Problem with blocking portably on sockets and Queue?

[Tero Saarni]

Hi,

I have several threads communicating with each other using events
stored in Queues. Threads block on Queue.get() until somebody
publishes an event in thread's event queue.

I need to add support for sockets to the system. Thread needs to
unblock when:

- there is socket ready to be read, or
- there is event waiting in the queue

My first tought was to replace blocking on Queue.get() with blocking
on poll or select and dedicating file descriptors (created with os.pipe())
as an semaphore. Event publisher would write something to the write
end of the pipe when it puts an event to the queue, effectively
unblocking the receiver.

BUT I noticed that select.poll() is not available on Windows and
secondly Windows version of select.select() will accept only socket
descriptors.

What options do I have that are still portable also to Windows
platform?

 

[Alex Martelli]

<posted & mailed>

Hi,

I have several threads communicating with each other using events
stored in Queues. Threads block on Queue.get() until somebody
publishes an event in thread's event queue.

So far so wonderful -- excellent architecture.

I need to add support for sockets to the system. Thread needs to
unblock when:

- there is socket ready to be read, or
- there is event waiting in the queue

My first tought was to replace blocking on Queue.get() with blocking
on poll or select and dedicating file descriptors (created with os.pipe())
as an semaphore. Event publisher would write something to the write
end of the pipe when it puts an event to the queue, effectively
unblocking the receiver.

BUT I noticed that select.poll() is not available on Windows and
secondly Windows version of select.select() will accept only socket
descriptors.

What options do I have that are still portable also to Windows
platform?

I think you could devote a thread specifically to the task of handling
sockets, only. That special thread would not block on any queue but
just on select; when its select shows that a socket is ready, you can
have the thread itself do the reading and post the appropriate special
message including the data read to the appropriate thread, or you could
pass the ready-for-reading socket object to the appropriate thread, or
the like. This needs a bit more care if the set of sockets to be
select'ed on changes with time; in this case, I think the socket-handler
thread would have to use a timeout on its select so the set of sockets
can be updated once in a while (either by another thread, in which
case you need a lock to protect the "set of sockets" shared data, or
by the socket-handling thread (SHT) itself in response to a queued
message -- the SHT would read non-blockingly from that queue periodically
when the select times out) -- or you could perhaps use a special
identifiable socket for the SHT to receive from other threads requests
to change the set of ('true') sockets to be selected on.


Alex

 

[Tero Saarni]

<posted & mailed>



So far so wonderful -- excellent architecture.



I think you could devote a thread specifically to the task of handling
sockets, only. That special thread would not block on any queue but
just on select; when its select shows that a socket is ready, you can
have the thread itself do the reading and post the appropriate special
message including the data read to the appropriate thread, or you could
pass the ready-for-reading socket object to the appropriate thread, or
the like. This needs a bit more care if the set of sockets to be
select'ed on changes with time; in this case, I think the socket-handler
thread would have to use a timeout on its select so the set of sockets
can be updated once in a while (either by another thread, in which
case you need a lock to protect the "set of sockets" shared data, or
by the socket-handling thread (SHT) itself in response to a queued
message -- the SHT would read non-blockingly from that queue periodically
when the select times out) -- or you could perhaps use a special
identifiable socket for the SHT to receive from other threads requests
to change the set of ('true') sockets to be selected on.


Alex

Good idea, thanks! I will try dedicating a special thread for socket
handling, although breaking out of select() for modifying the set of
sockets gets a little tricky as you pointed out.

Momentarily I was even thinking that I could break out of select() by
sending a signal when I want to modify the set of sockets, but
as I was almost expecting, it appeared that also signal support is
quite limited on Windows platform.

I guess that fallback solution would be to learn Windows
sychronization primitives (wait functions etc) and write two separate
implementations of relevant parts of the system. With some work
that could probably be hidden behind matching interfaces and a
factory method choosing the implementation on the fly.