/python/google/appengine/tools/devappserver2/wsgi_server.py

#!/usr/bin/env python
#
# Copyright 2007 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
"""A WSGI server implementation using a shared thread pool."""



import collections
import errno
import httplib
import logging
import select
import socket
import threading
import time

import google

from cherrypy import wsgiserver

from google.appengine.tools.devappserver2 import errors
from google.appengine.tools.devappserver2 import http_runtime_constants
from google.appengine.tools.devappserver2 import shutdown
from google.appengine.tools.devappserver2 import thread_executor


_HAS_POLL = hasattr(select, 'poll')

# TODO: the only reason we need to timeout is to pick up added or remove
# descriptors. But AFAICT, we only add descriptors at startup and remove them at
# shutdown so for the bulk of the run, the timeout is useless and just simply
# wastes CPU. For startup, if we wait to start the thread until after all
# WSGI servers are created, we are good (although may need to be careful in the
# runtime instances depending on when servers are created relative to the
# sandbox being enabled). For shutdown, more research is needed (one idea is
# simply not remove descriptors as the process is about to exit).
_READINESS_TIMEOUT_SECONDS = 1
_SECONDS_TO_MILLISECONDS = 1000

# Due to reports of failure to find a consistent port, trying a higher value
# to see if that reduces the problem sufficiently.  If it doesn't we can try
# increasing it (on my circa 2010 desktop, it takes about 1/2 second per 1024
# tries) but it would probably be better to either figure out a better
# algorithm or make it possible for code to work with inconsistent ports.


_PORT_0_RETRIES = 2048


class BindError(errors.Error):
  """The server failed to bind its address."""

_THREAD_POOL = thread_executor.ThreadExecutor()


class _SharedCherryPyThreadPool(object):
  """A mimic of wsgiserver.ThreadPool that delegates to a shared thread pool."""

  def __init__(self):
    self._condition = threading.Condition()
    self._connections = set()  # Protected by self._condition.

  def stop(self, timeout=5):
    timeout_time = time.time() + timeout
    with self._condition:
      while self._connections and time.time() < timeout_time:
        self._condition.wait(timeout_time - time.time())
      for connection in self._connections:
        self._shutdown_connection(connection)

  @staticmethod
  def _shutdown_connection(connection):
    if not connection.rfile.closed:
      connection.socket.shutdown(socket.SHUT_RD)

  def put(self, obj):
    with self._condition:
      self._connections.add(obj)
    _THREAD_POOL.submit(self._handle, obj)

  def _handle(self, obj):
    try:
      obj.communicate()
    finally:
      obj.close()
      with self._condition:
        self._connections.remove(obj)
        self._condition.notify()


class SelectThread(object):
  """A thread that selects on sockets and calls corresponding callbacks."""

  def __init__(self):
    self._lock = threading.Lock()
    # self._file_descriptors is a frozenset and
    # self._file_descriptor_to_callback is never mutated so they can be
    # snapshotted by the select thread without needing to copy.
    self._file_descriptors = frozenset()
    self._file_descriptor_to_callback = {}
    self._select_thread = threading.Thread(
        target=self._loop_forever, name='WSGI select')
    self._select_thread.daemon = True

  def start(self):
    self._select_thread.start()

  def add_socket(self, s, callback):
    """Add a new socket to watch.

    Args:
      s: A socket to select on.
      callback: A callable with no args to be called when s is ready for a read.
    """
    with self._lock:
      self._file_descriptors = self._file_descriptors.union([s.fileno()])
      new_file_descriptor_to_callback = self._file_descriptor_to_callback.copy()
      new_file_descriptor_to_callback[s.fileno()] = callback
      self._file_descriptor_to_callback = new_file_descriptor_to_callback

  def remove_socket(self, s):
    """Remove a watched socket."""
    with self._lock:
      self._file_descriptors = self._file_descriptors.difference([s.fileno()])
      new_file_descriptor_to_callback = self._file_descriptor_to_callback.copy()
      del new_file_descriptor_to_callback[s.fileno()]
      self._file_descriptor_to_callback = new_file_descriptor_to_callback

  def _loop_forever(self):
    while shutdown and not shutdown.shutting_down():
      # Check shutdown as it may be gc-ed during shutdown. See
      # http://stackoverflow.com/questions/17084260/imported-modules-become-none-when-running-a-function
      self._select()

  def _select(self):
    with self._lock:
      fds = self._file_descriptors
      fd_to_callback = self._file_descriptor_to_callback
    if fds:
      if _HAS_POLL:
        # With 100 file descriptors, it is approximately 5x slower to
        # recreate and reinitialize the Poll object on every call to _select
        # rather reuse one. But the absolute cost of contruction,
        # initialization and calling poll(0) is ~25us so code simplicity
        # wins.
        poll = select.poll()
        for fd in fds:
          poll.register(fd, select.POLLIN)
        ready_file_descriptors = [fd for fd, _ in poll.poll(
            _READINESS_TIMEOUT_SECONDS * _SECONDS_TO_MILLISECONDS)]
      else:
        ready_file_descriptors, _, _ = select.select(fds, [], [],
                                                     _READINESS_TIMEOUT_SECONDS)
      for fd in ready_file_descriptors:
        fd_to_callback[fd]()
    else:
      # select([], [], [], 1) is not supported on Windows.
      time.sleep(_READINESS_TIMEOUT_SECONDS)

_SELECT_THREAD = SelectThread()
_SELECT_THREAD.start()


class _SingleAddressWsgiServer(wsgiserver.CherryPyWSGIServer):
  """A WSGI server that uses a shared SelectThread and thread pool."""

  def __init__(self, host, app):
    """Constructs a _SingleAddressWsgiServer.

    Args:
      host: A (hostname, port) tuple containing the hostname and port to bind.
          The port can be 0 to allow any port.
      app: A WSGI app to handle requests.
    """
    super(_SingleAddressWsgiServer, self).__init__(host, self)
    self._lock = threading.Lock()
    self._app = app  # Protected by _lock.
    self._error = None  # Protected by _lock.
    self.requests = _SharedCherryPyThreadPool()
    self.software = http_runtime_constants.SERVER_SOFTWARE
    # Some servers, especially the API server, may receive many simultaneous
    # requests so set the listen() backlog to something high to reduce the
    # likelihood of refused connections.
    self.request_queue_size = 100

  def start(self):
    """Starts the _SingleAddressWsgiServer.

    This is a modified version of the base class implementation. Changes:
      - Removed unused functionality (Unix domain socket and SSL support).
      - Raises BindError instead of socket.error.
      - Uses _SharedCherryPyThreadPool instead of wsgiserver.ThreadPool.
      - Calls _SELECT_THREAD.add_socket instead of looping forever.

    Raises:
      BindError: The address could not be bound.
    """
    # AF_INET or AF_INET6 socket
    # Get the correct address family for our host (allows IPv6 addresses)
    host, port = self.bind_addr
    try:
      info = socket.getaddrinfo(host, port, socket.AF_UNSPEC,
                                socket.SOCK_STREAM, 0, socket.AI_PASSIVE)
    except socket.gaierror:
      if ':' in host:
        info = [(socket.AF_INET6, socket.SOCK_STREAM, 0, '',
                 self.bind_addr + (0, 0))]
      else:
        info = [(socket.AF_INET, socket.SOCK_STREAM, 0, '', self.bind_addr)]

    self.socket = None
    for res in info:
      af, socktype, proto, _, _ = res
      try:
        self.bind(af, socktype, proto)
      except socket.error as socket_error:
        if self.socket:
          self.socket.close()
        self.socket = None
        continue
      break
    if not self.socket:
      raise BindError('Unable to bind %s:%s' % self.bind_addr, socket_error)

    # Timeout so KeyboardInterrupt can be caught on Win32
    self.socket.settimeout(1)
    self.socket.listen(self.request_queue_size)

    self.ready = True
    self._start_time = time.time()
    _SELECT_THREAD.add_socket(self.socket, self.tick)

  def quit(self):
    """Quits the _SingleAddressWsgiServer."""
    _SELECT_THREAD.remove_socket(self.socket)
    self.requests.stop(timeout=1)

  @property
  def port(self):
    """Returns the port that the server is bound to."""
    return self.socket.getsockname()[1]

  def set_app(self, app):
    """Sets the PEP-333 app to use to serve requests."""
    with self._lock:
      self._app = app

  def set_error(self, error):
    """Sets the HTTP status code to serve for all requests."""
    with self._lock:
      self._error = error
      self._app = None

  def __call__(self, environ, start_response):
    with self._lock:
      app = self._app
      error = self._error
    if app:
      return app(environ, start_response)
    else:
      start_response('%d %s' % (error, httplib.responses[error]), [])
      return []


class WsgiServer(object):

  def __init__(self, host, app):
    """Constructs a WsgiServer.

    Args:
      host: A (hostname, port) tuple containing the hostname and port to bind.
          The port can be 0 to allow any port.
      app: A WSGI app to handle requests.
    """
    self.bind_addr = host
    self._app = app
    self._servers = []

  def start(self):
    """Starts the WsgiServer.

    This starts multiple _SingleAddressWsgiServers to bind the address in all
    address families.

    Raises:
      BindError: The address could not be bound.
    """
    host, port = self.bind_addr
    try:
      addrinfo = socket.getaddrinfo(host, port, socket.AF_UNSPEC,
                                    socket.SOCK_STREAM, 0, socket.AI_PASSIVE)
      sockaddrs = [addr[-1] for addr in addrinfo]
      host_ports = [sockaddr[:2] for sockaddr in sockaddrs]
      # Remove duplicate addresses caused by bad hosts file. Retain the
      # order to minimize behavior change (and so we don't have to tweak
      # unit tests to deal with different order).
      host_ports = list(collections.OrderedDict.fromkeys(host_ports))
    except socket.gaierror:
      host_ports = [self.bind_addr]

    if port != 0:
      self._start_all_fixed_port(host_ports)
    else:
      for _ in range(_PORT_0_RETRIES):
        if self._start_all_dynamic_port(host_ports):
          break
      else:
        raise BindError('Unable to find a consistent port for %s' % host)

  def _start_all_fixed_port(self, host_ports):
    """Starts a server for each specified address with a fixed port.

    Does the work of actually trying to create a _SingleAddressWsgiServer for
    each specified address.

    Args:
      host_ports: An iterable of host, port tuples.

    Raises:
      BindError: The address could not be bound.
    """
    for host, port in host_ports:
      assert port != 0
      server = _SingleAddressWsgiServer((host, port), self._app)
      try:
        server.start()
      except BindError as bind_error:
        # TODO: I'm not sure about the behavior of quietly ignoring an
        # EADDRINUSE as long as the bind succeeds on at least one interface. I
        # think we should either:
        # - Fail (just like we do now when bind fails on every interface).
        # - Retry on next highest port.
        logging.debug('Failed to bind "%s:%s": %s', host, port, bind_error)
        continue
      else:
        self._servers.append(server)

    if not self._servers:
      raise BindError('Unable to bind %s:%s' % self.bind_addr)

  def _start_all_dynamic_port(self, host_ports):
    """Starts a server for each specified address with a dynamic port.

    Does the work of actually trying to create a _SingleAddressWsgiServer for
    each specified address.

    Args:
      host_ports: An iterable of host, port tuples.

    Returns:
      The list of all servers (also saved as self._servers). A non empty list
      indicates success while an empty list indicates failure.
    """
    port = 0
    for host, _ in host_ports:
      server = _SingleAddressWsgiServer((host, port), self._app)
      try:
        server.start()
        if port == 0:
          port = server.port
      except BindError as bind_error:
        if bind_error[1][0] == errno.EADDRINUSE:
          # The port picked at random for first interface was not available
          # on one of the other interfaces. Forget them and try again.
          for server in self._servers:
            server.quit()
          self._servers = []
          break
        else:
          # Ignore the interface if we get an error other than EADDRINUSE.
          logging.debug('Failed to bind "%s:%s": %s', host, port, bind_error)
          continue
      else:
        self._servers.append(server)
    return self._servers

  def quit(self):
    """Quits the WsgiServer."""
    for server in self._servers:
      server.quit()

  @property
  def host(self):
    """Returns the host that the server is bound to."""
    return self._servers[0].socket.getsockname()[0]

  @property
  def port(self):
    """Returns the port that the server is bound to."""
    return self._servers[0].socket.getsockname()[1]

  def set_app(self, app):
    """Sets the PEP-333 app to use to serve requests."""
    self._app = app

    for server in self._servers:
      server.set_app(app)

  def set_error(self, error):
    """Sets the HTTP status code to serve for all requests."""
    self._error = error
    self._app = None
    for server in self._servers:
      server.set_error(error)

  @property
  def ready(self):
    return all(server.ready for server in self._servers)