JezK
Edit File: selectors.py
"""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. """ if sys.version_info >= (3, 5): 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 __getitem__(self, fileobj): try: fd = self._selector._fileobj_lookup(fileobj) return self._selector._fd_to_key[fd] except KeyError: raise KeyError("{!r} is not registered".format(fileobj)) from None 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 def _key_from_fd(self, fd): """Return the key associated to a given file descriptor. Parameters: fd -- file descriptor Returns: corresponding key, or None if not found """ try: return self._fd_to_key[fd] except KeyError: return None 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 = set(r) w = set(w) for fd in r | w: events = 0 if fd in r: events |= EVENT_READ if fd in w: events |= EVENT_WRITE key = self._key_from_fd(fd) if key: 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 = 0 if events & EVENT_READ: poller_events |= self._EVENT_READ if events & EVENT_WRITE: poller_events |= 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 = 0 if events & EVENT_READ: selector_events |= self._EVENT_READ if events & EVENT_WRITE: selector_events |= 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 for fd, event in fd_event_list: events = 0 if event & ~self._EVENT_READ: events |= EVENT_WRITE if event & ~self._EVENT_WRITE: events |= EVENT_READ key = self._key_from_fd(fd) if key: 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'): 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 = max(len(self._fd_to_key), 1) ready = [] try: fd_event_list = self._selector.poll(timeout, max_ev) except InterruptedError: return ready for fd, event in fd_event_list: events = 0 if event & ~select.EPOLLIN: events |= EVENT_WRITE if event & ~select.EPOLLOUT: events |= EVENT_READ key = self._key_from_fd(fd) if key: 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() 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) if events & EVENT_WRITE: kev = select.kevent(key.fd, select.KQ_FILTER_WRITE, select.KQ_EV_ADD) self._selector.control([kev], 0, 0) 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) 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) 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 = max(len(self._fd_to_key), 1) ready = [] try: kev_list = self._selector.control(None, max_ev, timeout) except InterruptedError: return ready for kev in kev_list: fd = kev.ident flag = kev.filter events = 0 if flag == select.KQ_FILTER_READ: events |= EVENT_READ if flag == select.KQ_FILTER_WRITE: events |= EVENT_WRITE key = self._key_from_fd(fd) if key: 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