Roy Smith wrote:
In article <gvb8fn$7g...@lust.ihug.co.nz>,
Lawrence D'Oliveiro <l...@geek-central.gen.new_zealand> wrote:
In message <mailman.651.1243154739.8015.python-l...@python.org>, Igor Katson
wrote:
I have written a socket server and some arbitrary clients. When I
shutdown the server, and do socket.close(), I cannot immediately start
it again cause it has some open sockets in TIME_WAIT state. It throws
address already in use exception at me.
There's a reason for that. It's to ensure that there are no leftover packets
floating around the Internet somewhere, that you might mistakenly receive
and think they were part of a new connection, when they were in fact part of
an old one.
In theory, that is indeed the reason for the TIME_WAIT state. In practice,
however, using SO_REUSEADDR is pretty safe, and common practice.
You've got several things working in your favor. First, late-delivery of
packets is pretty rare. Second, if some late packet were to arrive, the
chances of them having the same local and remote port numbers as an
existing connection is slim. And, finally, the TCP sequence number won't
line up.
One thing to be aware of is that SO_REUSEADDR isn't 100% portable. There
are some systems (ISTR HP-UX) which use SO_REUSEPORT instead of
SO_REUSEADDR. The original specifications weren't very clear, and some
implementers read them in strange ways. Some of that old code continues in
use today. I only mention this because if you try SO_REUSEADDR and it's
not doing what you expect, it's worth trying SO_REUSEPORT (or both) to see
what happens on your particular system.
The right thing to do is try to ensure that all your connections are
properly closed at shutdown. That may not be enough (if your server crashes
due to bugs), so the other thing you need to do is retry the socket open,
say, at 30-second intervals, until it succeeds.
That may be a reasonable thing to do for production code, but when you're
building and debugging a server, it's a real pain to not be able to restart
it quickly whenever you want (or need) to.
Thanks for a great answer, Roy!
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