from __future__ import annotations
import select
import socket
from functools import partial
__all__ = ["wait_for_read", "wait_for_write"]
# 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.
def select_wait_for_socket(
sock: socket.socket,
read: bool = False,
write: bool = False,
timeout: float | None = None,
) -> bool:
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 = fn(timeout)
return bool(rready or wready or xready)
def poll_wait_for_socket(
sock: socket.socket,
read: bool = False,
write: bool = False,
timeout: float | None = None,
) -> bool:
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: float | None) -> list[tuple[int, int]]:
if t is not None:
t *= 1000
return poll_obj.poll(t)
return bool(do_poll(timeout))
def _have_working_poll() -> bool:
# 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()
poll_obj.poll(0)
except (AttributeError, OSError):
return False
else:
return True
def wait_for_socket(
sock: socket.socket,
read: bool = False,
write: bool = False,
timeout: float | None = None,
) -> bool:
# 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
return wait_for_socket(sock, read, write, timeout)
[docs]def wait_for_read(sock: socket.socket, timeout: float | None = None) -> bool:
"""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)
[docs]def wait_for_write(sock: socket.socket, timeout: float | None = None) -> bool:
"""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)