Queue enhancement suggestion

Antoon Pardon apardon at forel.vub.ac.be
Tue Apr 17 08:47:51 CEST 2007

On 2007-04-17, Hendrik van Rooyen <mail at microcorp.co.za> wrote:
> "Antoon Pardon" <a..n at forel.vub.ac.be> wrote:
>> The problem is this doesn't work well if you have multiple producers.
>> One producer can be finished while the other is still putting values
>> on the queue.
>> The solution I have been thinking on is the following.
>> Add an open and close operation. Only threads that have the queue
>> open can access it. The open call should specify whether you
>> want to read or write to the queue or both. When all writers
>> have closed the queue and the queue is empty a q.get will
>> raise an exception. This may be done by putting a sentinel
>> on the queue when the last writer closed the queue.
> This is beginning to look like a named pipe to me.
> The nice thing about queues is that there is currently so little
> BS about them - you just import the module, create one by binding
> a name to it, and you are in business, and anyone can read and/or
> write to it.

And if you are not carefull you have a deadlock. I tried queues
in a threaded gui application. Al the advise you get about such
applications tell you to have one thread doing all the gui-stuff.
So you basically have n producers and one consumer. Unfortunatly
the gui thread sometimes has things of its own it want to show.
So when the gui thread wants to put things on the queue you
risk a deadlock.

> If I were faced with the sort of thing addressed by this thread, I would
> probably use some sort of time out to decide when the end has happened.
> After all - if the task is long running, it never stops (hopefully), and if its
> a batch type job, it runs out of input and stops putting stuff on the queue.

This is unworkable for worker threads in a gui environment.

Antoon Pardon

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