text: """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
  saves 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 read 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 a selector 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
  
  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
  
  __version__ = "0.4"
  
  
  import socket
  import selectors
  import os
  import sys
  import threading
  from io import BufferedIOBase
  from time import monotonic as time
  
  __all__ = ["BaseServer", "TCPServer", "UDPServer",
             "ThreadingUDPServer", "ThreadingTCPServer",
             "BaseRequestHandler", "StreamRequestHandler",
             "DatagramRequestHandler", "ThreadingMixIn"]
  if hasattr(os, "fork"):
      __all__.extend(["ForkingUDPServer","ForkingTCPServer", "ForkingMixIn"])
  if hasattr(socket, "AF_UNIX"):
      __all__.extend(["UnixStreamServer","UnixDatagramServer",
                      "ThreadingUnixStreamServer",
                      "ThreadingUnixDatagramServer"])
  
  # poll/select have the advantage of not requiring any extra file descriptor,
  # contrarily to epoll/kqueue (also, they require a single syscall).
  if hasattr(selectors, 'PollSelector'):
      _ServerSelector = selectors.PollSelector
  else:
      _ServerSelector = selectors.SelectSelector
  
  
  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 selector
  
      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)
      - shutdown_request(request)
      - close_request(request)
      - service_actions()
      - 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.__shutdown_request = 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.__is_shut_down.clear()
          try:
              # 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.
              with _ServerSelector() as selector:
                  selector.register(self, selectors.EVENT_READ)
  
                  while not self.__shutdown_request:
                      ready = selector.select(poll_interval)
                      # bpo-35017: shutdown() called during select(), exit immediately.
                      if self.__shutdown_request:
                          break
                      if ready:
                          self._handle_request_noblock()
  
                      self.service_actions()
          finally:
              self.__shutdown_request = False
              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.__shutdown_request = True
          self.__is_shut_down.wait()
  
      def service_actions(self):
          """Called by the serve_forever() loop.
  
          May be overridden by a subclass / Mixin to implement any code that
          needs to be run during the loop.
          """
          pass
  
      # The distinction between handling, getting, processing and finishing a
      # request is fairly arbitrary.  Remember:
      #
      # - handle_request() is the top-level call.  It calls selector.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)
          if timeout is not None:
              deadline = time() + timeout
  
          # Wait until a request arrives or the timeout expires - the loop is
          # necessary to accommodate early wakeups due to EINTR.
          with _ServerSelector() as selector:
              selector.register(self, selectors.EVENT_READ)
  
              while True:
                  ready = selector.select(timeout)
                  if ready:
                      return self._handle_request_noblock()
                  else:
                      if timeout is not None:
                          timeout = deadline - time()
                          if timeout < 0:
                              return self.handle_timeout()
  
      def _handle_request_noblock(self):
          """Handle one request, without blocking.
  
          I assume that selector.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 OSError:
              return
          if self.verify_request(request, client_address):
              try:
                  self.process_request(request, client_address)
              except Exception:
                  self.handle_error(request, client_address)
                  self.shutdown_request(request)
              except:
                  self.shutdown_request(request)
                  raise
          else:
              self.shutdown_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.shutdown_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 shutdown_request(self, request):
          """Called to shutdown and close an individual request."""
          self.close_request(request)
  
      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, file=sys.stderr)
          print('Exception occurred during processing of request from',
              client_address, file=sys.stderr)
          import traceback
          traceback.print_exc()
          print('-'*40, file=sys.stderr)
  
      def __enter__(self):
          return self
  
      def __exit__(self, *args):
          self.server_close()
  
  
  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 selector
  
      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)
      - shutdown_request(request)
      - 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:
              try:
                  self.server_bind()
                  self.server_activate()
              except:
                  self.server_close()
                  raise
  
      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 selector.
  
          """
          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 shutdown_request(self, request):
          """Called to shutdown and close an individual request."""
          try:
              #explicitly shutdown.  socket.close() merely releases
              #the socket and waits for GC to perform the actual close.
              request.shutdown(socket.SHUT_WR)
          except OSError:
              pass #some platforms may raise ENOTCONN here
          self.close_request(request)
  
      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 shutdown_request(self, request):
          # No need to shutdown anything.
          self.close_request(request)
  
      def close_request(self, request):
          # No need to close anything.
          pass
  
  if hasattr(os, "fork"):
      class ForkingMixIn:
          """Mix-in class to handle each request in a new process."""
  
          timeout = 300
          active_children = None
          max_children = 40
          # If true, server_close() waits until all child processes complete.
          block_on_close = True
  
          def collect_children(self, *, blocking=False):
              """Internal routine to wait for children that have exited."""
              if self.active_children is None:
                  return
  
              # If we're above the max number of children, wait and reap them until
              # we go back below threshold. Note that we use waitpid(-1) below to be
              # able to collect children in size(<defunct children>) syscalls instead
              # of size(<children>): the downside is that this might reap children
              # which we didn't spawn, which is why we only resort to this when we're
              # above max_children.
              while len(self.active_children) >= self.max_children:
                  try:
                      pid, _ = os.waitpid(-1, 0)
                      self.active_children.discard(pid)
                  except ChildProcessError:
                      # we don't have any children, we're done
                      self.active_children.clear()
                  except OSError:
                      break
  
              # Now reap all defunct children.
              for pid in self.active_children.copy():
                  try:
                      flags = 0 if blocking else os.WNOHANG
                      pid, _ = os.waitpid(pid, flags)
                      # if the child hasn't exited yet, pid will be 0 and ignored by
                      # discard() below
                      self.active_children.discard(pid)
                  except ChildProcessError:
                      # someone else reaped it
                      self.active_children.discard(pid)
                  except OSError:
                      pass
  
          def handle_timeout(self):
              """Wait for zombies after self.timeout seconds of inactivity.
  
              May be extended, do not override.
              """
              self.collect_children()
  
          def service_actions(self):
              """Collect the zombie child processes regularly in the ForkingMixIn.
  
              service_actions is called in the BaseServer's serve_forever loop.
              """
              self.collect_children()
  
          def process_request(self, request, client_address):
              """Fork a new subprocess to process the request."""
              pid = os.fork()
              if pid:
                  # Parent process
                  if self.active_children is None:
                      self.active_children = set()
                  self.active_children.add(pid)
                  self.close_request(request)
                  return
              else:
                  # Child process.
                  # This must never return, hence os._exit()!
                  status = 1
                  try:
                      self.finish_request(request, client_address)
                      status = 0
                  except Exception:
                      self.handle_error(request, client_address)
                  finally:
                      try:
                          self.shutdown_request(request)
                      finally:
                          os._exit(status)
  
          def server_close(self):
              super().server_close()
              self.collect_children(blocking=self.block_on_close)
  
  
  class ThreadingMixIn:
      """Mix-in class to handle each request in a new thread."""
  
      # Decides how threads will act upon termination of the
      # main process
      daemon_threads = False
      # If true, server_close() waits until all non-daemonic threads terminate.
      block_on_close = True
      # For non-daemonic threads, list of threading.Threading objects
      # used by server_close() to wait for all threads completion.
      _threads = None
  
      def process_request_thread(self, request, client_address):
          """Same as in BaseServer but as a thread.
  
          In addition, exception handling is done here.
  
          """
          try:
              self.finish_request(request, client_address)
          except Exception:
              self.handle_error(request, client_address)
          finally:
              self.shutdown_request(request)
  
      def process_request(self, request, client_address):
          """Start a new thread to process the request."""
          t = threading.Thread(target = self.process_request_thread,
                               args = (request, client_address))
          t.daemon = self.daemon_threads
          if not t.daemon and self.block_on_close:
              if self._threads is None:
                  self._threads = []
              self._threads.append(t)
          t.start()
  
      def server_close(self):
          super().server_close()
          if self.block_on_close:
              threads = self._threads
              self._threads = None
              if threads:
                  for thread in threads:
                      thread.join()
  
  
  if hasattr(os, "fork"):
      class ForkingUDPServer(ForkingMixIn, UDPServer): pass
      class ForkingTCPServer(ForkingMixIn, TCPServer): pass
  
  class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
  class ThreadingTCPServer(ThreadingMixIn, TCPServer): pass
  
  if hasattr(socket, 'AF_UNIX'):
  
      class UnixStreamServer(TCPServer):
          address_family = socket.AF_UNIX
  
      class UnixDatagramServer(UDPServer):
          address_family = socket.AF_UNIX
  
      class ThreadingUnixStreamServer(ThreadingMixIn, UnixStreamServer): pass
  
      class ThreadingUnixDatagramServer(ThreadingMixIn, UnixDatagramServer): pass
  
  class BaseRequestHandler:
  
      """Base class for request handler classes.
  
      This class is instantiated for each request to be handled.  The
      constructor sets the instance variables request, client_address
      and server, and then calls the handle() method.  To implement a
      specific service, all you need to do is to derive a class which
      defines a handle() method.
  
      The handle() method can find the request as self.request, the
      client address as self.client_address, and the server (in case it
      needs access to per-server information) as self.server.  Since a
      separate instance is created for each request, the handle() method
      can define other arbitrary instance variables.
  
      """
  
      def __init__(self, request, client_address, server):
          self.request = request
          self.client_address = client_address
          self.server = server
          self.setup()
          try:
              self.handle()
          finally:
              self.finish()
  
      def setup(self):
          pass
  
      def handle(self):
          pass
  
      def finish(self):
          pass
  
  
  # The following two classes make it possible to use the same service
  # class for stream or datagram servers.
  # Each class sets up these instance variables:
  # - rfile: a file object from which receives the request is read
  # - wfile: a file object to which the reply is written
  # When the handle() method returns, wfile is flushed properly
  
  
  class StreamRequestHandler(BaseRequestHandler):
  
      """Define self.rfile and self.wfile for stream sockets."""
  
      # Default buffer sizes for rfile, wfile.
      # We default rfile to buffered because otherwise it could be
      # really slow for large data (a getc() call per byte); we make
      # wfile unbuffered because (a) often after a write() we want to
      # read and we need to flush the line; (b) big writes to unbuffered
      # files are typically optimized by stdio even when big reads
      # aren't.
      rbufsize = -1
      wbufsize = 0
  
      # A timeout to apply to the request socket, if not None.
      timeout = None
  
      # Disable nagle algorithm for this socket, if True.
      # Use only when wbufsize != 0, to avoid small packets.
      disable_nagle_algorithm = False
  
      def setup(self):
          self.connection = self.request
          if self.timeout is not None:
              self.connection.settimeout(self.timeout)
          if self.disable_nagle_algorithm:
              self.connection.setsockopt(socket.IPPROTO_TCP,
                                         socket.TCP_NODELAY, True)
          self.rfile = self.connection.makefile('rb', self.rbufsize)
          if self.wbufsize == 0:
              self.wfile = _SocketWriter(self.connection)
          else:
              self.wfile = self.connection.makefile('wb', self.wbufsize)
  
      def finish(self):
          if not self.wfile.closed:
              try:
                  self.wfile.flush()
              except socket.error:
                  # A final socket error may have occurred here, such as
                  # the local error ECONNABORTED.
                  pass
          self.wfile.close()
          self.rfile.close()
  
  class _SocketWriter(BufferedIOBase):
      """Simple writable BufferedIOBase implementation for a socket
  
      Does not hold data in a buffer, avoiding any need to call flush()."""
  
      def __init__(self, sock):
          self._sock = sock
  
      def writable(self):
          return True
  
      def write(self, b):
          self._sock.sendall(b)
          with memoryview(b) as view:
              return view.nbytes
  
      def fileno(self):
          return self._sock.fileno()
  
  class DatagramRequestHandler(BaseRequestHandler):
  
      """Define self.rfile and self.wfile for datagram sockets."""
  
      def setup(self):
          from io import BytesIO
          self.packet, self.socket = self.request
          self.rfile = BytesIO(self.packet)
          self.wfile = BytesIO()
  
      def finish(self):
          self.socket.sendto(self.wfile.getvalue(), self.client_address)
  