[issue7674] select.select() corner cases: duplicate fds, out-of-range fds

[Berker Peksag]

Change by Berker Peksag :


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[issue7674] select.select() corner cases: duplicate fds, out-of-range fds

[Mark Lawrence]

Changes by Mark Lawrence breamore...@yahoo.co.uk:


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[issue7674] select.select() corner cases: duplicate fds, out-of-range fds

[Tshepang Lekhonkhobe]

Changes by Tshepang Lekhonkhobe tshep...@gmail.com:


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[issue7674] select.select() corner cases: duplicate fds, out-of-range fds

[Mark Lawrence]

Mark Lawrence breamore...@yahoo.co.uk added the comment:

Chris, to me it's as clear as mud but please produce a doc patch anyway. :)

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[issue7674] select.select() corner cases: duplicate fds, out-of-range fds

[Antoine Pitrou]

Changes by Antoine Pitrou pit...@free.fr:


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components:  -Documentation
nosy: +exarkun
versions: +Python 3.2

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[issue7674] select.select() corner cases: duplicate fds, out-of-range fds

[Chris Leary]

New submission from Chris Leary cd...@cornell.edu:

I was just reading through this ACM article that enumerates some of the issues 
with the select function in .NET: 
http://cacm.acm.org/magazines/2009/5/24646-api-design-matters/fulltext

select.select() currently suffers from the same documentation problem where the 
behavior with duplicate and/or out-of-range file descriptors in one of the 
sequences (i.e. rlist) is not described.

Given the current implementation of seq2set in trunk it appears that:

1. A ValueError is raised when a given file descriptor is out of range. 
(Typically a result of the programmer passing a non-fd value, since FD_SETSIZE 
is normally at least equal to the maximum number of descriptors supported by 
the system.)

2. Duplicate file descriptor numbers are collapsed into the fd_set, and are 
therefore idempotent at a system API level.

However, the language-level support code generally assumes no duplication, as 
there is a fixed size array of (FD_SETSIZE + 1) pylist entries (one additional 
for a sentinel value). Although there is a TODO to dynamically size that to the 
largest targeted file descriptor number, that would still assume one PyObject 
per file descriptor in the input sequences.

The set2list function used to produce a return value will, however, return 
duplicates: for each value in the input list, if the corresponding fd is set, 
that pyobject is added to the return list.


Proposed Changes


At a glance it would seem that the Right Thing to do is to collapse duplicates 
in the input, as if we created a set(AsFileDescriptor(o) for o in input_list), 
so that no duplicates will be returned in the result; however, you *can* have a 
heterogeneous input list with a fileno like 5 and a file-like object whose 
fileno() resolved to 5, in which case you don't want to arbitrarily choose only 
one of those PyObjects to return. Therefore, I'm thinking it's probably best to 
leave it as-is and document it.

In any case, if we want to explicitly allow duplicates in the input list we 
should probably make the pylist arrays into dynamically sized structures in the 
sizes of the corresponding input lists for correctness.

If this all makes sense I'll be happy to come up with a 
module/documentation/unit test patch.

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assignee: georg.brandl
components: Documentation, Extension Modules
messages: 97578
nosy: cdleary, georg.brandl
severity: normal
status: open
title: select.select() corner cases: duplicate fds, out-of-range fds
type: behavior

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