Lib/SocketServer.py

   1 """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 reqd 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 try:
 137     import threading
 138 except ImportError:
 139     import dummy_threading as threading
 140
 141 __all__ = ["TCPServer","UDPServer","ForkingUDPServer","ForkingTCPServer",
 142            "ThreadingUDPServer","ThreadingTCPServer","BaseRequestHandler",
 143            "StreamRequestHandler","DatagramRequestHandler",
 144            "ThreadingMixIn", "ForkingMixIn"]
 145 if hasattr(socket, "AF_UNIX"):
 146     __all__.extend(["UnixStreamServer","UnixDatagramServer",
 147                     "ThreadingUnixStreamServer",
 148                     "ThreadingUnixDatagramServer"])
 149
 150 class BaseServer:
 151
 152     """Base class for server classes.
 153
 154     Methods for the caller:
 155
 156     - __init__(server_address, RequestHandlerClass)
 157     - serve_forever(poll_interval=0.5)
 158     - shutdown()
 159     - handle_request()  # if you do not use serve_forever()
 160     - fileno() -> int   # for select()
 161
 162     Methods that may be overridden:
 163
 164     - server_bind()
 165     - server_activate()
 166     - get_request() -> request, client_address
 167     - handle_timeout()
 168     - verify_request(request, client_address)
 169     - server_close()
 170     - process_request(request, client_address)
 171     - close_request(request)
 172     - handle_error()
 173
 174     Methods for derived classes:
 175
 176     - finish_request(request, client_address)
 177
 178     Class variables that may be overridden by derived classes or
 179     instances:
 180
 181     - timeout
 182     - address_family
 183     - socket_type
 184     - allow_reuse_address
 185
 186     Instance variables:
 187
 188     - RequestHandlerClass
 189     - socket
 190
 191     """
 192
 193     timeout = None
 194
 195     def __init__(self, server_address, RequestHandlerClass):
 196         """Constructor.  May be extended, do not override."""
 197         self.server_address = server_address
 198         self.RequestHandlerClass = RequestHandlerClass
 199         self.__is_shut_down = threading.Event()
 200         self.__serving = False
 201
 202     def server_activate(self):
 203         """Called by constructor to activate the server.
 204
 205         May be overridden.
 206
 207         """
 208         pass
 209
 210     def serve_forever(self, poll_interval=0.5):
 211         """Handle one request at a time until shutdown.
 212
 213         Polls for shutdown every poll_interval seconds. Ignores
 214         self.timeout. If you need to do periodic tasks, do them in
 215         another thread.
 216         """
 217         self.__serving = True
 218         self.__is_shut_down.clear()
 219         while self.__serving:
 220             # XXX: Consider using another file descriptor or
 221             # connecting to the socket to wake this up instead of
 222             # polling. Polling reduces our responsiveness to a
 223             # shutdown request and wastes cpu at all other times.
 224             r, w, e = select.select([self], [], [], poll_interval)
 225             if r:
 226                 self._handle_request_noblock()
 227         self.__is_shut_down.set()
 228
 229     def shutdown(self):
 230         """Stops the serve_forever loop.
 231
 232         Blocks until the loop has finished. This must be called while
 233         serve_forever() is running in another thread, or it will
 234         deadlock.
 235         """
 236         self.__serving = False
 237         self.__is_shut_down.wait()
 238
 239     # The distinction between handling, getting, processing and
 240     # finishing a request is fairly arbitrary.  Remember:
 241     #
 242     # - handle_request() is the top-level call.  It calls
 243     #   select, get_request(), verify_request() and process_request()
 244     # - get_request() is different for stream or datagram sockets
 245     # - process_request() is the place that may fork a new process
 246     #   or create a new thread to finish the request
 247     # - finish_request() instantiates the request handler class;
 248     #   this constructor will handle the request all by itself
 249
 250     def handle_request(self):
 251         """Handle one request, possibly blocking.
 252
 253         Respects self.timeout.
 254         """
 255         # Support people who used socket.settimeout() to escape
 256         # handle_request before self.timeout was available.
 257         timeout = self.socket.gettimeout()
 258         if timeout is None:
 259             timeout = self.timeout
 260         elif self.timeout is not None:
 261             timeout = min(timeout, self.timeout)
 262         fd_sets = select.select([self], [], [], timeout)
 263         if not fd_sets[0]:
 264             self.handle_timeout()
 265             return
 266         self._handle_request_noblock()
 267
 268     def _handle_request_noblock(self):
 269         """Handle one request, without blocking.
 270
 271         I assume that select.select has returned that the socket is
 272         readable before this function was called, so there should be
 273         no risk of blocking in get_request().
 274         """
 275         try:
 276             request, client_address = self.get_request()
 277         except socket.error:
 278             return
 279         if self.verify_request(request, client_address):
 280             try:
 281                 self.process_request(request, client_address)
 282             except:
 283                 self.handle_error(request, client_address)
 284                 self.close_request(request)
 285
 286     def handle_timeout(self):
 287         """Called if no new request arrives within self.timeout.
 288
 289         Overridden by ForkingMixIn.
 290         """
 291         pass
 292
 293     def verify_request(self, request, client_address):
 294         """Verify the request.  May be overridden.
 295
 296         Return True if we should proceed with this request.
 297
 298         """
 299         return True
 300
 301     def process_request(self, request, client_address):
 302         """Call finish_request.
 303
 304         Overridden by ForkingMixIn and ThreadingMixIn.
 305
 306         """
 307         self.finish_request(request, client_address)
 308         self.close_request(request)
 309
 310     def server_close(self):
 311         """Called to clean-up the server.
 312
 313         May be overridden.
 314
 315         """
 316         pass
 317
 318     def finish_request(self, request, client_address):
 319         """Finish one request by instantiating RequestHandlerClass."""
 320         self.RequestHandlerClass(request, client_address, self)
 321
 322     def close_request(self, request):
 323         """Called to clean up an individual request."""
 324         pass
 325
 326     def handle_error(self, request, client_address):
 327         """Handle an error gracefully.  May be overridden.
 328
 329         The default is to print a traceback and continue.
 330
 331         """
 332         print '-'*40
 333         print 'Exception happened during processing of request from',
 334         print client_address
 335         import traceback
 336         traceback.print_exc() # XXX But this goes to stderr!
 337         print '-'*40
 338
 339
 340 class TCPServer(BaseServer):
 341
 342     """Base class for various socket-based server classes.
 343
 344     Defaults to synchronous IP stream (i.e., TCP).
 345
 346     Methods for the caller:
 347
 348     - __init__(server_address, RequestHandlerClass, bind_and_activate=True)
 349     - serve_forever(poll_interval=0.5)
 350     - shutdown()
 351     - handle_request()  # if you don't use serve_forever()
 352     - fileno() -> int   # for select()
 353
 354     Methods that may be overridden:
 355
 356     - server_bind()
 357     - server_activate()
 358     - get_request() -> request, client_address
 359     - handle_timeout()
 360     - verify_request(request, client_address)
 361     - process_request(request, client_address)
 362     - close_request(request)
 363     - handle_error()
 364
 365     Methods for derived classes:
 366
 367     - finish_request(request, client_address)
 368
 369     Class variables that may be overridden by derived classes or
 370     instances:
 371
 372     - timeout
 373     - address_family
 374     - socket_type
 375     - request_queue_size (only for stream sockets)
 376     - allow_reuse_address
 377
 378     Instance variables:
 379
 380     - server_address
 381     - RequestHandlerClass
 382     - socket
 383
 384     """
 385
 386     address_family = socket.AF_INET
 387
 388     socket_type = socket.SOCK_STREAM
 389
 390     request_queue_size = 5
 391
 392     allow_reuse_address = False
 393
 394     def __init__(self, server_address, RequestHandlerClass, bind_and_activate=True):
 395         """Constructor.  May be extended, do not override."""
 396         BaseServer.__init__(self, server_address, RequestHandlerClass)
 397         self.socket = socket.socket(self.address_family,
 398                                     self.socket_type)
 399         if bind_and_activate:
 400             self.server_bind()
 401             self.server_activate()
 402
 403     def server_bind(self):
 404         """Called by constructor to bind the socket.
 405
 406         May be overridden.
 407
 408         """
 409         if self.allow_reuse_address:
 410             self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
 411         self.socket.bind(self.server_address)
 412         self.server_address = self.socket.getsockname()
 413
 414     def server_activate(self):
 415         """Called by constructor to activate the server.
 416
 417         May be overridden.
 418
 419         """
 420         self.socket.listen(self.request_queue_size)
 421
 422     def server_close(self):
 423         """Called to clean-up the server.
 424
 425         May be overridden.
 426
 427         """
 428         self.socket.close()
 429
 430     def fileno(self):
 431         """Return socket file number.
 432
 433         Interface required by select().
 434
 435         """
 436         return self.socket.fileno()
 437
 438     def get_request(self):
 439         """Get the request and client address from the socket.
 440
 441         May be overridden.
 442
 443         """
 444         return self.socket.accept()
 445
 446     def close_request(self, request):
 447         """Called to clean up an individual request."""
 448         request.close()
 449
 450
 451 class UDPServer(TCPServer):
 452
 453     """UDP server class."""
 454
 455     allow_reuse_address = False
 456
 457     socket_type = socket.SOCK_DGRAM
 458
 459     max_packet_size = 8192
 460
 461     def get_request(self):
 462         data, client_addr = self.socket.recvfrom(self.max_packet_size)
 463         return (data, self.socket), client_addr
 464
 465     def server_activate(self):
 466         # No need to call listen() for UDP.
 467         pass
 468
 469     def close_request(self, request):
 470         # No need to close anything.
 471         pass
 472
 473 class ForkingMixIn:
 474
 475     """Mix-in class to handle each request in a new process."""
 476
 477     timeout = 300
 478     active_children = None
 479     max_children = 40
 480
 481     def collect_children(self):
 482         """Internal routine to wait for children that have exited."""
 483         if self.active_children is None: return
 484         while len(self.active_children) >= self.max_children:
 485             # XXX: This will wait for any child process, not just ones
 486             # spawned by this library. This could confuse other
 487             # libraries that expect to be able to wait for their own
 488             # children.
 489             try:
 490                 pid, status = os.waitpid(0, options=0)
 491             except os.error:
 492                 pid = None
 493             if pid not in self.active_children: continue
 494             self.active_children.remove(pid)
 495
 496         # XXX: This loop runs more system calls than it ought
 497         # to. There should be a way to put the active_children into a
 498         # process group and then use os.waitpid(-pgid) to wait for any
 499         # of that set, but I couldn't find a way to allocate pgids
 500         # that couldn't collide.
 501         for child in self.active_children:
 502             try:
 503                 pid, status = os.waitpid(child, os.WNOHANG)
 504             except os.error:
 505                 pid = None
 506             if not pid: continue
 507             try:
 508                 self.active_children.remove(pid)
 509             except ValueError, e:
 510                 raise ValueError('%s. x=%d and list=%r' % (e.message, pid,
 511                                                            self.active_children))
 512
 513     def handle_timeout(self):
 514         """Wait for zombies after self.timeout seconds of inactivity.
 515
 516         May be extended, do not override.
 517         """
 518         self.collect_children()
 519
 520     def process_request(self, request, client_address):
 521         """Fork a new subprocess to process the request."""
 522         self.collect_children()
 523         pid = os.fork()
 524         if pid:
 525             # Parent process
 526             if self.active_children is None:
 527                 self.active_children = []
 528             self.active_children.append(pid)
 529             self.close_request(request)
 530             return
 531         else:
 532             # Child process.
 533             # This must never return, hence os._exit()!
 534             try:
 535                 self.finish_request(request, client_address)
 536                 os._exit(0)
 537             except:
 538                 try:
 539                     self.handle_error(request, client_address)
 540                 finally:
 541                     os._exit(1)
 542
 543
 544 class ThreadingMixIn:
 545     """Mix-in class to handle each request in a new thread."""
 546
 547     # Decides how threads will act upon termination of the
 548     # main process
 549     daemon_threads = False
 550
 551     def process_request_thread(self, request, client_address):
 552         """Same as in BaseServer but as a thread.
 553
 554         In addition, exception handling is done here.
 555
 556         """
 557         try:
 558             self.finish_request(request, client_address)
 559             self.close_request(request)
 560         except:
 561             self.handle_error(request, client_address)
 562             self.close_request(request)
 563
 564     def process_request(self, request, client_address):
 565         """Start a new thread to process the request."""
 566         t = threading.Thread(target = self.process_request_thread,
 567                              args = (request, client_address))
 568         if self.daemon_threads:
 569             t.setDaemon (1)
 570         t.start()
 571
 572
 573 class ForkingUDPServer(ForkingMixIn, UDPServer): pass
 574 class ForkingTCPServer(ForkingMixIn, TCPServer): pass
 575
 576 class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
 577 class ThreadingTCPServer(ThreadingMixIn, TCPServer): pass
 578
 579 if hasattr(socket, 'AF_UNIX'):
 580
 581     class UnixStreamServer(TCPServer):
 582         address_family = socket.AF_UNIX
 583
 584     class UnixDatagramServer(UDPServer):
 585         address_family = socket.AF_UNIX
 586
 587     class ThreadingUnixStreamServer(ThreadingMixIn, UnixStreamServer): pass
 588
 589     class ThreadingUnixDatagramServer(ThreadingMixIn, UnixDatagramServer): pass
 590
 591 class BaseRequestHandler:
 592
 593     """Base class for request handler classes.
 594
 595     This class is instantiated for each request to be handled.  The
 596     constructor sets the instance variables request, client_address
 597     and server, and then calls the handle() method.  To implement a
 598     specific service, all you need to do is to derive a class which
 599     defines a handle() method.
 600
 601     The handle() method can find the request as self.request, the
 602     client address as self.client_address, and the server (in case it
 603     needs access to per-server information) as self.server.  Since a
 604     separate instance is created for each request, the handle() method
 605     can define arbitrary other instance variariables.
 606
 607     """
 608
 609     def __init__(self, request, client_address, server):
 610         self.request = request
 611         self.client_address = client_address
 612         self.server = server
 613         try:
 614             self.setup()
 615             self.handle()
 616             self.finish()
 617         finally:
 618             sys.exc_traceback = None    # Help garbage collection
 619
 620     def setup(self):
 621         pass
 622
 623     def handle(self):
 624         pass
 625
 626     def finish(self):
 627         pass
 628
 629
 630 # The following two classes make it possible to use the same service
 631 # class for stream or datagram servers.
 632 # Each class sets up these instance variables:
 633 # - rfile: a file object from which receives the request is read
 634 # - wfile: a file object to which the reply is written
 635 # When the handle() method returns, wfile is flushed properly
 636
 637
 638 class StreamRequestHandler(BaseRequestHandler):
 639
 640     """Define self.rfile and self.wfile for stream sockets."""
 641
 642     # Default buffer sizes for rfile, wfile.
 643     # We default rfile to buffered because otherwise it could be
 644     # really slow for large data (a getc() call per byte); we make
 645     # wfile unbuffered because (a) often after a write() we want to
 646     # read and we need to flush the line; (b) big writes to unbuffered
 647     # files are typically optimized by stdio even when big reads
 648     # aren't.
 649     rbufsize = -1
 650     wbufsize = 0
 651
 652     def setup(self):
 653         self.connection = self.request
 654         self.rfile = self.connection.makefile('rb', self.rbufsize)
 655         self.wfile = self.connection.makefile('wb', self.wbufsize)
 656
 657     def finish(self):
 658         if not self.wfile.closed:
 659             self.wfile.flush()
 660         self.wfile.close()
 661         self.rfile.close()
 662
 663
 664 class DatagramRequestHandler(BaseRequestHandler):
 665
 666     # XXX Regrettably, I cannot get this working on Linux;
 667     # s.recvfrom() doesn't return a meaningful client address.
 668
 669     """Define self.rfile and self.wfile for datagram sockets."""
 670
 671     def setup(self):
 672         try:
 673             from cStringIO import StringIO
 674         except ImportError:
 675             from StringIO import StringIO
 676         self.packet, self.socket = self.request
 677         self.rfile = StringIO(self.packet)
 678         self.wfile = StringIO()
 679
 680     def finish(self):
 681         self.socket.sendto(self.wfile.getvalue(), self.client_address)