandlerClass(request, client_address, self)

    def close_request(self, request):

        """Called to clean up an individual request."""

        pass

    def handle_error(self, request, client_address):

        """Handle an error gracefully.  May be overridden.

        The default is to print a traceback and continue.

        """

        print '-'*40

        print 'Exception happened during processing of request from',

        print client_address

        import traceback

        traceback.print_exc() # XXX But this goes to stderr!

        print '-'*40

class TCPServer(BaseServer):

    """Base class for various socket-based server classes.

    Defaults to synchronous IP stream (i.e., TCP).

    Methods for the caller:

    - __init__(server_address, RequestHandlerClass, bind_and_activate=True)

    - serve_forever(poll_interval=0.5)

    - shutdown()

    - handle_request()  # if you don't use serve_forever()

    - fileno() -> int   # for select()

    Methods that may be overridden:

    - server_bind()

    - server_activate()

    - get_request() -> request, client_address

    - handle_timeout()

    - verify_request(request, client_address)

    - process_request(request, client_address)

    - close_request(request)

    - handle_error()

    Methods for derived classes:

    - finish_request(request, client_address)

    Class variables that may be overridden by derived classes or

    instances:

    - timeout

    - address_family

    - socket_type

    - request_queue_size (only for stream sockets)

    - allow_reuse_address

    Instance variables:

    - server_address

    - RequestHandlerClass

    - socket

    """

    address_family = socket.AF_INET

    socket_type = socket.SOCK_STREAM

    request_queue_size = 5

    allow_reuse_address = False

    def __init__(self, server_address, RequestHandlerClass, bind_and_activate=True):

        """Constructor.  May be extended, do not override."""

        BaseServer.__init__(self, server_address, RequestHandlerClass)

        self.socket = socket.socket(self.address_family,

                                    self.socket_type)

        if bind_and_activate:

            self.server_bind()

            self.server_activate()

    def server_bind(self):

        """Called by constructor to bind the socket.

        May be overridden.

        """

        if self.allow_reuse_address:

            self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

        self.socket.bind(self.server_address)

        self.server_address = self.socket.getsockname()

    def server_activate(self):

        """Called by constructor to activate the server.

        May be overridden.

        """

        self.socket.listen(self.request_queue_size)

    def server_close(self):

        """Called to clean-up the server.

        May be overridden.

        """

        self.socket.close()

    def fileno(self):

        """Return socket file number.

        Interface required by select().

        """

        return self.socket.fileno()

    def get_request(self):

        """Get the request and client address from the socket.

        May be overridden.

        """

        return self.socket.accept()

    def close_request(self, request):

        """Called to clean up an individual request."""

        request.close()

class UDPServer(TCPServer):

    """UDP server class."""

    allow_reuse_address = False

    socket_type = socket.SOCK_DGRAM

    max_packet_size = 8192

    def get_request(self):

        data, client_addr = self.socket.recvfrom(self.max_packet_size)

        return (data, self.socket), client_addr

    def server_activate(self):

        # No need to call listen() for UDP.

        pass

    def close_request(self, request):

        # No need to close anything.

        pass

class ForkingMixIn:

    """Mix-in class to handle each request in a new process."""

    timeout = 300

    active_children = None

    max_children = 40

    def collect_children(self):

        """Internal routine to wait for children that have exited."""

        if self.active_children is None: return

        while len(self.active_children) >= self.max_children:

            # XXX: This will wait for any child process, not just ones

            # spawned by this library. This could confuse other

            # libraries that expect to be able to wait for their own

            # children.

            try:

                pid, status = os.waitpid(0, options=0)

            except os.error:

                pid = None

            if pid not in self.active_children: continue

            self.active_children.remove(pid)

        # XXX: This loop runs more system calls than it ought

        # to. There should be a way to put the active_children into a

        # process group and then use os.waitpid(-pgid) to wait for any

        # of that set, but I couldn't find a way to allocate pgids

        # that couldn't collide.

        for child in self.active_children:

            try:

                pid, status = os.waitpid(child, os.WNOHANG)

            except os.error:

                pid = None

            if not pid: continue

            try:

                self.active_children.remove(pid)

            except ValueError, e:

                raise ValueError('%s. x=%d and list=%r' % (e.message, pid,

                                                           self.active_children))

    def handle_timeout(self):

        """Wait for zombies after self.timeout seconds of inactivity.

        May be extended, do not override.

        """

        self.collect_children()

    def process_request(self, request, client_address):

        """Fork a new subprocess to process the request."""

        self.collect_children()

        pid = os.fork()

        if pid:

            # Parent process

            if self.active_children is None:

                self.active_children = []

            self.active_children.append(pid)

            self.close_request(request)