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)