New submission from Zack Weinberg:
This is pretty esoteric, please bear with me. I'm attempting to enhance a
transparent-SOCKS module (https://github.com/Anorov/PySocks) to support
non-blocking connect(). This means, you should be able to do this:
socks.set_default_proxy(socks.SOCKS5, "localhost")
s = socks.socksocket(socket.AF_INET, socket.SOCK_STREAM)
s.setblocking(False)
err = s.connect_ex(address)
if err == errno.EINPROGRESS:
select.select([], [s], []) # returns when connection completes
Note the position of s in the select() arguments: the documented behavior of
non-blocking connect(), at the operating system level, is that the socket will
become *writable* when the connection resolves (whether successfully or not).
However, in this case, under the hood, the socket is *already connected* -- to
the proxy server -- after socksocket() returns. When connect_ex() returns
EINPROGRESS, the thing we're really waiting for is a SOCKS-protocol reply
message, i.e. the socket needs to become *readable* before the application can
continue. An application that used the above code (with a hypothetical version
of PySocks where this was supported) would get woken up immediately, since the
initial SOCKS client->server message doesn't even come close to filling up the
TCP send buffer.
There's no practical way to hide this divergence with the current library.
What would be needed, I think, is a pair of new special methods on filelikes,
which rewrite the set of events to listen for and the set of events to report.
Hypothetical, for my use case:
def __preselect__(self, events):
if not self._connecting: return events
return selectors.EVENT_READ
def __postselect__(self, events):
if not self._connecting: return events
return selectors.EVENT_WRITE
(I'm using the high-level selectors.EVENT_* constants for illustration. This
needs to happen in the low-level select-module methods, because callers can't
be expected to use selectors.)
There are a bunch of awkward corner cases to worry about with this, and I'm not
even sure the feature is worth it, but I thought I'd write up the problem and
see what other people think.
----------
components: IO
messages: 262612
nosy: zwol
priority: normal
severity: normal
status: open
title: File object hook to modify select(ors) event mask
type: enhancement
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue26666>
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