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wait.py
import errno from functools import partial import select import sys try: from time import monotonic except ImportError: from time import time as monotonic __all__ = ["NoWayToWaitForSocketError", "wait_for_read", "wait_for_write"] class NoWayToWaitForSocketError(Exception): pass # How should we wait on sockets? # # There are two types of APIs you can use for waiting on sockets: the fancy # modern stateful APIs like epoll/kqueue, and the older stateless APIs like # select/poll. The stateful APIs are more efficient when you have a lots of # sockets to keep track of, because you can set them up once and then use them # lots of times. But we only ever want to wait on a single socket at a time # and don't want to keep track of state, so the stateless APIs are actually # more efficient. So we want to use select() or poll(). # # Now, how do we choose between select() and poll()? On traditional Unixes, # select() has a strange calling convention that makes it slow, or fail # altogether, for high-numbered file descriptors. The point of poll() is to fix # that, so on Unixes, we prefer poll(). # # On Windows, there is no poll() (or at least Python doesn't provide a wrapper # for it), but that's OK, because on Windows, select() doesn't have this # strange calling convention; plain select() works fine. # # So: on Windows we use select(), and everywhere else we use poll(). We also # fall back to select() in case poll() is somehow broken or missing. if sys.version_info >= (3, 5): # Modern Python, that retries syscalls by default def _retry_on_intr(fn, timeout): return fn(timeout) else: # Old and broken Pythons. def _retry_on_intr(fn, timeout): if timeout is None: deadline = float("inf") else: deadline = monotonic() + timeout while True: try: return fn(timeout) # OSError for 3 <= pyver < 3.5, select.error for pyver <= 2.7 except (OSError, select.error) as e: # 'e.args[0]' incantation works for both OSError and select.error if e.args[0] != errno.EINTR: raise else: timeout = deadline - monotonic() if timeout < 0: timeout = 0 if timeout == float("inf"): timeout = None continue def select_wait_for_socket(sock, read=False, write=False, timeout=None): if not read and not write: raise RuntimeError("must specify at least one of read=True, write=True") rcheck = [] wcheck = [] if read: rcheck.append(sock) if write: wcheck.append(sock) # When doing a non-blocking connect, most systems signal success by # marking the socket writable. Windows, though, signals success by marked # it as "exceptional". We paper over the difference by checking the write # sockets for both conditions. (The stdlib selectors module does the same # thing.) fn = partial(select.select, rcheck, wcheck, wcheck) rready, wready, xready = _retry_on_intr(fn, timeout) return bool(rready or wready or xready) def poll_wait_for_socket(sock, read=False, write=False, timeout=None): if not read and not write: raise RuntimeError("must specify at least one of read=True, write=True") mask = 0 if read: mask |= select.POLLIN if write: mask |= select.POLLOUT poll_obj = select.poll() poll_obj.register(sock, mask) # For some reason, poll() takes timeout in milliseconds def do_poll(t): if t is not None: t *= 1000 return poll_obj.poll(t) return bool(_retry_on_intr(do_poll, timeout)) def null_wait_for_socket(*args, **kwargs): raise NoWayToWaitForSocketError("no select-equivalent available") def _have_working_poll(): # Apparently some systems have a select.poll that fails as soon as you try # to use it, either due to strange configuration or broken monkeypatching # from libraries like eventlet/greenlet. try: poll_obj = select.poll() _retry_on_intr(poll_obj.poll, 0) except (AttributeError, OSError): return False else: return True def wait_for_socket(*args, **kwargs): # We delay choosing which implementation to use until the first time we're # called. We could do it at import time, but then we might make the wrong # decision if someone goes wild with monkeypatching select.poll after # we're imported. global wait_for_socket if _have_working_poll(): wait_for_socket = poll_wait_for_socket elif hasattr(select, "select"): wait_for_socket = select_wait_for_socket else: # Platform-specific: Appengine. wait_for_socket = null_wait_for_socket return wait_for_socket(*args, **kwargs) def wait_for_read(sock, timeout=None): """ Waits for reading to be available on a given socket. Returns True if the socket is readable, or False if the timeout expired. """ return wait_for_socket(sock, read=True, timeout=timeout) def wait_for_write(sock, timeout=None): """ Waits for writing to be available on a given socket. Returns True if the socket is readable, or False if the timeout expired. """ return wait_for_socket(sock, write=True, timeout=timeout)
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