"""Selectors module.

This module allows high-level and efficient I/O multiplexing, built upon the

`select` module primitives.

"""

from abc import ABCMeta, abstractmethod

from collections import namedtuple

from collections.abc import Mapping

import math

import select

import sys

# generic events, that must be mapped to implementation-specific ones

EVENT_READ = (1 << 0)

EVENT_WRITE = (1 << 1)

def _fileobj_to_fd(fileobj):

"""Return a file descriptor from a file object.

Parameters:

fileobj -- file object or file descriptor

Returns:

corresponding file descriptor

Raises:

ValueError if the object is invalid

"""

if isinstance(fileobj, int):

fd = fileobj

else:

try:

fd = int(fileobj.fileno())

except (AttributeError, TypeError, ValueError):

raise ValueError("Invalid file object: "

"{!r}".format(fileobj)) from None

if fd < 0:

raise ValueError("Invalid file descriptor: {}".format(fd))

return fd

SelectorKey = namedtuple('SelectorKey', ['fileobj', 'fd', 'events', 'data'])

SelectorKey.__doc__ = """SelectorKey(fileobj, fd, events, data)

Object used to associate a file object to its backing

file descriptor, selected event mask, and attached data.

"""

SelectorKey.fileobj.__doc__ = 'File object registered.'

SelectorKey.fd.__doc__ = 'Underlying file descriptor.'

SelectorKey.events.__doc__ = 'Events that must be waited for on this file object.'

SelectorKey.data.__doc__ = ('''Optional opaque data associated to this file object.

For example, this could be used to store a per-client session ID.''')

class _SelectorMapping(Mapping):

"""Mapping of file objects to selector keys."""

def __init__(self, selector):

self._selector = selector

def __len__(self):

return len(self._selector._fd_to_key)

def get(self, fileobj, default=None):

fd = self._selector._fileobj_lookup(fileobj)

return self._selector._fd_to_key.get(fd, default)

def __getitem__(self, fileobj):

fd = self._selector._fileobj_lookup(fileobj)

key = self._selector._fd_to_key.get(fd)

if key is None:

raise KeyError("{!r} is not registered".format(fileobj))

return key

def __iter__(self):

return iter(self._selector._fd_to_key)

class BaseSelector(metaclass=ABCMeta):

"""Selector abstract base class.

A selector supports registering file objects to be monitored for specific

I/O events.

A file object is a file descriptor or any object with a `fileno()` method.

An arbitrary object can be attached to the file object, which can be used

for example to store context information, a callback, etc.

A selector can use various implementations (select(), poll(), epoll()...)

depending on the platform. The default `Selector` class uses the most

efficient implementation on the current platform.

"""

@abstractmethod

def register(self, fileobj, events, data=None):

"""Register a file object.

Parameters:

fileobj -- file object or file descriptor

events -- events to monitor (bitwise mask of EVENT_READ|EVENT_WRITE)

data -- attached data

Returns:

SelectorKey instance

Raises:

ValueError if events is invalid

KeyError if fileobj is already registered

OSError if fileobj is closed or otherwise is unacceptable to

the underlying system call (if a system call is made)

Note:

OSError may or may not be raised

"""

raise NotImplementedError

@abstractmethod

def unregister(self, fileobj):

"""Unregister a file object.

Parameters:

fileobj -- file object or file descriptor

Returns:

SelectorKey instance

Raises:

KeyError if fileobj is not registered

Note:

If fileobj is registered but has since been closed this does

*not* raise OSError (even if the wrapped syscall does)

"""

raise NotImplementedError

def modify(self, fileobj, events, data=None):

"""Change a registered file object monitored events or attached data.

Parameters:

fileobj -- file object or file descriptor

events -- events to monitor (bitwise mask of EVENT_READ|EVENT_WRITE)

data -- attached data

Returns:

SelectorKey instance

Raises:

Anything that unregister() or register() raises

"""

self.unregister(fileobj)

return self.register(fileobj, events, data)

@abstractmethod

def select(self, timeout=None):

"""Perform the actual selection, until some monitored file objects are

ready or a timeout expires.

Parameters:

timeout -- if timeout > 0, this specifies the maximum wait time, in

seconds

if timeout <= 0, the select() call won't block, and will

report the currently ready file objects

if timeout is None, select() will block until a monitored

file object becomes ready

Returns:

list of (key, events) for ready file objects

`events` is a bitwise mask of EVENT_READ|EVENT_WRITE

"""

raise NotImplementedError

def close(self):

"""Close the selector.

This must be called to make sure that any underlying resource is freed.

"""

pass

def get_key(self, fileobj):

"""Return the key associated to a registered file object.

Returns:

SelectorKey for this file object

"""

mapping = self.get_map()

if mapping is None:

raise RuntimeError('Selector is closed')

try:

return mapping[fileobj]

except KeyError:

raise KeyError("{!r} is not registered".format(fileobj)) from None

@abstractmethod

def get_map(self):

"""Return a mapping of file objects to selector keys."""

raise NotImplementedError

def __enter__(self):

return self

def __exit__(self, *args):

self.close()

class _BaseSelectorImpl(BaseSelector):

"""Base selector implementation."""

def __init__(self):

# this maps file descriptors to keys

self._fd_to_key = {}

# read-only mapping returned by get_map()

self._map = _SelectorMapping(self)

def _fileobj_lookup(self, fileobj):

"""Return a file descriptor from a file object.

This wraps _fileobj_to_fd() to do an exhaustive search in case

the object is invalid but we still have it in our map. This

is used by unregister() so we can unregister an object that

was previously registered even if it is closed. It is also

used by _SelectorMapping.

"""

try:

return _fileobj_to_fd(fileobj)

except ValueError:

# Do an exhaustive search.

for key in self._fd_to_key.values():

if key.fileobj is fileobj:

return key.fd

# Raise ValueError after all.

raise

def register(self, fileobj, events, data=None):

if (not events) or (events & ~(EVENT_READ | EVENT_WRITE)):

raise ValueError("Invalid events: {!r}".format(events))

key = SelectorKey(fileobj, self._fileobj_lookup(fileobj), events, data)

if key.fd in self._fd_to_key:

raise KeyError("{!r} (FD {}) is already registered"

.format(fileobj, key.fd))

self._fd_to_key[key.fd] = key

return key

def unregister(self, fileobj):

try:

key = self._fd_to_key.pop(self._fileobj_lookup(fileobj))

except KeyError:

raise KeyError("{!r} is not registered".format(fileobj)) from None

return key

def modify(self, fileobj, events, data=None):

try:

key = self._fd_to_key[self._fileobj_lookup(fileobj)]

except KeyError:

raise KeyError("{!r} is not registered".format(fileobj)) from None

if events != key.events:

self.unregister(fileobj)

key = self.register(fileobj, events, data)

elif data != key.data:

# Use a shortcut to update the data.

key = key._replace(data=data)

self._fd_to_key[key.fd] = key

return key

def close(self):

self._fd_to_key.clear()

self._map = None

def get_map(self):

return self._map

class SelectSelector(_BaseSelectorImpl):

"""Select-based selector."""

def __init__(self):

super().__init__()

self._readers = set()

self._writers = set()

def register(self, fileobj, events, data=None):

key = super().register(fileobj, events, data)

if events & EVENT_READ:

self._readers.add(key.fd)

if events & EVENT_WRITE:

self._writers.add(key.fd)

return key

def unregister(self, fileobj):

key = super().unregister(fileobj)

self._readers.discard(key.fd)

self._writers.discard(key.fd)

return key

if sys.platform == 'win32':

def _select(self, r, w, _, timeout=None):

r, w, x = select.select(r, w, w, timeout)

return r, w + x, []

else:

_select = select.select

def select(self, timeout=None):

timeout = None if timeout is None else max(timeout, 0)

ready = []

try:

r, w, _ = self._select(self._readers, self._writers, [], timeout)

except InterruptedError:

return ready

r = frozenset(r)

w = frozenset(w)

rw = r | w

fd_to_key_get = self._fd_to_key.get

for fd in rw:

key = fd_to_key_get(fd)

if key:

events = ((fd in r and EVENT_READ)

| (fd in w and EVENT_WRITE))

ready.append((key, events & key.events))

return ready

class _PollLikeSelector(_BaseSelectorImpl):

"""Base class shared between poll, epoll and devpoll selectors."""

_selector_cls = None

_EVENT_READ = None

_EVENT_WRITE = None

def __init__(self):

super().__init__()

self._selector = self._selector_cls()

def register(self, fileobj, events, data=None):

key = super().register(fileobj, events, data)

poller_events = ((events & EVENT_READ and self._EVENT_READ)

| (events & EVENT_WRITE and self._EVENT_WRITE) )

try:

self._selector.register(key.fd, poller_events)

except:

super().unregister(fileobj)

raise

return key

def unregister(self, fileobj):

key = super().unregister(fileobj)

try:

self._selector.unregister(key.fd)

except OSError:

# This can happen if the FD was closed since it

# was registered.

pass

return key

def modify(self, fileobj, events, data=None):

try:

key = self._fd_to_key[self._fileobj_lookup(fileobj)]

except KeyError:

raise KeyError(f"{fileobj!r} is not registered") from None

changed = False

if events != key.events:

selector_events = ((events & EVENT_READ and self._EVENT_READ)

| (events & EVENT_WRITE and self._EVENT_WRITE))

try:

self._selector.modify(key.fd, selector_events)

except:

super().unregister(fileobj)

raise

changed = True

if data != key.data:

changed = True

if changed:

key = key._replace(events=events, data=data)

self._fd_to_key[key.fd] = key

return key

def select(self, timeout=None):

# This is shared between poll() and epoll().

# epoll() has a different signature and handling of timeout parameter.

if timeout is None:

timeout = None

elif timeout <= 0:

timeout = 0

else:

# poll() has a resolution of 1 millisecond, round away from

# zero to wait *at least* timeout seconds.

timeout = math.ceil(timeout * 1e3)

ready = []

try:

fd_event_list = self._selector.poll(timeout)

except InterruptedError:

return ready

fd_to_key_get = self._fd_to_key.get

for fd, event in fd_event_list:

key = fd_to_key_get(fd)

if key:

events = ((event & ~self._EVENT_READ and EVENT_WRITE)

| (event & ~self._EVENT_WRITE and EVENT_READ))

ready.append((key, events & key.events))

return ready

if hasattr(select, 'poll'):

class PollSelector(_PollLikeSelector):

"""Poll-based selector."""

_selector_cls = select.poll

_EVENT_READ = select.POLLIN

_EVENT_WRITE = select.POLLOUT

if hasattr(select, 'epoll'):

_NOT_EPOLLIN = ~select.EPOLLIN

_NOT_EPOLLOUT = ~select.EPOLLOUT

class EpollSelector(_PollLikeSelector):

"""Epoll-based selector."""

_selector_cls = select.epoll

_EVENT_READ = select.EPOLLIN

_EVENT_WRITE = select.EPOLLOUT

def fileno(self):

return self._selector.fileno()

def select(self, timeout=None):

if timeout is None:

timeout = -1

elif timeout <= 0:

timeout = 0

else:

# epoll_wait() has a resolution of 1 millisecond, round away

# from zero to wait *at least* timeout seconds.

timeout = math.ceil(timeout * 1e3) * 1e-3

# epoll_wait() expects `maxevents` to be greater than zero;

# we want to make sure that `select()` can be called when no

# FD is registered.

max_ev = len(self._fd_to_key) or 1

ready = []

try:

fd_event_list = self._selector.poll(timeout, max_ev)

except InterruptedError:

return ready

fd_to_key = self._fd_to_key

for fd, event in fd_event_list:

key = fd_to_key.get(fd)

if key:

events = ((event & _NOT_EPOLLIN and EVENT_WRITE)

| (event & _NOT_EPOLLOUT and EVENT_READ))

ready.append((key, events & key.events))

return ready

def close(self):

self._selector.close()

super().close()

if hasattr(select, 'devpoll'):

class DevpollSelector(_PollLikeSelector):

"""Solaris /dev/poll selector."""

_selector_cls = select.devpoll

_EVENT_READ = select.POLLIN

_EVENT_WRITE = select.POLLOUT

def fileno(self):

return self._selector.fileno()

def close(self):

self._selector.close()

super().close()

if hasattr(select, 'kqueue'):

class KqueueSelector(_BaseSelectorImpl):

"""Kqueue-based selector."""

def __init__(self):

super().__init__()

self._selector = select.kqueue()

self._max_events = 0

def fileno(self):

return self._selector.fileno()

def register(self, fileobj, events, data=None):

key = super().register(fileobj, events, data)

try:

if events & EVENT_READ:

kev = select.kevent(key.fd, select.KQ_FILTER_READ,

select.KQ_EV_ADD)

self._selector.control([kev], 0, 0)

self._max_events += 1

if events & EVENT_WRITE:

kev = select.kevent(key.fd, select.KQ_FILTER_WRITE,

select.KQ_EV_ADD)

self._selector.control([kev], 0, 0)

self._max_events += 1

except:

super().unregister(fileobj)

raise

return key

def unregister(self, fileobj):

key = super().unregister(fileobj)

if key.events & EVENT_READ:

kev = select.kevent(key.fd, select.KQ_FILTER_READ,

select.KQ_EV_DELETE)

self._max_events -= 1

try:

self._selector.control([kev], 0, 0)

except OSError:

# This can happen if the FD was closed since it

# was registered.

pass

if key.events & EVENT_WRITE:

kev = select.kevent(key.fd, select.KQ_FILTER_WRITE,

select.KQ_EV_DELETE)

self._max_events -= 1

try:

self._selector.control([kev], 0, 0)

except OSError:

# See comment above.

pass

return key

def select(self, timeout=None):

timeout = None if timeout is None else max(timeout, 0)

# If max_ev is 0, kqueue will ignore the timeout. For consistent

# behavior with the other selector classes, we prevent that here

# (using max). See https://bugs.python.org/issue29255

max_ev = self._max_events or 1

ready = []

try:

kev_list = self._selector.control(None, max_ev, timeout)

except InterruptedError:

return ready

fd_to_key_get = self._fd_to_key.get

for kev in kev_list:

fd = kev.ident

flag = kev.filter

key = fd_to_key_get(fd)

if key:

events = ((flag == select.KQ_FILTER_READ and EVENT_READ)

| (flag == select.KQ_FILTER_WRITE and EVENT_WRITE))

ready.append((key, events & key.events))

return ready

def close(self):

self._selector.close()

super().close()

def _can_use(method):

"""Check if we can use the selector depending upon the

operating system. """

# Implementation based upon https://github.com/sethmlarson/selectors2/blob/master/selectors2.py

selector = getattr(select, method, None)

if selector is None:

# select module does not implement method

return False

# check if the OS and Kernel actually support the method. Call may fail with

# OSError: [Errno 38] Function not implemented

try:

selector_obj = selector()

if method == 'poll':

# check that poll actually works

selector_obj.poll(0)

else:

# close epoll, kqueue, and devpoll fd

selector_obj.close()

return True

except OSError:

return False

# Choose the best implementation, roughly:

# epoll|kqueue|devpoll > poll > select.

# select() also can't accept a FD > FD_SETSIZE (usually around 1024)

if _can_use('kqueue'):

DefaultSelector = KqueueSelector

elif _can_use('epoll'):

DefaultSelector = EpollSelector

elif _can_use('devpoll'):

DefaultSelector = DevpollSelector

elif _can_use('poll'):

DefaultSelector = PollSelector

else:

DefaultSelector = SelectSelector