SocketServer.py

"""Generic socket server classes.
     2 
     3 This module tries to capture the various aspects of defining a server:
     4 
     5 For socket-based servers:
     6 
     7 - address family:
     8         - AF_INET{,6}: IP (Internet Protocol) sockets (default)
     9         - AF_UNIX: Unix domain sockets
    10         - others, e.g. AF_DECNET are conceivable (see <socket.h>
    11 - socket type:
    12         - SOCK_STREAM (reliable stream, e.g. TCP)
    13         - SOCK_DGRAM (datagrams, e.g. UDP)
    14 
    15 For request-based servers (including socket-based):
    16 
    17 - client address verification before further looking at the request
    18         (This is actually a hook for any processing that needs to look
    19          at the request before anything else, e.g. logging)
    20 - how to handle multiple requests:
    21         - synchronous (one request is handled at a time)
    22         - forking (each request is handled by a new process)
    23         - threading (each request is handled by a new thread)
    24 
    25 The classes in this module favor the server type that is simplest to
    26 write: a synchronous TCP/IP server.  This is bad class design, but
    27 save some typing.  (There's also the issue that a deep class hierarchy
    28 slows down method lookups.)
    29 
    30 There are five classes in an inheritance diagram, four of which represent
    31 synchronous servers of four types:
    32 
    33         +------------+
    34         | BaseServer |
    35         +------------+
    36               |
    37               v
    38         +-----------+        +------------------+
    39         | TCPServer |------->| UnixStreamServer |
    40         +-----------+        +------------------+
    41               |
    42               v
    43         +-----------+        +--------------------+
    44         | UDPServer |------->| UnixDatagramServer |
    45         +-----------+        +--------------------+
    46 
    47 Note that UnixDatagramServer derives from UDPServer, not from
    48 UnixStreamServer -- the only difference between an IP and a Unix
    49 stream server is the address family, which is simply repeated in both
    50 unix server classes.
    51 
    52 Forking and threading versions of each type of server can be created
    53 using the ForkingMixIn and ThreadingMixIn mix-in classes.  For
    54 instance, a threading UDP server class is created as follows:
    55 
    56         class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
    57 
    58 The Mix-in class must come first, since it overrides a method defined
    59 in UDPServer! Setting the various member variables also changes
    60 the behavior of the underlying server mechanism.
    61 
    62 To implement a service, you must derive a class from
    63 BaseRequestHandler and redefine its handle() method.  You can then run
    64 various versions of the service by combining one of the server classes
    65 with your request handler class.
    66 
    67 The request handler class must be different for datagram or stream
    68 services.  This can be hidden by using the request handler
    69 subclasses StreamRequestHandler or DatagramRequestHandler.
    70 
    71 Of course, you still have to use your head!
    72 
    73 For instance, it makes no sense to use a forking server if the service
    74 contains state in memory that can be modified by requests (since the
    75 modifications in the child process would never reach the initial state
    76 kept in the parent process and passed to each child).  In this case,
    77 you can use a threading server, but you will probably have to use
    78 locks to avoid two requests that come in nearly simultaneous to apply
    79 conflicting changes to the server state.
    80 
    81 On the other hand, if you are building e.g. an HTTP server, where all
    82 data is stored externally (e.g. in the file system), a synchronous
    83 class will essentially render the service "deaf" while one request is
    84 being handled -- which may be for a very long time if a client is slow
    85 to read all the data it has requested.  Here a threading or forking
    86 server is appropriate.
    87 
    88 In some cases, it may be appropriate to process part of a request
    89 synchronously, but to finish processing in a forked child depending on
    90 the request data.  This can be implemented by using a synchronous
    91 server and doing an explicit fork in the request handler class
    92 handle() method.
    93 
    94 Another approach to handling multiple simultaneous requests in an
    95 environment that supports neither threads nor fork (or where these are
    96 too expensive or inappropriate for the service) is to maintain an
    97 explicit table of partially finished requests and to use select() to
    98 decide which request to work on next (or whether to handle a new
    99 incoming request).  This is particularly important for stream services
   100 where each client can potentially be connected for a long time (if
   101 threads or subprocesses cannot be used).
   102 
   103 Future work:
   104 - Standard classes for Sun RPC (which uses either UDP or TCP)
   105 - Standard mix-in classes to implement various authentication
   106   and encryption schemes
   107 - Standard framework for select-based multiplexing
   108 
   109 XXX Open problems:
   110 - What to do with out-of-band data?
   111 
   112 BaseServer:
   113 - split generic "request" functionality out into BaseServer class.
   114   Copyright (C) 2000  Luke Kenneth Casson Leighton <lkcl@samba.org>
   115 
   116   example: read entries from a SQL database (requires overriding
   117   get_request() to return a table entry from the database).
   118   entry is processed by a RequestHandlerClass.
   119 
   120 """
   121 
   122 # Author of the BaseServer patch: Luke Kenneth Casson Leighton
   123 
   124 # XXX Warning!
   125 # There is a test suite for this module, but it cannot be run by the
   126 # standard regression test.
   127 # To run it manually, run Lib/test/test_socketserver.py.
   128 
   129 __version__ = "0.4"
   130 
   131 
   132 import socket
   133 import select
   134 import sys
   135 import os
   136 import errno
   137 try:
   138     import threading
   139 except ImportError:
   140     import dummy_threading as threading
   141 
   142 __all__ = ["TCPServer","UDPServer","ForkingUDPServer","ForkingTCPServer",
   143            "ThreadingUDPServer","ThreadingTCPServer","BaseRequestHandler",
   144            "StreamRequestHandler","DatagramRequestHandler",
   145            "ThreadingMixIn", "ForkingMixIn"]
   146 if hasattr(socket, "AF_UNIX"):
   147     __all__.extend(["UnixStreamServer","UnixDatagramServer",
   148                     "ThreadingUnixStreamServer",
   149                     "ThreadingUnixDatagramServer"])
   150 
   151 def _eintr_retry(func, *args):
   152     """restart a system call interrupted by EINTR"""
   153     while True:
   154         try:
   155             return func(*args)
   156         except (OSError, select.error) as e:
   157             if e.args[0] != errno.EINTR:
   158                 raise
   159 
   160 class BaseServer:
   161 
   162     """Base class for server classes.
   163 
   164     Methods for the caller:
   165 
   166     - __init__(server_address, RequestHandlerClass)
   167     - serve_forever(poll_interval=0.5)
   168     - shutdown()
   169     - handle_request()  # if you do not use serve_forever()
   170     - fileno() -> int   # for select()
   171 
   172     Methods that may be overridden:
   173 
   174     - server_bind()
   175     - server_activate()
   176     - get_request() -> request, client_address
   177     - handle_timeout()
   178     - verify_request(request, client_address)
   179     - server_close()
   180     - process_request(request, client_address)
   181     - shutdown_request(request)
   182     - close_request(request)
   183     - handle_error()
   184 
   185     Methods for derived classes:
   186 
   187     - finish_request(request, client_address)
   188 
   189     Class variables that may be overridden by derived classes or
   190     instances:
   191 
   192     - timeout
   193     - address_family
   194     - socket_type
   195     - allow_reuse_address
   196 
   197     Instance variables:
   198 
   199     - RequestHandlerClass
   200     - socket
   201 
   202     """
   203 
   204     timeout = None
   205 
   206     def __init__(self, server_address, RequestHandlerClass):
   207         """Constructor.  May be extended, do not override."""
   208         self.server_address = server_address
   209         self.RequestHandlerClass = RequestHandlerClass
   210         self.__is_shut_down = threading.Event()
   211         self.__shutdown_request = False
   212 
   213     def server_activate(self):
   214         """Called by constructor to activate the server.
   215 
   216         May be overridden.
   217 
   218         """
   219         pass
   220 
   221     def serve_forever(self, poll_interval=0.5):
   222         """Handle one request at a time until shutdown.
   223 
   224         Polls for shutdown every poll_interval seconds. Ignores
   225         self.timeout. If you need to do periodic tasks, do them in
   226         another thread.
   227         """
   228         self.__is_shut_down.clear()
   229         try:
   230             while not self.__shutdown_request:
   231                 # XXX: Consider using another file descriptor or
   232                 # connecting to the socket to wake this up instead of
   233                 # polling. Polling reduces our responsiveness to a
   234                 # shutdown request and wastes cpu at all other times.
   235                 r, w, e = _eintr_retry(select.select, [self], [], [],
   236                                        poll_interval)
   237                 if self in r:
   238                     self._handle_request_noblock()
   239         finally:
   240             self.__shutdown_request = False
   241             self.__is_shut_down.set()
   242 
   243     def shutdown(self):
   244         """Stops the serve_forever loop.
   245 
   246         Blocks until the loop has finished. This must be called while
   247         serve_forever() is running in another thread, or it will
   248         deadlock.
   249         """
   250         self.__shutdown_request = True
   251         self.__is_shut_down.wait()
   252 
   253     # The distinction between handling, getting, processing and
   254     # finishing a request is fairly arbitrary.  Remember:
   255     #
   256     # - handle_request() is the top-level call.  It calls
   257     #   select, get_request(), verify_request() and process_request()
   258     # - get_request() is different for stream or datagram sockets
   259     # - process_request() is the place that may fork a new process
   260     #   or create a new thread to finish the request
   261     # - finish_request() instantiates the request handler class;
   262     #   this constructor will handle the request all by itself
   263 
   264     def handle_request(self):
   265         """Handle one request, possibly blocking.
   266 
   267         Respects self.timeout.
   268         """
   269         # Support people who used socket.settimeout() to escape
   270         # handle_request before self.timeout was available.
   271         timeout = self.socket.gettimeout()
   272         if timeout is None:
   273             timeout = self.timeout
   274         elif self.timeout is not None:
   275             timeout = min(timeout, self.timeout)
   276         fd_sets = _eintr_retry(select.select, [self], [], [], timeout)
   277         if not fd_sets[0]:
   278             self.handle_timeout()
   279             return
   280         self._handle_request_noblock()
   281 
   282     def _handle_request_noblock(self):
   283         """Handle one request, without blocking.
   284 
   285         I assume that select.select has returned that the socket is
   286         readable before this function was called, so there should be
   287         no risk of blocking in get_request().
   288         """
   289         try:
   290             request, client_address = self.get_request()
   291         except socket.error:
   292             return
   293         if self.verify_request(request, client_address):
   294             try:
   295                 self.process_request(request, client_address)
   296             except:
   297                 self.handle_error(request, client_address)
   298                 self.shutdown_request(request)
   299 
   300     def handle_timeout(self):
   301         """Called if no new request arrives within self.timeout.
   302 
   303         Overridden by ForkingMixIn.
   304         """
   305         pass
   306 
   307     def verify_request(self, request, client_address):
   308         """Verify the request.  May be overridden.
   309 
   310         Return True if we should proceed with this request.
   311 
   312         """
   313         return True
   314 
   315     def process_request(self, request, client_address):
   316         """Call finish_request.
   317 
   318         Overridden by ForkingMixIn and ThreadingMixIn.
   319 
   320         """
   321         self.finish_request(request, client_address)
   322         self.shutdown_request(request)
   323 
   324     def server_close(self):
   325         """Called to clean-up the server.
   326 
   327         May be overridden.
   328 
   329         """
   330         pass
   331 
   332     def finish_request(self, request, client_address):
   333         """Finish one request by instantiating RequestHandlerClass."""
   334         self.RequestHandlerClass(request, client_address, self)
   335 
   336     def shutdown_request(self, request):
   337         """Called to shutdown and close an individual request."""
   338         self.close_request(request)
   339 
   340     def close_request(self, request):
   341         """Called to clean up an individual request."""
   342         pass
   343 
   344     def handle_error(self, request, client_address):
   345         """Handle an error gracefully.  May be overridden.
   346 
   347         The default is to print a traceback and continue.
   348 
   349         """
   350         print '-'*40
   351         print 'Exception happened during processing of request from',
   352         print client_address
   353         import traceback
   354         traceback.print_exc() # XXX But this goes to stderr!
   355         print '-'*40
   356 
   357 
   358 class TCPServer(BaseServer):
   359 
   360     """Base class for various socket-based server classes.
   361 
   362     Defaults to synchronous IP stream (i.e., TCP).
   363 
   364     Methods for the caller:
   365 
   366     - __init__(server_address, RequestHandlerClass, bind_and_activate=True)
   367     - serve_forever(poll_interval=0.5)
   368     - shutdown()
   369     - handle_request()  # if you don't use serve_forever()
   370     - fileno() -> int   # for select()
   371 
   372     Methods that may be overridden:
   373 
   374     - server_bind()
   375     - server_activate()
   376     - get_request() -> request, client_address
   377     - handle_timeout()
   378     - verify_request(request, client_address)
   379     - process_request(request, client_address)
   380     - shutdown_request(request)
   381     - close_request(request)
   382     - handle_error()
   383 
   384     Methods for derived classes:
   385 
   386     - finish_request(request, client_address)
   387 
   388     Class variables that may be overridden by derived classes or
   389     instances:
   390 
   391     - timeout
   392     - address_family
   393     - socket_type
   394     - request_queue_size (only for stream sockets)
   395     - allow_reuse_address
   396 
   397     Instance variables:
   398 
   399     - server_address
   400     - RequestHandlerClass
   401     - socket
   402 
   403     """
   404 
   405     address_family = socket.AF_INET
   406 
   407     socket_type = socket.SOCK_STREAM
   408 
   409     request_queue_size = 5
   410 
   411     allow_reuse_address = False
   412 
   413     def __init__(self, server_address, RequestHandlerClass, bind_and_activate=True):
   414         """Constructor.  May be extended, do not override."""
   415         BaseServer.__init__(self, server_address, RequestHandlerClass)
   416         self.socket = socket.socket(self.address_family,
   417                                     self.socket_type)
   418         if bind_and_activate:
   419             self.server_bind()
   420             self.server_activate()
   421 
   422     def server_bind(self):
   423         """Called by constructor to bind the socket.
   424 
   425         May be overridden.
   426 
   427         """
   428         if self.allow_reuse_address:
   429             self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
   430         self.socket.bind(self.server_address)
   431         self.server_address = self.socket.getsockname()
   432 
   433     def server_activate(self):
   434         """Called by constructor to activate the server.
   435 
   436         May be overridden.
   437 
   438         """
   439         self.socket.listen(self.request_queue_size)
   440 
   441     def server_close(self):
   442         """Called to clean-up the server.
   443 
   444         May be overridden.
   445 
   446         """
   447         self.socket.close()
   448 
   449     def fileno(self):
   450         """Return socket file number.
   451 
   452         Interface required by select().
   453 
   454         """
   455         return self.socket.fileno()
   456 
   457     def get_request(self):
   458         """Get the request and client address from the socket.
   459 
   460         May be overridden.
   461 
   462         """
   463         return self.socket.accept()
   464 
   465     def shutdown_request(self, request):
   466         """Called to shutdown and close an individual request."""
   467         try:
   468             #explicitly shutdown.  socket.close() merely releases
   469             #the socket and waits for GC to perform the actual close.
   470             request.shutdown(socket.SHUT_WR)
   471         except socket.error:
   472             pass #some platforms may raise ENOTCONN here
   473         self.close_request(request)
   474 
   475     def close_request(self, request):
   476         """Called to clean up an individual request."""
   477         request.close()
   478 
   479 
   480 class UDPServer(TCPServer):
   481 
   482     """UDP server class."""
   483 
   484     allow_reuse_address = False
   485 
   486     socket_type = socket.SOCK_DGRAM
   487 
   488     max_packet_size = 8192
   489 
   490     def get_request(self):
   491         data, client_addr = self.socket.recvfrom(self.max_packet_size)
   492         return (data, self.socket), client_addr
   493 
   494     def server_activate(self):
   495         # No need to call listen() for UDP.
   496         pass
   497 
   498     def shutdown_request(self, request):
   499         # No need to shutdown anything.
   500         self.close_request(request)
   501 
   502     def close_request(self, request):
   503         # No need to close anything.
   504         pass
   505 
   506 class ForkingMixIn:
   507 
   508     """Mix-in class to handle each request in a new process."""
   509 
   510     timeout = 300
   511     active_children = None
   512     max_children = 40
   513 
   514     def collect_children(self):
   515         """Internal routine to wait for children that have exited."""
   516         if self.active_children is None: return
   517         while len(self.active_children) >= self.max_children:
   518             # XXX: This will wait for any child process, not just ones
   519             # spawned by this library. This could confuse other
   520             # libraries that expect to be able to wait for their own
   521             # children.
   522             try:
   523                 pid, status = os.waitpid(0, 0)
   524             except os.error:
   525                 pid = None
   526             if pid not in self.active_children: continue
   527             self.active_children.remove(pid)
   528 
   529         # XXX: This loop runs more system calls than it ought
   530         # to. There should be a way to put the active_children into a
   531         # process group and then use os.waitpid(-pgid) to wait for any
   532         # of that set, but I couldn't find a way to allocate pgids
   533         # that couldn't collide.
   534         for child in self.active_children:
   535             try:
   536                 pid, status = os.waitpid(child, os.WNOHANG)
   537             except os.error:
   538                 pid = None
   539             if not pid: continue
   540             try:
   541                 self.active_children.remove(pid)
   542             except ValueError, e:
   543                 raise ValueError('%s. x=%d and list=%r' % (e.message, pid,
   544                                                            self.active_children))
   545 
   546     def handle_timeout(self):
   547         """Wait for zombies after self.timeout seconds of inactivity.
   548 
   549         May be extended, do not override.
   550         """
   551         self.collect_children()
   552 
   553     def process_request(self, request, client_address):
   554         """Fork a new subprocess to process the request."""
   555         self.collect_children()
   556         pid = os.fork()
   557         if pid:
   558             # Parent process
   559             if self.active_children is None:
   560                 self.active_children = []
   561             self.active_children.append(pid)
   562             self.close_request(request) #close handle in parent process
   563             return
   564         else:
   565             # Child process.
   566             # This must never return, hence os._exit()!
   567             try:
   568                 self.finish_request(request, client_address)
   569                 self.shutdown_request(request)
   570                 os._exit(0)
   571             except:
   572                 try:
   573                     self.handle_error(request, client_address)
   574                     self.shutdown_request(request)
   575                 finally:
   576                     os._exit(1)
   577 
   578 
   579 class ThreadingMixIn:
   580     """Mix-in class to handle each request in a new thread."""
   581 
   582     # Decides how threads will act upon termination of the
   583     # main process
   584     daemon_threads = False
   585 
   586     def process_request_thread(self, request, client_address):
   587         """Same as in BaseServer but as a thread.
   588 
   589         In addition, exception handling is done here.
   590 
   591         """
   592         try:
   593             self.finish_request(request, client_address)
   594             self.shutdown_request(request)
   595         except:
   596             self.handle_error(request, client_address)
   597             self.shutdown_request(request)
   598 
   599     def process_request(self, request, client_address):
   600         """Start a new thread to process the request."""
   601         t = threading.Thread(target = self.process_request_thread,
   602                              args = (request, client_address))
   603         t.daemon = self.daemon_threads
   604         t.start()
   605 
   606 
   607 class ForkingUDPServer(ForkingMixIn, UDPServer): pass
   608 class ForkingTCPServer(ForkingMixIn, TCPServer): pass
   609 
   610 class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
   611 class ThreadingTCPServer(ThreadingMixIn, TCPServer): pass
   612 
   613 if hasattr(socket, 'AF_UNIX'):
   614 
   615     class UnixStreamServer(TCPServer):
   616         address_family = socket.AF_UNIX
   617 
   618     class UnixDatagramServer(UDPServer):
   619         address_family = socket.AF_UNIX
   620 
   621     class ThreadingUnixStreamServer(ThreadingMixIn, UnixStreamServer): pass
   622 
   623     class ThreadingUnixDatagramServer(ThreadingMixIn, UnixDatagramServer): pass
   624 
   625 class BaseRequestHandler:
   626 
   627     """Base class for request handler classes.
   628 
   629     This class is instantiated for each request to be handled.  The
   630     constructor sets the instance variables request, client_address
   631     and server, and then calls the handle() method.  To implement a
   632     specific service, all you need to do is to derive a class which
   633     defines a handle() method.
   634 
   635     The handle() method can find the request as self.request, the
   636     client address as self.client_address, and the server (in case it
   637     needs access to per-server information) as self.server.  Since a
   638     separate instance is created for each request, the handle() method
   639     can define arbitrary other instance variariables.
   640 
   641     """
   642 
   643     def __init__(self, request, client_address, server):
   644         self.request = request
   645         self.client_address = client_address
   646         self.server = server
   647         self.setup()
   648         try:
   649             self.handle()
   650         finally:
   651             self.finish()
   652 
   653     def setup(self):
   654         pass
   655 
   656     def handle(self):
   657         pass
   658 
   659     def finish(self):
   660         pass
   661 
   662 
   663 # The following two classes make it possible to use the same service
   664 # class for stream or datagram servers.
   665 # Each class sets up these instance variables:
   666 # - rfile: a file object from which receives the request is read
   667 # - wfile: a file object to which the reply is written
   668 # When the handle() method returns, wfile is flushed properly
   669 
   670 
   671 class StreamRequestHandler(BaseRequestHandler):
   672 
   673     """Define self.rfile and self.wfile for stream sockets."""
   674 
   675     # Default buffer sizes for rfile, wfile.
   676     # We default rfile to buffered because otherwise it could be
   677     # really slow for large data (a getc() call per byte); we make
   678     # wfile unbuffered because (a) often after a write() we want to
   679     # read and we need to flush the line; (b) big writes to unbuffered
   680     # files are typically optimized by stdio even when big reads
   681     # aren't.
   682     rbufsize = -1
   683     wbufsize = 0
   684 
   685     # A timeout to apply to the request socket, if not None.
   686     timeout = None
   687 
   688     # Disable nagle algorithm for this socket, if True.
   689     # Use only when wbufsize != 0, to avoid small packets.
   690     disable_nagle_algorithm = False
   691 
   692     def setup(self):
   693         self.connection = self.request
   694         if self.timeout is not None:
   695             self.connection.settimeout(self.timeout)
   696         if self.disable_nagle_algorithm:
   697             self.connection.setsockopt(socket.IPPROTO_TCP,
   698                                        socket.TCP_NODELAY, True)
   699         self.rfile = self.connection.makefile('rb', self.rbufsize)
   700         self.wfile = self.connection.makefile('wb', self.wbufsize)
   701 
   702     def finish(self):
   703         if not self.wfile.closed:
   704             self.wfile.flush()
   705         self.wfile.close()
   706         self.rfile.close()
   707 
   708 
   709 class DatagramRequestHandler(BaseRequestHandler):
   710 
   711     # XXX Regrettably, I cannot get this working on Linux;
   712     # s.recvfrom() doesn't return a meaningful client address.
   713 
   714     """Define self.rfile and self.wfile for datagram sockets."""
   715 
   716     def setup(self):
   717         try:
   718             from cStringIO import StringIO
   719         except ImportError:
   720             from StringIO import StringIO
   721         self.packet, self.socket = self.request
   722         self.rfile = StringIO(self.packet)
   723         self.wfile = StringIO()
   724 
   725     def finish(self):
   726         self.socket.sendto(self.wfile.getvalue(), self.client_address)