New submission from Raymond Hettinger:
I was working through the problem sets in The Little Book of Semaphores
(http://www.greenteapress.com/semaphores/downey08semaphores.pdf) and ran into
an issue because Python's threading.Semaphore has an unnecessary restriction
against having negative values.
That precludes use cases such as simple barriers (i.e. wait on five signals
before a wait is released).
Various descriptions of Semaphores allow their counts to be set to arbitrary
integer values. Here's one definition:
1. When you create the semaphore, you can initialize its
value to any integer, but after that the only operations
you are allowed to perform are increment (increase by one)
and decrement (decrease by one). You cannot read the current
value of the semaphore.
2. When a thread decrements the semaphore, if the result is negative,
the thread blocks itself and cannot continue until another
thread increments the semaphore.
3. When a thread increments the semaphore, if there are other
threads waiting, one of the waiting threads gets unblocked.
The patch is simple, remove the guard in the initialization and change the
value==0 test with value<=0 to trigger blocking.
A bit of demonstration code is attached.
----------
components: Library (Lib)
files: barrier.py
messages: 183642
nosy: rhettinger
priority: normal
severity: normal
status: open
title: Remove restriction against Semaphore having a negative value
type: enhancement
versions: Python 3.4
Added file: http://bugs.python.org/file29334/barrier.py
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue17374>
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