| 1 | # SPDX-License-Identifier: MIT |
| 2 | # Backport of selectors.py from Python 3.5+ to support Python < 3.4 |
| 3 | # Also has the behavior specified in PEP 475 which is to retry syscalls |
| 4 | # in the case of an EINTR error. This module is required because selectors34 |
| 5 | # does not follow this behavior and instead returns that no dile descriptor |
| 6 | # events have occurred rather than retry the syscall. The decision to drop |
| 7 | # support for select.devpoll is made to maintain 100% test coverage. |
| 8 | |
| 9 | import errno |
| 10 | import math |
| 11 | import select |
| 12 | import socket |
| 13 | import sys |
| 14 | import time |
| 15 | |
| 16 | from collections import namedtuple |
| 17 | |
| 18 | try: |
| 19 | from collections import Mapping |
| 20 | except ImportError: |
| 21 | from collections.abc import Mapping |
| 22 | |
| 23 | try: |
| 24 | monotonic = time.monotonic |
| 25 | except (AttributeError, ImportError): # Python 3.3< |
| 26 | monotonic = time.time |
| 27 | |
| 28 | EVENT_READ = (1 << 0) |
| 29 | EVENT_WRITE = (1 << 1) |
| 30 | |
| 31 | HAS_SELECT = True # Variable that shows whether the platform has a selector. |
| 32 | _SYSCALL_SENTINEL = object() # Sentinel in case a system call returns None. |
| 33 | _DEFAULT_SELECTOR = None |
| 34 | |
| 35 | |
| 36 | class SelectorError(Exception): |
| 37 | def __init__(self, errcode): |
| 38 | super(SelectorError, self).__init__() |
| 39 | self.errno = errcode |
| 40 | |
| 41 | def __repr__(self): |
| 42 | return "<SelectorError errno={0}>".format(self.errno) |
| 43 | |
| 44 | def __str__(self): |
| 45 | return self.__repr__() |
| 46 | |
| 47 | |
| 48 | def _fileobj_to_fd(fileobj): |
| 49 | """ Return a file descriptor from a file object. If |
| 50 | given an integer will simply return that integer back. """ |
| 51 | if isinstance(fileobj, int): |
| 52 | fd = fileobj |
| 53 | else: |
| 54 | try: |
| 55 | fd = int(fileobj.fileno()) |
| 56 | except (AttributeError, TypeError, ValueError): |
| 57 | raise ValueError("Invalid file object: {0!r}".format(fileobj)) |
| 58 | if fd < 0: |
| 59 | raise ValueError("Invalid file descriptor: {0}".format(fd)) |
| 60 | return fd |
| 61 | |
| 62 | |
| 63 | # Determine which function to use to wrap system calls because Python 3.5+ |
| 64 | # already handles the case when system calls are interrupted. |
| 65 | if sys.version_info >= (3, 5): |
| 66 | def _syscall_wrapper(func, _, *args, **kwargs): |
| 67 | """ This is the short-circuit version of the below logic |
| 68 | because in Python 3.5+ all system calls automatically restart |
| 69 | and recalculate their timeouts. """ |
| 70 | try: |
| 71 | return func(*args, **kwargs) |
| 72 | except (OSError, IOError, select.error) as e: |
| 73 | errcode = None |
| 74 | if hasattr(e, "errno"): |
| 75 | errcode = e.errno |
| 76 | raise SelectorError(errcode) |
| 77 | else: |
| 78 | def _syscall_wrapper(func, recalc_timeout, *args, **kwargs): |
| 79 | """ Wrapper function for syscalls that could fail due to EINTR. |
| 80 | All functions should be retried if there is time left in the timeout |
| 81 | in accordance with PEP 475. """ |
| 82 | timeout = kwargs.get("timeout", None) |
| 83 | if timeout is None: |
| 84 | expires = None |
| 85 | recalc_timeout = False |
| 86 | else: |
| 87 | timeout = float(timeout) |
| 88 | if timeout < 0.0: # Timeout less than 0 treated as no timeout. |
| 89 | expires = None |
| 90 | else: |
| 91 | expires = monotonic() + timeout |
| 92 | |
| 93 | args = list(args) |
| 94 | if recalc_timeout and "timeout" not in kwargs: |
| 95 | raise ValueError( |
| 96 | "Timeout must be in args or kwargs to be recalculated") |
| 97 | |
| 98 | result = _SYSCALL_SENTINEL |
| 99 | while result is _SYSCALL_SENTINEL: |
| 100 | try: |
| 101 | result = func(*args, **kwargs) |
| 102 | # OSError is thrown by select.select |
| 103 | # IOError is thrown by select.epoll.poll |
| 104 | # select.error is thrown by select.poll.poll |
| 105 | # Aren't we thankful for Python 3.x rework for exceptions? |
| 106 | except (OSError, IOError, select.error) as e: |
| 107 | # select.error wasn't a subclass of OSError in the past. |
| 108 | errcode = None |
| 109 | if hasattr(e, "errno"): |
| 110 | errcode = e.errno |
| 111 | elif hasattr(e, "args"): |
| 112 | errcode = e.args[0] |
| 113 | |
| 114 | # Also test for the Windows equivalent of EINTR. |
| 115 | is_interrupt = (errcode == errno.EINTR or (hasattr(errno, "WSAEINTR") and |
| 116 | errcode == errno.WSAEINTR)) |
| 117 | |
| 118 | if is_interrupt: |
| 119 | if expires is not None: |
| 120 | current_time = monotonic() |
| 121 | if current_time > expires: |
| 122 | raise OSError(errno=errno.ETIMEDOUT) |
| 123 | if recalc_timeout: |
| 124 | if "timeout" in kwargs: |
| 125 | kwargs["timeout"] = expires - current_time |
| 126 | continue |
| 127 | if errcode: |
| 128 | raise SelectorError(errcode) |
| 129 | else: |
| 130 | raise |
| 131 | return result |
| 132 | |
| 133 | |
| 134 | SelectorKey = namedtuple('SelectorKey', ['fileobj', 'fd', 'events', 'data']) |
| 135 | |
| 136 | |
| 137 | class _SelectorMapping(Mapping): |
| 138 | """ Mapping of file objects to selector keys """ |
| 139 | |
| 140 | def __init__(self, selector): |
| 141 | self._selector = selector |
| 142 | |
| 143 | def __len__(self): |
| 144 | return len(self._selector._fd_to_key) |
| 145 | |
| 146 | def __getitem__(self, fileobj): |
| 147 | try: |
| 148 | fd = self._selector._fileobj_lookup(fileobj) |
| 149 | return self._selector._fd_to_key[fd] |
| 150 | except KeyError: |
| 151 | raise KeyError("{0!r} is not registered.".format(fileobj)) |
| 152 | |
| 153 | def __iter__(self): |
| 154 | return iter(self._selector._fd_to_key) |
| 155 | |
| 156 | |
| 157 | class BaseSelector(object): |
| 158 | """ Abstract Selector class |
| 159 | |
| 160 | A selector supports registering file objects to be monitored |
| 161 | for specific I/O events. |
| 162 | |
| 163 | A file object is a file descriptor or any object with a |
| 164 | `fileno()` method. An arbitrary object can be attached to the |
| 165 | file object which can be used for example to store context info, |
| 166 | a callback, etc. |
| 167 | |
| 168 | A selector can use various implementations (select(), poll(), epoll(), |
| 169 | and kqueue()) depending on the platform. The 'DefaultSelector' class uses |
| 170 | the most efficient implementation for the current platform. |
| 171 | """ |
| 172 | def __init__(self): |
| 173 | # Maps file descriptors to keys. |
| 174 | self._fd_to_key = {} |
| 175 | |
| 176 | # Read-only mapping returned by get_map() |
| 177 | self._map = _SelectorMapping(self) |
| 178 | |
| 179 | def _fileobj_lookup(self, fileobj): |
| 180 | """ Return a file descriptor from a file object. |
| 181 | This wraps _fileobj_to_fd() to do an exhaustive |
| 182 | search in case the object is invalid but we still |
| 183 | have it in our map. Used by unregister() so we can |
| 184 | unregister an object that was previously registered |
| 185 | even if it is closed. It is also used by _SelectorMapping |
| 186 | """ |
| 187 | try: |
| 188 | return _fileobj_to_fd(fileobj) |
| 189 | except ValueError: |
| 190 | |
| 191 | # Search through all our mapped keys. |
| 192 | for key in self._fd_to_key.values(): |
| 193 | if key.fileobj is fileobj: |
| 194 | return key.fd |
| 195 | |
| 196 | # Raise ValueError after all. |
| 197 | raise |
| 198 | |
| 199 | def register(self, fileobj, events, data=None): |
| 200 | """ Register a file object for a set of events to monitor. """ |
| 201 | if (not events) or (events & ~(EVENT_READ | EVENT_WRITE)): |
| 202 | raise ValueError("Invalid events: {0!r}".format(events)) |
| 203 | |
| 204 | key = SelectorKey(fileobj, self._fileobj_lookup(fileobj), events, data) |
| 205 | |
| 206 | if key.fd in self._fd_to_key: |
| 207 | raise KeyError("{0!r} (FD {1}) is already registered" |
| 208 | .format(fileobj, key.fd)) |
| 209 | |
| 210 | self._fd_to_key[key.fd] = key |
| 211 | return key |
| 212 | |
| 213 | def unregister(self, fileobj): |
| 214 | """ Unregister a file object from being monitored. """ |
| 215 | try: |
| 216 | key = self._fd_to_key.pop(self._fileobj_lookup(fileobj)) |
| 217 | except KeyError: |
| 218 | raise KeyError("{0!r} is not registered".format(fileobj)) |
| 219 | |
| 220 | # Getting the fileno of a closed socket on Windows errors with EBADF. |
| 221 | except socket.error as e: # Platform-specific: Windows. |
| 222 | if e.errno != errno.EBADF: |
| 223 | raise |
| 224 | else: |
| 225 | for key in self._fd_to_key.values(): |
| 226 | if key.fileobj is fileobj: |
| 227 | self._fd_to_key.pop(key.fd) |
| 228 | break |
| 229 | else: |
| 230 | raise KeyError("{0!r} is not registered".format(fileobj)) |
| 231 | return key |
| 232 | |
| 233 | def modify(self, fileobj, events, data=None): |
| 234 | """ Change a registered file object monitored events and data. """ |
| 235 | # NOTE: Some subclasses optimize this operation even further. |
| 236 | try: |
| 237 | key = self._fd_to_key[self._fileobj_lookup(fileobj)] |
| 238 | except KeyError: |
| 239 | raise KeyError("{0!r} is not registered".format(fileobj)) |
| 240 | |
| 241 | if events != key.events: |
| 242 | self.unregister(fileobj) |
| 243 | key = self.register(fileobj, events, data) |
| 244 | |
| 245 | elif data != key.data: |
| 246 | # Use a shortcut to update the data. |
| 247 | key = key._replace(data=data) |
| 248 | self._fd_to_key[key.fd] = key |
| 249 | |
| 250 | return key |
| 251 | |
| 252 | def select(self, timeout=None): |
| 253 | """ Perform the actual selection until some monitored file objects |
| 254 | are ready or the timeout expires. """ |
| 255 | raise NotImplementedError() |
| 256 | |
| 257 | def close(self): |
| 258 | """ Close the selector. This must be called to ensure that all |
| 259 | underlying resources are freed. """ |
| 260 | self._fd_to_key.clear() |
| 261 | self._map = None |
| 262 | |
| 263 | def get_key(self, fileobj): |
| 264 | """ Return the key associated with a registered file object. """ |
| 265 | mapping = self.get_map() |
| 266 | if mapping is None: |
| 267 | raise RuntimeError("Selector is closed") |
| 268 | try: |
| 269 | return mapping[fileobj] |
| 270 | except KeyError: |
| 271 | raise KeyError("{0!r} is not registered".format(fileobj)) |
| 272 | |
| 273 | def get_map(self): |
| 274 | """ Return a mapping of file objects to selector keys """ |
| 275 | return self._map |
| 276 | |
| 277 | def _key_from_fd(self, fd): |
| 278 | """ Return the key associated to a given file descriptor |
| 279 | Return None if it is not found. """ |
| 280 | try: |
| 281 | return self._fd_to_key[fd] |
| 282 | except KeyError: |
| 283 | return None |
| 284 | |
| 285 | def __enter__(self): |
| 286 | return self |
| 287 | |
| 288 | def __exit__(self, *args): |
| 289 | self.close() |
| 290 | |
| 291 | |
| 292 | # Almost all platforms have select.select() |
| 293 | if hasattr(select, "select"): |
| 294 | class SelectSelector(BaseSelector): |
| 295 | """ Select-based selector. """ |
| 296 | def __init__(self): |
| 297 | super(SelectSelector, self).__init__() |
| 298 | self._readers = set() |
| 299 | self._writers = set() |
| 300 | |
| 301 | def register(self, fileobj, events, data=None): |
| 302 | key = super(SelectSelector, self).register(fileobj, events, data) |
| 303 | if events & EVENT_READ: |
| 304 | self._readers.add(key.fd) |
| 305 | if events & EVENT_WRITE: |
| 306 | self._writers.add(key.fd) |
| 307 | return key |
| 308 | |
| 309 | def unregister(self, fileobj): |
| 310 | key = super(SelectSelector, self).unregister(fileobj) |
| 311 | self._readers.discard(key.fd) |
| 312 | self._writers.discard(key.fd) |
| 313 | return key |
| 314 | |
| 315 | def _select(self, r, w, timeout=None): |
| 316 | """ Wrapper for select.select because timeout is a positional arg """ |
| 317 | return select.select(r, w, [], timeout) |
| 318 | |
| 319 | def select(self, timeout=None): |
| 320 | # Selecting on empty lists on Windows errors out. |
| 321 | if not len(self._readers) and not len(self._writers): |
| 322 | return [] |
| 323 | |
| 324 | timeout = None if timeout is None else max(timeout, 0.0) |
| 325 | ready = [] |
| 326 | r, w, _ = _syscall_wrapper(self._select, True, self._readers, |
| 327 | self._writers, timeout) |
| 328 | r = set(r) |
| 329 | w = set(w) |
| 330 | for fd in r | w: |
| 331 | events = 0 |
| 332 | if fd in r: |
| 333 | events |= EVENT_READ |
| 334 | if fd in w: |
| 335 | events |= EVENT_WRITE |
| 336 | |
| 337 | key = self._key_from_fd(fd) |
| 338 | if key: |
| 339 | ready.append((key, events & key.events)) |
| 340 | return ready |
| 341 | |
| 342 | |
| 343 | if hasattr(select, "poll"): |
| 344 | class PollSelector(BaseSelector): |
| 345 | """ Poll-based selector """ |
| 346 | def __init__(self): |
| 347 | super(PollSelector, self).__init__() |
| 348 | self._poll = select.poll() |
| 349 | |
| 350 | def register(self, fileobj, events, data=None): |
| 351 | key = super(PollSelector, self).register(fileobj, events, data) |
| 352 | event_mask = 0 |
| 353 | if events & EVENT_READ: |
| 354 | event_mask |= select.POLLIN |
| 355 | if events & EVENT_WRITE: |
| 356 | event_mask |= select.POLLOUT |
| 357 | self._poll.register(key.fd, event_mask) |
| 358 | return key |
| 359 | |
| 360 | def unregister(self, fileobj): |
| 361 | key = super(PollSelector, self).unregister(fileobj) |
| 362 | self._poll.unregister(key.fd) |
| 363 | return key |
| 364 | |
| 365 | def _wrap_poll(self, timeout=None): |
| 366 | """ Wrapper function for select.poll.poll() so that |
| 367 | _syscall_wrapper can work with only seconds. """ |
| 368 | if timeout is not None: |
| 369 | if timeout <= 0: |
| 370 | timeout = 0 |
| 371 | else: |
| 372 | # select.poll.poll() has a resolution of 1 millisecond, |
| 373 | # round away from zero to wait *at least* timeout seconds. |
| 374 | timeout = math.ceil(timeout * 1e3) |
| 375 | |
| 376 | result = self._poll.poll(timeout) |
| 377 | return result |
| 378 | |
| 379 | def select(self, timeout=None): |
| 380 | ready = [] |
| 381 | fd_events = _syscall_wrapper(self._wrap_poll, True, timeout=timeout) |
| 382 | for fd, event_mask in fd_events: |
| 383 | events = 0 |
| 384 | if event_mask & ~select.POLLIN: |
| 385 | events |= EVENT_WRITE |
| 386 | if event_mask & ~select.POLLOUT: |
| 387 | events |= EVENT_READ |
| 388 | |
| 389 | key = self._key_from_fd(fd) |
| 390 | if key: |
| 391 | ready.append((key, events & key.events)) |
| 392 | |
| 393 | return ready |
| 394 | |
| 395 | |
| 396 | if hasattr(select, "epoll"): |
| 397 | class EpollSelector(BaseSelector): |
| 398 | """ Epoll-based selector """ |
| 399 | def __init__(self): |
| 400 | super(EpollSelector, self).__init__() |
| 401 | self._epoll = select.epoll() |
| 402 | |
| 403 | def fileno(self): |
| 404 | return self._epoll.fileno() |
| 405 | |
| 406 | def register(self, fileobj, events, data=None): |
| 407 | key = super(EpollSelector, self).register(fileobj, events, data) |
| 408 | events_mask = 0 |
| 409 | if events & EVENT_READ: |
| 410 | events_mask |= select.EPOLLIN |
| 411 | if events & EVENT_WRITE: |
| 412 | events_mask |= select.EPOLLOUT |
| 413 | _syscall_wrapper(self._epoll.register, False, key.fd, events_mask) |
| 414 | return key |
| 415 | |
| 416 | def unregister(self, fileobj): |
| 417 | key = super(EpollSelector, self).unregister(fileobj) |
| 418 | try: |
| 419 | _syscall_wrapper(self._epoll.unregister, False, key.fd) |
| 420 | except SelectorError: |
| 421 | # This can occur when the fd was closed since registry. |
| 422 | pass |
| 423 | return key |
| 424 | |
| 425 | def select(self, timeout=None): |
| 426 | if timeout is not None: |
| 427 | if timeout <= 0: |
| 428 | timeout = 0.0 |
| 429 | else: |
| 430 | # select.epoll.poll() has a resolution of 1 millisecond |
| 431 | # but luckily takes seconds so we don't need a wrapper |
| 432 | # like PollSelector. Just for better rounding. |
| 433 | timeout = math.ceil(timeout * 1e3) * 1e-3 |
| 434 | timeout = float(timeout) |
| 435 | else: |
| 436 | timeout = -1.0 # epoll.poll() must have a float. |
| 437 | |
| 438 | # We always want at least 1 to ensure that select can be called |
| 439 | # with no file descriptors registered. Otherwise will fail. |
| 440 | max_events = max(len(self._fd_to_key), 1) |
| 441 | |
| 442 | ready = [] |
| 443 | fd_events = _syscall_wrapper(self._epoll.poll, True, |
| 444 | timeout=timeout, |
| 445 | maxevents=max_events) |
| 446 | for fd, event_mask in fd_events: |
| 447 | events = 0 |
| 448 | if event_mask & ~select.EPOLLIN: |
| 449 | events |= EVENT_WRITE |
| 450 | if event_mask & ~select.EPOLLOUT: |
| 451 | events |= EVENT_READ |
| 452 | |
| 453 | key = self._key_from_fd(fd) |
| 454 | if key: |
| 455 | ready.append((key, events & key.events)) |
| 456 | return ready |
| 457 | |
| 458 | def close(self): |
| 459 | self._epoll.close() |
| 460 | super(EpollSelector, self).close() |
| 461 | |
| 462 | |
| 463 | if hasattr(select, "kqueue"): |
| 464 | class KqueueSelector(BaseSelector): |
| 465 | """ Kqueue / Kevent-based selector """ |
| 466 | def __init__(self): |
| 467 | super(KqueueSelector, self).__init__() |
| 468 | self._kqueue = select.kqueue() |
| 469 | |
| 470 | def fileno(self): |
| 471 | return self._kqueue.fileno() |
| 472 | |
| 473 | def register(self, fileobj, events, data=None): |
| 474 | key = super(KqueueSelector, self).register(fileobj, events, data) |
| 475 | if events & EVENT_READ: |
| 476 | kevent = select.kevent(key.fd, |
| 477 | select.KQ_FILTER_READ, |
| 478 | select.KQ_EV_ADD) |
| 479 | |
| 480 | _syscall_wrapper(self._kqueue.control, False, [kevent], 0, 0) |
| 481 | |
| 482 | if events & EVENT_WRITE: |
| 483 | kevent = select.kevent(key.fd, |
| 484 | select.KQ_FILTER_WRITE, |
| 485 | select.KQ_EV_ADD) |
| 486 | |
| 487 | _syscall_wrapper(self._kqueue.control, False, [kevent], 0, 0) |
| 488 | |
| 489 | return key |
| 490 | |
| 491 | def unregister(self, fileobj): |
| 492 | key = super(KqueueSelector, self).unregister(fileobj) |
| 493 | if key.events & EVENT_READ: |
| 494 | kevent = select.kevent(key.fd, |
| 495 | select.KQ_FILTER_READ, |
| 496 | select.KQ_EV_DELETE) |
| 497 | try: |
| 498 | _syscall_wrapper(self._kqueue.control, False, [kevent], 0, 0) |
| 499 | except SelectorError: |
| 500 | pass |
| 501 | if key.events & EVENT_WRITE: |
| 502 | kevent = select.kevent(key.fd, |
| 503 | select.KQ_FILTER_WRITE, |
| 504 | select.KQ_EV_DELETE) |
| 505 | try: |
| 506 | _syscall_wrapper(self._kqueue.control, False, [kevent], 0, 0) |
| 507 | except SelectorError: |
| 508 | pass |
| 509 | |
| 510 | return key |
| 511 | |
| 512 | def select(self, timeout=None): |
| 513 | if timeout is not None: |
| 514 | timeout = max(timeout, 0) |
| 515 | |
| 516 | max_events = len(self._fd_to_key) * 2 |
| 517 | ready_fds = {} |
| 518 | |
| 519 | kevent_list = _syscall_wrapper(self._kqueue.control, True, |
| 520 | None, max_events, timeout) |
| 521 | |
| 522 | for kevent in kevent_list: |
| 523 | fd = kevent.ident |
| 524 | event_mask = kevent.filter |
| 525 | events = 0 |
| 526 | if event_mask == select.KQ_FILTER_READ: |
| 527 | events |= EVENT_READ |
| 528 | if event_mask == select.KQ_FILTER_WRITE: |
| 529 | events |= EVENT_WRITE |
| 530 | |
| 531 | key = self._key_from_fd(fd) |
| 532 | if key: |
| 533 | if key.fd not in ready_fds: |
| 534 | ready_fds[key.fd] = (key, events & key.events) |
| 535 | else: |
| 536 | old_events = ready_fds[key.fd][1] |
| 537 | ready_fds[key.fd] = (key, (events | old_events) & key.events) |
| 538 | |
| 539 | return list(ready_fds.values()) |
| 540 | |
| 541 | def close(self): |
| 542 | self._kqueue.close() |
| 543 | super(KqueueSelector, self).close() |
| 544 | |
| 545 | |
| 546 | if not hasattr(select, 'select'): # Platform-specific: AppEngine |
| 547 | HAS_SELECT = False |
| 548 | |
| 549 | |
| 550 | def _can_allocate(struct): |
| 551 | """ Checks that select structs can be allocated by the underlying |
| 552 | operating system, not just advertised by the select module. We don't |
| 553 | check select() because we'll be hopeful that most platforms that |
| 554 | don't have it available will not advertise it. (ie: GAE) """ |
| 555 | try: |
| 556 | # select.poll() objects won't fail until used. |
| 557 | if struct == 'poll': |
| 558 | p = select.poll() |
| 559 | p.poll(0) |
| 560 | |
| 561 | # All others will fail on allocation. |
| 562 | else: |
| 563 | getattr(select, struct)().close() |
| 564 | return True |
| 565 | except (OSError, AttributeError) as e: |
| 566 | return False |
| 567 | |
| 568 | |
| 569 | # Choose the best implementation, roughly: |
| 570 | # kqueue == epoll > poll > select. Devpoll not supported. (See above) |
| 571 | # select() also can't accept a FD > FD_SETSIZE (usually around 1024) |
| 572 | def DefaultSelector(): |
| 573 | """ This function serves as a first call for DefaultSelector to |
| 574 | detect if the select module is being monkey-patched incorrectly |
| 575 | by eventlet, greenlet, and preserve proper behavior. """ |
| 576 | global _DEFAULT_SELECTOR |
| 577 | if _DEFAULT_SELECTOR is None: |
| 578 | if _can_allocate('kqueue'): |
| 579 | _DEFAULT_SELECTOR = KqueueSelector |
| 580 | elif _can_allocate('epoll'): |
| 581 | _DEFAULT_SELECTOR = EpollSelector |
| 582 | elif _can_allocate('poll'): |
| 583 | _DEFAULT_SELECTOR = PollSelector |
| 584 | elif hasattr(select, 'select'): |
| 585 | _DEFAULT_SELECTOR = SelectSelector |
| 586 | else: # Platform-specific: AppEngine |
| 587 | raise ValueError('Platform does not have a selector') |
| 588 | return _DEFAULT_SELECTOR() |